Potentiometric and Speciation Studies on the Complex Formation Reactions of [Pd(2-methylaminomethyl)-pyridine)(H2O)2]2+ with Some Bio-active Ligands and Displacement Reaction of Coordinated Inosine

Article abstract of DOI:10.1007/s10953-017-0621-z

The stoichiometry and stability constants of the complexes formed between [Pd(MAMP)(H₂O)₂]²⁺ and various biologically relevant ligands containing different functional groups were investigated. The ligands used are amino acids, peptides and DNA constituents. The results show the formation of 1:1 complexes with amino acids and peptides and the corresponding deprotonated amide species. Structural effects of peptides on amide deprotonation were investigated. The purine and pyrimidine bases uracil, uridine, cytosine, inosine, inosine 5′-monophosphate (5′-IMP) and thymine form 1:1 and 1:2 complexes. The concentration distribution of the various complex species was calculated as a function of pH. The effect of chloride ion concentration on the formation constant of CBDCA with Pd(MAMP)²⁺ was also reported. The results show ring opening of CBDCA and monodentate complexation of the DNA constituent with the formation of [Pd(MAMP)(CBDCA-O)DNA], where (CBDCA-O) represents cyclobutane dicarboxylate coordinated by one carboxylate oxygen. The equilibrium constant of the displacement reaction of coordinated inosine, as a typical DNA constituent, by SMC and/or methionine was calculated. The results are expected to contribute to the chemistry of antitumor agents. The calculated parameters of the optimized complexes support the measured formation constants.

Full text of DOI:10.1007/s10953-017-0621-z

J Solution Chem (2017) 46:1024–1047
DOI 10.1007/s10953-017-0621-z
Potentiometric and Speciation Studies on the Complex
Formation Reactions of [Pd(2-methylaminomethyl)-
pyridine)(H O) ] with Some Bio-active Ligands
2
+
2
2
and Displacement Reaction of Coordinated Inosine
1
1
•
Abeer T. Abd El-Karim
1
•
Islam R. El-Sherif
2
3
•
Wafaa M. Hosny
Ahmed A. El-Sherif
•
Eyad K. Alkhadhairi
1
•
Mutlaq S. Aljahdali
,4
Received: 19 November 2016 / Accepted: 13 March 2017 / Published online: 10 May 2017
Ó Springer Science+Business Media New York 2017
Abstract The stoichiometry and stability constants of the complexes formed between
[Pd(MAMP)(H O) ]
2?
and various biologically relevant ligands containing different
2
2
functional groups were investigated. The ligands used are amino acids, peptides and DNA
constituents. The results show the formation of 1:1 complexes with amino acids and
peptides and the corresponding deprotonated amide species. Structural effects of peptides
on amide deprotonation were investigated. The purine and pyrimidine bases uracil, uridine,
0
0
cytosine, inosine, inosine 5 -monophosphate (5 -IMP) and thymine form 1:1 and 1:2
complexes. The concentration distribution of the various complex species was calculated
as a function of pH. The effect of chloride ion concentration on the formation constant of
2
?
CBDCA with Pd(MAMP) was also reported. The results show ring opening of CBDCA
and monodentate complexation of the DNA constituent with the formation of
[Pd(MAMP)(CBDCA-O)DNA], where (CBDCA-O) represents cyclobutane dicarboxylate
coordinated by one carboxylate oxygen. The equilibrium constant of the displacement
reaction of coordinated inosine, as a typical DNA constituent, by SMC and/or methionine
was calculated. The results are expected to contribute to the chemistry of antitumor agents.
The calculated parameters of the optimized complexes support the measured formation
constants.
Electronic supplementary material The online version of this article (doi:10.1007/s10953-017-0621-z)
contains supplementary material, which is available to authorized users.
&
&
Abeer T. Abd El-Karim
tabdelkarim@yahoo.com
Ahmed A. El-Sherif
aelsherif72@yahoo.com
1
2
3
4
Faculty of Science, Chemistry Department, Cairo University, Giza, Egypt
Faculty of Pharmacy, Northern Border University, Rafha, Saudi Arabia
Faculty of Science, Chemistry Department, King AbdAl-Aziz University, Jeddah, Saudi Arabia
Faculty of Arts and Science, Chemistry Department, Northern Border University, Rafha, Saudi
Arabia
1
23

J Solution Chem (2017) 46:1024–1047
1025
Keywords 2-(Methylamino)methylpyridine Á Amino acids Á Peptides Á DNA
constituents Á Potentiometric titration
Abbreviations
CBDCA 1,1-Cyclobutanedicarboxylic acid
MAMP
2-(Methylamino)methylpyridine
0
0
5
5
-IMP
-GMP
Inosine-5 -monophosphate
0
0
Guanosine-5 -monophosphate
S-methyl-L-cysteine
SMC
DMEN
Thr
0
N,N -Dimethylethylenediamine
Threonine
Serine
Ethylenediamine
Ser
En
1
,3-DAP 1,3-Diaminopropane
0
BPY
a-Ala
b-Ala
PM3
UV
HOMO
LUMO
v
N,N -Bipyridyl
a-Alanine
b-Alanine
Parametric Method-3
Ultraviolet
Highest occupied molecular orbital energy
Lowest unoccupied molecular orbital energy
Mulliken electronegativity
Chemical potential
Pi
g
S
Global hardness
Global softness
Global electrophilicity
x
1
Introduction
Since the discovery of the antitumor properties of platinum compounds by Rosenberg et al.
1], cisplatin has demonstrated a remarkable chemotherapeutic potential in a large variety
[
of human solid cancers, including testicular, ovarian, bladder, lung and stomach carcino-
mas [2, 3]. A large number of analogs have been tested. The replacement of ammonia by
other amines such as cyclopentylamine and cyclohexylamine has sresulted in higher
therapeutic indices than cisplatin and some of these are potential second-generation drugs
[
4, 5]. Cisplatin and other second-generation platinum drugs have draw-backs such as high
nephorotoxicity, low water solubility and relative inactivity against gastrointestinal tumors
6]. Furthermore, the development of an acquired resistance to cisplatin is frequently
observed during chemotherapy [7]. In recent years, work from our groups and others has
[
II
II
concentrated on the reactions of Pt and Pd complexes with nitrogen containing bio-
molecules [8–21], which could be of fundamental importance in the understanding of the
mechanism of antitumor activity of related platinum complexes. For mechanistic studies of
II
II
the action of Pt antitumor drugs, their Pd analogues are usually good model compounds
4
5
since they exhibit a 10 –10 fold higher reactivity, whereas coordination geometry and
complex formation processes of palladium(II) are very similar to those of platinum(II);
thus palladium(II) ions are frequently used to mimic the binding properties of various
platinum(II) species [22]. On this basis Gill [6] has reported several palladium complexes
123

1026
J Solution Chem (2017) 46:1024–1047
with bidentate amine ligands that have shown anticancer activity comparable to or greater
than cisplatin. Much work has been carried out on the equilibrium studies of palladium(II)
complexes with aliphatic amines [23–25] and it is of considerable interest to extend this
work to other palladium complexes with aromatic heterocycles such as 2-(methy-
lamino)methylpyridine. The aromatic heterocycles act as r-donors and can also function as
fairly effective p-acceptors. The p-accepting properties can be involved in p–p stacking
effects with purine and pyrimidine bases. This will enhance complex formation with DNA
subunits, which is the principal target in the chemotherapy of tumors. As part of our
II
interest in the synthesis, structure and reactivity of coordination complexes of Pd with
chelating ligands [23–26], and with the aim of extending our earlier work in this area, we
report here a detailed study on the complex formation equilibrium of [Pd(MAMP)
2?
(H O) ] with biologically relevant ligands. The effect of chloride ion concentration on
the formation of Pd(MAMP)–CBDCA (as a representative example), is also investigated.
2 2
2
Experimental
2
.1 Materials
All chemicals used were of the analytical reagent grade (AR) and of highest purity. Ligand
stock solutions were prepared shortly before use by dissolving the chemicals in deionized
water. Amino acids, peptides and DNA constituents investigated are: glycine (Gly,
Aldrich, assay C 99.0%), alanine (Ala, Aldrich, assay C 98.0%), b-alanine (b-Ala,
Aldrich, assay = 99.0%), valine (Val, Aldrich, assay C 98.0%), proline (Pro, Aldrich,
assay C 99.0%), threonine (Thr, Aldrich, assay C 98.0%), serine (Ser, Aldrich,
assay C 99.0%), S-methylcysteine (SMC, Aldrich, assay C 98.0%), ornithine (Orn,
Aldrich, assay C 99.0%), L-methionine (Met, Fluka, assay [ 99.0%), glycinamide (GA,
Aldrich, assay = 99.0%), glycylglycine (GlyGly, Aldrich, assay C 99.0%), glutamine
(
Glu, Aldrich, assay C 99.0%), glycylalanine (GlyAla, Aldrich, assay C 99.0%), uracil
Aldrich, assay C 99.0%), uridine (Aldrich, assay C 99.0%), thymine (Aldrich,
(
assay C 99.0%), cytosine (Aldrich, assay C 99.0%), inosine (Aldrich, assay C 99.0%)
0
and inosine 5 -monophosphate (Aldrich, assay C 99.0%). Additionally, 1,1-cyclobu-
tanedicarboxylic acid (CBDCAH , Aldrich, assay = 99.0%) was provided also by Aldrich
2
Chemical Company. These materials were provided by Sigma Chemicals Company and
used without further purification. Also, methylamine hydrochloride (Aldrich,
assay [ 98.0%) and imidazole (Aldrich, assay [ 98.0%) were provided by Sigma
Chemicals Company. Carbonate-free NaOH (titrant) was prepared and standardized
against potassium hydrogen phthalate solution daily. All solutions were prepared in
deionized H O.
2
2
.2 Preparation of the Diaqua [Pd(MAMP)(H O) ](NO ) Complex
2 2 3 2
In this study, [Pd(MAMP)Cl ] was prepared by heating PdCl (532.2 mg, 3 mmol) in
2
2
5
0 mL water and KCl (447 mg, 6 mmol) to 50 °C for 30 min. After the K [PdCl ] solution
2
4
cooled, 2-(methylamino)methyl pyridine (366.51 mg, 3 mmol), dissolved in 10 mL water,
was added dropwise with stirring. A yellow precipitate formed, and the mixture was stirred
for a further 1 h at 25 °C. After the precipitate was filtered off, it was washed sequentially
with water, ethanol and diethyl ether.
1
23

J Solution Chem (2017) 46:1024–1047
1027
The diaqua-complex [Pd(MAMP)(H O) ](NO ) was prepared in solution by stirring
2
2
3 2
the chloro complex with two equivalents of AgNO overnight, and removing the AgCl
3
precipitate by filtration through a 0.1 lm pore membrane filter. Great care was taken to
ensure that the resulting solution was free of Ag ions and that the chloro complex had
?
been converted into the aqua species. The filtrate was made up to the desired volume in a
standard volumetric flask. The ligands in the form of hydrochlorides were converted to the
corresponding hydronitrate in the same way as described above.
2
.3 Apparatus
Potentiometric measurements were made using a Metrohm 686 titroprocessor equipped
with a 665 Dosimat (Switzerland-Herisau). The thermostatted glass cell was equipped with
a magnetic stirring system, a Metrohm glass-calomel combined electrode, a thermometric
probe and a microburet delivery tube. The titroprocessor and electrode were calibrated
daily with standard buffer solutions prepared according to NBS specifications at
-
3
2
and a mixture of KH PO and Na HPO (pH = 6.865).
5.0 ± 0.1 °C [27] and I = 0.1 molÁdm , potassium hydrogen phthalate (pH = 4.008)
2
4
2
4
2
.4 Procedure and Measuring Techniques
The acid dissociation constants of the ligands were determined potentiometrically by
3
-3
-3
titrating 40 cm of ligand solution (1.25 9 10 molÁdm ). The acid-dissociation con-
2?
stants of the coordinated water molecules in [Pd(MAMP)(H O) )] were determined by
2
2
-
3
-3
titrating a 1.25 9 10 molÁdm solution of the complex. The formation constants of the
2
?
complexes were determined by titrating solution mixtures of [Pd(MAMP)(H O) ]
2
2
-
3
-3
(
1.25 9 10 molÁdm ) and the ligand in concentration ratios of 1:1 for amino acids and
peptides, and for concentration ratios of 1:1 and 1:2 (metal:ligand) for DNA constituents.
The titration solution mixtures had a volume of 40 mL. Temperature was maintained
constant inside the cell at (25.0 ± 0.1) °C, by circulating thermostated water through the
double-wall of the titration vessel and titrations were performed with a slow and constant
stream of N over the test solutions. The pH meter readings were converted into hydrogen
2
-
3
ion concentrations by titrating a standard acid solution (0.05 molÁdm ), the ionic strength
-
3
-3
of which was adjusted to 0.1 molÁdm , with standard base solution (0.05 molÁdm ) at
25 °C. The pH was plotted against p[H]. The relationship pH – p[H] = 0.05 was observed.
The [OH ] values were calculated using a pK value of 13.997 [28]. The ionic strength
-
w
-
3
was adjusted to 0.1 molÁdm using NaNO . The equilibrium constants evaluated from the
3
titration data (defined by Eqs. 1 and 2) are:
blpqr
lM þ pL1 þ qL2 þ rH ꢀ MlL1pL2qHr
ð1Þ
ð2Þ
Â
Ã
MlL1pL2qHr
blpqr
¼
l
p
q
r
½
MŠ ½L1Š ½L2Š ½HŠ
2
where M, L and H stand for the [Pd(MAMP)(H O) ] ion, ligand and proton, respec-
?
2
2
tively. The calculations were carried out using ca. 100 data points in each titration, using
the computer program MINIQUAD-75 [29]. The stoichiometry and stability constants of
the complexes formed were determined by trying various possible composition models.
The model selected gave the best statistical fit and was chemically consistent with the
titration data without giving any systematic drifts in the magnitudes of various residuals, as
123

1028
J Solution Chem (2017) 46:1024–1047
described elsewhere [29]. The fitted model was tested by comparing the experimental
titration data points and the theoretical curve calculated from the values of the acid dis-
sociation constant of the ligand and the formation constants of the corresponding com-
plexes. The species distribution diagrams were obtained using the program SPECIES [30]
under the experimental conditions employed.
2
.5 Molecular modeling studies
An attempt to gain a better insight on the molecular structure of the compounds, geometry
optimization and conformation analysis was performed using the semi-empirical method
-1
˚
PM3 as implemented in HyperChem 7.5 [31]. A gradient of 0.04 kJÁA was set as a
convergence criterion in all the molecular mechanics and quantum calculations. The lowest
energy structure was used for each molecule to calculate physicochemical properties.
2
.6 Spectrophotometric Measurements
Spectrophotometric measurements of the Pd(MAMP)–glycyinamide complex were per-
formed by recording the UV–visible spectra of solutions (A–C) as follows:
-
3
2?
(
A) 0.001 molÁdm [Pd(MAMP)(H O) ]
;
2
2
-
3
2?
-3
(
B) 0.001 molÁdm
[Pd(MAMP)(H O) ] ? 0.001 molÁdm
glycyinamide ?
glycyinamide ?
2
2
-
3
0
.001 molÁdm of NaOH
-
-
3
3
2?
-3
(
C) 0.001 molÁdm
of [Pd(MAMP)(H O) ] ? 0.001 molÁdm
2
2
0
.002 molÁdm NaOH.
Under these experimental conditions, and after neutralization of the hydrogen ions
released associated with complex formation, it is supposed that the complexes have been
completely formed. In each mixture, the volume was brought to 10 mL by addition of
-
3
deionized water and ionic strength was kept constant at 0.1 molÁdm with NaNO3.
3
Results and Discussion
The acid dissociation constants of the ligands were determined under the experimental
-3
conditions of (25 ± 0.1) °C and a constant ionic strength of 0.1 molÁdm , which were
(
II)
also used to determine the stability constants of the Pd complexes. The values obtained
are consistent with data reported in the literature [32].
2
1
3
.1 Hydrolysis of [Pd(MAMP)(H O) ]
2 2
2?
The hydrolysis of the [Pd(MAMP)(H O) ] complex was studied by fitting the poten-
2
2
tiometric data to various models. The best fit model was found to be consistent with the
species 10–1, 10–2 and 20–1, as given in Eqs. 3–5:
Â
Ã
2þ
þ
þ
PdðMAMPÞðH2OÞ
ꢀ ½PdðMAMPÞðH2OÞðOHފ þH
ð3Þ
ð4Þ
2
100
10À1
Â
Ã
PdðMAMPÞðH2OÞðOHފ ꢀ PdðMAMPÞðOHÞ þHþ
þ
0
½
2
10À1
10À2
1
23

J Solution Chem (2017) 46:1024–1047
1029
Â
Ã
ÂÀ
Á
Ã3þþH2O
ð5Þ
2þ
þ
PdðMAMPÞðH2OÞ
þ ½PdðMAMPÞðH2OÞðOHފ
ꢀ
PdðMAMPÞðH2OÞ2 2ðOHÞ
2
1
00
10À1
20À1
The first two species are due to deprotonation of the two coordinated water molecules,
as given by Eqs. 3 and 4, and the negative numbers refer to proton loss. The hydroxo
bridged-dimer (20–1) with a single hydroxyl bridge may be formed as result of the
combination of the monoaqua hydroxo species (10–1) with the diaqua species (100)
2
?
[
Pd(MAMP)(H O) ] as given by Eq. 5.
2 2
The pKa1 and pKa2 values were found to be 4.69 and 10.08, respectively. The pK value
a1
2
?
is lower than that for [Pd(1,3-DAP)(H O) ] (pK = 5.96) [21]. This shows that the first
2
2
a
2
?
coordinated water molecule in [Pd(MAMP)(H O) ]
2
is more acidic than that of
2
2
?
[
Pd(en)(H O) ] , which may be attributed to the p-acceptor properties of the aromatic
2
2
moiety of MAMP, leading to an increase in the electrophilicity of the Pd ion and a
consequent decrease of the pK of the coordinated water molecule. The pK (4.69) of
a
a1
2?
2?
Pd(MAMP)(H O) ] is higher than that of [Pd(BPY)(H O) ] (3.81) and pKa2 (10.08)
[
is higher also than that of [Pd(BPY)(H O) ] (8.39) [33], this may be due to the presence
2 2 2 2
2
?
2
2
of the two aromatic moieties in bipyridyl, which increases the the p-acceptor properties
and the electrophilicity of the Pd ion more than presence of the one aromatic ring in the
case of 2-(methylamino)methylpyridine. The l-hydroxo species (20–1) is assumed to form
(II)
through dimerization of the Pd complex via a hydroxo group.
The equilibrium constant for the dimerization reaction (Eq. 5) can be calculated by
Eq. 6 from the formation constants listed in Table 1, and amounts to log10 K = 3.95.
d
log10 Kd ¼ log10
b
À log10
b
:
ð6Þ
20À1
10À1
2
The distribution diagram for [Pd(MAMP)(H O) ] and its hydrolyzed form reveals that
?
2
2
the concentration of the monohydroxo species (10–1) and the l-hydroxo species (20–1)
increase with increasing pH. The monohydroxo (10–1) complex reaches its maximum con-
centration of 98% at pH = 7.8 while the l-hydroxo species (20–1) reaches its maximum
concentration of 66% at pH = 4.6, i.e., they are approximately the main species present in
solution in thephysiologicalpH range. Afurtherincrease inpH isaccompanied byan increase
in the concentration of dihydroxo species, which is the main species above a pH of ca. 11, i.e.,
in the high pH range the inert dihydroxo complex would be the predominant species, so that
the reactivity of DNA to bind the Pd(amine) complex will considerably decrease.
3
.2 Amino Acid Complexes
Analysis of pH titration data for Pd(MAMP)–amino acid systems showed the formation of
:1 complexes with stability constants larger than for the corresponding monodentate
1
methylamine and imidazole complexes. This indicates that amino acids bind through the
amino and carboxylate groups as given in Scheme 1.
Comparison of the stability constant of the a-alanine (11.42) and b-alanine (8.79)
complexes indicates that the five-membered chelate ring formed in the a-alanine complex
is the most favored, whereas the six-membered chelate ring formed in the b-alanine amino
acid complex is least favored as given in Scheme 2.
Serine and threonine have an extra binding center on the b-alcoholate group. This group
was reported [34, 35] to participate in transition metal ion complex formation reactions.
The pK values of the b-alcoholate group incorporated in the Pd(II) complex (log b110
–
a
10
123

1030
J Solution Chem (2017) 46:1024–1047
Table 1 Protonation constants of some selected amino acids and their formation constants with
[Pd(MAMP)(H
2
?
-3
at 25 °C and 0.1 molÁdm ionic strength
2
O)
2
]
a
b
b
c
Ligand
l
p
r
log10
S
-
-7
OH
1
1
2
0
0
1
0
0
1
0
0
1
0
0
1
0
0
1
0
0
1
1
0
0
1
1
0
0
1
0
0
1
0
0
0
1
1
0
1
1
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
-1
-2
-1
1
-4.69 (0.05)
-14.77 (0.07)
-0.74 (0.09)
9.60 (0.01)
11.93 (0.03)
10.30 (0.07)
9.69 (0.01)
11.88 (0.02)
11.42 (0.05)
9.12 (0.01)
11.39 (0.02)
8.79 (0.05)
9.57 (0.01)
11.70 (0.03)
10.55 (0.1)
10.52 (0.01)
12.03 (0.03)
12.42 (0.08)
9.06 (0.01)
11.03 (0.02)
10.77 (0.08)
2.90 (0.09)
9.14 (0.01)
11.40 (0.01)
11.27 (0.09)
3.31 (0.09)
9.10 (0.01)
11.08 (0.03)
9.42 (0.05)
8.51 (0.01)
10.52 (0.03)
9.63 (0.07)
10.58 (0.00)
19.43 (0.02)
21.39 (0.02)
11.95 (0.05)
20.44 (0.05)
10.03 (0.04)
7.32 (0.03)
13.45 (0.05)
6.71 9 10
-7
Glycine
Alanine
b-alanine
Valine
1.50 9 10
2
-
7
8
0
3.53 9 10
9.21 9 10
-
1
2
-
8
7
0
6.70 9 10
3.5 9 10
-
1
2
-
7
8
0
3.73 9 10
9.90 9 10
-
1
2
-
6
8
0
2.85 9 10
4.42 9 10
-
Proline
1
2
-
7
8
0
7.61 9 10
7.93 9 10
-
Threonine
1
2
-7
-8
-7
-8
0
3.20 9 10
1.72 9 10
3.75 9 10
8.91 9 10
-1
1
Serine
2
0
-1
1
Methionine
2
-
7
8
0
4.07 9 10
3.44 9 10
-
S-methylcysteine
Orinthine
1
2
-
6
8
0
2.11 9 10
1.03 9 10
-
1
2
3
-7
0
1.02 9 10
1
-
7
7
Methylamine
1
4.40 9 10
1.81 9 10
-
0
0
1
23

J Solution Chem (2017) 46:1024–1047
1031
Table 1 continued
a
r
b
b
c
Ligand
l
p
log10
S
-
8
7
Imidazole
0
1
1
1
1
2
1
0
0
7.04 (0.01)
7.01 (0.04)
10.29 (0.07)
2?
3.15 9 10
5.01 9 10
-
a
?
l, p and r are the stoichiometric coefficients corresponding to Pd(MAMP) , amino acid, and H ,
respectively
b
Standard deviations are given in parentheses
c
Sum of square of residuals
Scheme 1 Coordination modes
of imidazole as monodentate
ligand and glycine as bidentate
ligand with
CH3
NH
CH3
NH
OH2
N
2
?
O
O
[Pd(MAMP)(H
2
O)
2
]
complex
Pd
Pd
N
CH2
N
N
H
NH2
(110)
(
110)
Scheme 2 Structural formulae
of [Pd(MAMP)-a-Ala] and
O
O
CH3
[Pd(MAMP)-b-Ala] complexes
6
O
NH2
5
O
NH2
Pd
Pd
H C NHH
N
H C NHH
N
3
3
log10 b11-1) for threonine and serine amino acid complexes are 7.87 and 7.96, respectively
Table 1), i.e., the alcohol group of serine is competing with the carboxylate group in
(
2?
binding to the Pd(MAMP) ion. Due to the donation of the electron pair on the oxygen to
the metal center, the OH bond is considerably weakened and thus the ionization of the
proton occurs at a lower pH with respect to the higher pK value (pK = *14) of the
a
a
hydroxo group of the free serine and threonine amino acids. These values are lower than
´
that of the Pd(N,N-dimethylethylenediamine)–serine complex (8.43) [23]. This may be
explained on the premise that the presence of the aromatic moiety in 2-(methyl-
amino)methylpyridine increases the electrophilic character of the Pd(II) ion and hence the
pK value of the coordinated alcoholate group decreases.
a
123

1032
J Solution Chem (2017) 46:1024–1047
The pKa values of the alcoholate group incorporated in [Pd(SMC)Ser] and
Pd(SMC)Thr] (where SMC = S-methyl-L-cysteine), are 9.26 and 9.29 (Table 1S) for
serine and threonine, respectively, as reported in literature [36]. These pK values reported
[
a
in the literature [36] for the [Pd(SMC)Ser] and [Pd(SMC)Thr] complexes are higher than
the corresponding values of the [Pd(MAMP)Ser] (pK = 7.96) and [Pd(MAMP)Thr]
a
(pK = 7.87) complexes, i.e., the pK value in case of the S-methyl-L-cysteine complex is
a a
higher than that of the dinitrogen complex. This is due to the strong trans labilization effect
of sulfur on the coordinated alcoholate group, which weakens the bond between the S atom
and the alcohol group and in turn hinders the induced proton ionization causing an in
increase the pK value of the alcoholate group in the case of the Pd(SMC)–serine complex.
a
The distribution diagram for the threonine complex is given in Fig. 1S. The complex
species with coefficients 110 reaches its maximum degree of formation (98%) at
pH = 4.4–6.6, i.e., in the physiological pH range. However, the species 11-1 attains a
maximum concentration of 97% at pH ca. 10.4.
S-methylcysteine forms a more stable complex than the methionine amino acid, plau-
sibly due to the fact that the five-membered chelate ring in the former complex is ener-
getically favored over the six-membered chelate ring in the latter complex.
Ornithine has one carboxylic and two amino groups. It coordinates as a bidentate ligand
using either a (N,N) or (N,O) donor set. The stability constant of its complex is higher than
those of a-amino acids (N,O donor sets). This may be taken as an indication that ornithine
chelates using the (N,N) donor set. This formulation is supported by the high affinity of
Pd(II) to nitrogen donor centers. Ornithine forms, in addition to 1:1 complexes, the
monoprotonated 111 complex species. The pK of the protonated complex was calculated
a
from Eq. 7:
pKa ¼ log10 b111 À log10 b110
The pK value of the protonated species is 8.49 for ornithine.
ð7Þ
a
The species distribution for ornithine complex, taken as representative, is given in
Fig. 2S. The protonated species 111 complex predominates (95% at pH ca. 5.6); the
deprotonated species 110 complex predominates (94% at pH ca. 10.2); the hydroxo
?
complex [Pd(MAMP)(H O)(OH)] plays a minor role in that region. This means that in
2
-
the physiological pH range the OH ion does not compete with the amino acid in the
2
?
reaction with the [Pd(MAMP)(H O) ] complex.
2
2
3
.3 Complex Formation Equilibria Involving Peptides
The potentiometric data for the Pd(MAMP)–peptide system were fitted to various models.
The most acceptable model, as given in Scheme 3, was found to be consistent with the
formation of complexes with stoichiometric coefficients 110 and 11–1.
In the 110 species, the peptide is bound through the amino and carbonyl groups. Upon
deprotonation of the amide group, the coordination sites could switch from the carbonyl
oxygen to the amide nitrogen such that the 11–1 complex is formed. Such changes in
coordination modes are well documented [37]. The glutamine complex (log b = 9.73)
1
0 110
is more stable than the glycinamide complex (log b = 8.11), presumably due to the
1
0 110
fact that glutamine carries a negative charge whereas glycinamide is neutral. The elec-
trostatic interaction between the glutaminate and the positively charged Pd(II) complex
will result in a lowering of the Gibbs energy of formation. The pK values of the amide
a
group, incorporated in the Pd(II) complex (log b - log10b11-1), are in the range
1
0 110
1
23

J Solution Chem (2017) 46:1024–1047
1033
+
2
+
NH
H
N
O
+
CH2
O
K
N
CH2
-
N
OH2
CH2
NH2
COO
Pd
-
2H O
-
2
Pd
CH2
CO2
NH
N
OH2
Glycylglycinate
H
2
NH
H C
3
H C
3
(110)
(
100)
pKa
-
COO
CH2
N
N
+
N
O
H
+
Pd
CH2
NH
H
2
H C
3
(
11-1)
Scheme 3 Coordination modes of GlyGly with the [Pd(MAMP)(H
2
O)
2
] complex
Table 2 Protonation constants
of some selected peptides and
their formation constants with
a
b
c
System
l
p
r
log10
b
pK
a
S
-
8
2
?
Glycinamide
0
1
1
0
1
1
0
1
1
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
7.68 (0.00) 4.59 4.51 9 10
[
Pd(MAMP)(H O) ] at 25 °C
2 2
-3
and 0.1 molÁdm ionic strength
8.11 (0.02)
-7
-8
-7
-1 3.52 (0.04)
1.72 9 10
7.98 (0.01) 8.04 4.03 9 10
8.31 (0.02) 1.01 9 10
Glycylglycine
Glutamine
1
0
-1 0.27 (0.03)
-
8
7
1
0
8.92 (0.00) 9.44 3.04 9 10
9.73 (0.06) 1.62 9 10
a
l, p and r are the stoichiometric
coefficients corresponding to
-
2
?
?
Pd(MAMP) , peptide and H ,
respectively
-1 0.29 (0.08)
-8
-7
Glycylalanine
1
0
8.04 (0.00) 5.38 4.41 9 10
8.46 (0.05) 9.15 9 10
b
Standard deviations are given
in parentheses
-1 3.08 (0.06)
c
Sum of square of residuals
4
affinity of Pd(II) for nitrogen donor ligands. It is interesting to note that the pK value for
.59–9.44 (Table 2). The low pK values in the present study are probably due to the high
a
a
the glycinamide complex is lower than those for other peptides. This can be explained on
the basis that the more bulky substituent on the peptide may hinder structural changes
when going from the protonated to the deprotonated complexes. The pK of the glutamine
a
complex is exceptionally higher than those for the other peptide complexes. This is due to
123

1034
J Solution Chem (2017) 46:1024–1047
1
00
90
80
70
60
50
40
30
20
10
0
1
00
110
1
0-2
11-1
1
0-1
9
2
0-1
1
3
5
7
11
pH
Fig. 1 Concentration distribution of various species as a function of pH in the Pd(MAMP)–glycylglycine
system
the formation of a seven-membered chelate ring which is more strained and therefore less
favored.
The distribution diagram for the Pd(MAMP)–glycylglycine system is given in Fig. 1.
?
[Pd(MAMP)L] (110) starts to form at lower pH, its concentration increases with
increasing pH and reaches a maximum of 92% at pH = 5.8. A further increase in pH is
?
accompanied by a decrease in the [Pd(MAMP)L] concentration and an increase in the
[Pd(MAMP)LH ] (11–1) concentration, reaching a maximum of 87% at pH = 9.8, i.e., in
the physiological pH range the deprotonated species (11–1) predominates.
-1
For metal complexes with a series of structurally related ligands, a linear relationship
holds between the logarithm of the stability constant of the complex and the logarithm of
the acid dissociation constant of the ligand. The importance of these plots is that they
afford a means of estimating the stabilities of metal complexes of closely related sub-
stances that have not yet been studied if their pK values are known. Figure 3S demon-
a
2
?
strates a relationship between log10 b for the [Pd(MAMP)(H O) ] complexes with
2
2
peptides and their pK values.
a
2
Spectral bands of [Pd(MAMP)(H O) ] and its glycyinamide complex are quite dif-
?
2
2
ferent in the position of the maximum wavelength and molar absorptivity. The spectrum of
2
the [Pd(MAMP)(H O) ] complex shows an absorption maximum at 289 nm. On the
?
2
2
other hand, the spectrum obtained for [Pd(MAMP)(glycyinamide)] (110 species), mixture
B, exhibits a band at 263 nm. The spectrum obtained for the [Pd(MAMP)(glycyi-
namide,H-1)] complex (11–1 species) (mixture C) exhibits a band at 255 nm. The pro-
gressive shift toward shorter wavelength in the absorption spectrum may be taken as
evidence supporting the potentiometric measurements for the induced ionization of amide
upon complex formation.
1
23

J Solution Chem (2017) 46:1024–1047
1035
3
.4 Complex Formation Equilibria Involving DNA Constituents
The pyrimidines: cytosine, uracil, uridine and thymine have only basic nitrogen donor
atoms (N3-C4O group). Consequently the pyrimidines ligate in the deprotonated form
through this site and they do not form protonated complexes. They form 1:1 and 1:2
2
?
complexes with [Pd(MAMP)(H O) ] . The thymine complex is more stable than that of
2
2
uridine, most probably owing to the high basicity of the N3 group of thymine resulting
from the extra electron donating methyl group. As a result of the high pK values of
a
pyrimidines (pK & 9) and the fact that they are monodentates, the complexes are formed
a
only above pH & 6, supporting the view that the negatively charged nitrogen donors of
pyrimidine bases are important binding sites in the neutral and slightly basic pH ranges.
Table 3 Protonation constants of some selected nucleoside and nucleotide bases and CBDCA and their
O)
2
?
-3
at 25 °C and 0.1 molÁdm ionic strength
formation constants with [Pd(MAMP)(H
2
2
]
a
b
c
System
l
p
r
log10
b
S
-
8
7
1
,1-Cyclobutane dicarboxylic
acid
0
0
1
1
0
1
1
0
1
1
0
1
1
0
1
1
0
1
1
1
0
0
1
1
1
1
1
1
1
1
1
1
1
2
1
1
2
1
1
2
1
1
2
1
1
2
1
1
1
1
2
1
2
1
1
2
0
1
1
0
0
1
0
0
1
0
0
1
0
0
1
0
0
1
1
2
0
0
1
1
2
5.62 (0.002)
8.66 (0.01)
7.20 (0.06)
11.35 (0.1)
9.28 (0.006)
9.13 (0.05)
15.51 (0.09)
9.01 (0.01)
9.19 (0.04)
13.04 (0.06)
9.58 (0.00)
10.07 (0.07)
13.93 (0.09)
4.59 (0.04)
4.87 (0.08)
7.38 (0.07)
8.81 (0.006)
7.46 (0.1)
1.6 9 10
8.6 9 10
-
-
8
7
Uracil
2.42 9 10
2.82 9 10
-
-
7
7
Uridine
Thymine
Cytosine
Inosine
1.10 9 10
4.36 9 10
-
-
8
7
8.14 9 10
8.33 9 10
-
-
8
7
1.79 9 10
2.25 9 10
-
-
8
6
4.26 9 10
8.17 9 10
2.6 9 10
-
11.32 (0.1)
13.42 (0.06)
9.15 (0.006)
15.21 (0.01)
10.28 (0.07)
19.65 (0.07)
16.33 (0.08)
25.61 (0.07)
19.34 (0.08)
2?
0
-8
Inosine 5 - monophosphate
0
-8
1.67 9 1
a
l, p and r are the stoichiometric coefficients corresponding to Pd(MAMP) , nucleoside/nucleotide bases
?
and H , respectively
b
Standard deviations are given in parentheses
c
Sum of square of residuals
123

1036
J Solution Chem (2017) 46:1024–1047
Cytosine undergoes N3 protonation under mildly acidic conditions. The value obtained
for its protonation constant is 4.59. The lower values of the stability constants of its
complexes, listed in Table 3, reflect the difference in the basicity of the donor sites.
0
The purines, inosine and inosine 5 -monophosphate have two metal ion binding centers,
the N1 and N7 nitrogens. The pH dependent binding of these N-donors has already been
reported. Inosine may protonate at N7 forming a (N1H–N7H) monocation. In the present
study, the pK of N1H was determined (pK = 8.81). The pK of N7H is 1.2 [38]. It was
a
a
a
reported [39] that, in the acidic pH range, the metal ion is coordinated to N7, while N1
remained protonated. The results show that inosine forms the complexes 110 and 111. The
species 111 is formed in the acidic pH region and it corresponds to a N7 coordinated complex,
while the N1 nitrogen is in a protonated form. In inosine, the pK of the protonated form (log10
a
b111 - log10 b110) is 5.96, which means that N1H is still protonated and is acidified by 2.85
pK units (8.81 – 5.96), Table 3. The lowering of the pK value of N1H upon coordination was
a
reported previously [40]. Raising the pH could switch the coordination sites and result in a
gradual change from N7 binding to N1 binding with an increase of pH as was suggested [39].
The monoprotonated 111 species reaches a maximum of 97% at pH = 3.6 (Fig. 2), which has
the N7 site unprotonated and this is the potential coordination site at low pH. The binding site
at the higher pH was disputed and a gradual change from N7-binding to N1-binding with an
increase of pH was suggested [39]. Thus, 110 species prevails at pH = 6.2–7.8 while the 120
species reaches a maximum of 60% at pH = 9.0.
0
0
Inosine-5 -monophosphate (5 -IMP) forms a stronger complex with [Pd(MAMP)
2?
(
H O) ] than does inosine; the log10 b110 value is 10.28 compared to 7.46 for the inosine
complex. The extra stabilization can be attributed to the triply negatively charged 5 -
3
IMP ) ion. The results also show that IMP forms 110, 120, 111, 112 and 121 complexes
2 2
0
-
0
(Table 3), i.e., the 5 -IMP complex has two protonated species associated with the 1:1
complex, namely 111 and 112, and their pK values are 6.05 and 3.01, respectively, with
a
only one protonated species with 1:2 sroichiometry, namely 121. The species 112 was not
detected with the Pd(aliphatic amines) complexes studied previously [41, 42]. This may be
100
1
0-2
90
80
70
60
50
40
30
20
10
0
111
1
10
1
20
1
00
10-1
2
0-1
6
1
2
3
4
5
7
8
9
10
11
12
pH
Fig. 2 Concentration distribution of various species as a function of pH in the Pd(MAMP)–inosine system
1
23

J Solution Chem (2017) 46:1024–1047
1037
O
7N
6
HN 1
O
P
+
-
N
9
N
3
N
N
H
CH3
O
O H
O
Pd
O
H O
2
OH2
O H O H
H O
2
N
N
H
CH3
Pd
O
OH2
7N
6
H N 1
O
9
N
3
N
O
P
O H
O
O
O H O H
(
112)
N1H-IM PN7-Pd(M AM P)
pH ~ 2.8
+
pK = ~ 3.01
- H
a
H C
3
O H2
Pd
NH
N
O
N
6
N 1
7
O
9
N
3
N
O
P
O H
O
O
O H O H
(
111)
IM PN1-Pd(M AM P)
pH ~ 4.0
2
Scheme 4 Proposed coordination process of Pd(MAMP) with 5 -IMP
?
0
123

1038
J Solution Chem (2017) 46:1024–1047
interpreted on the basis that the 112 is stabilized through back bonding to the p-system of
the pyridine rings. In the fully protonated species (112), the two protons bound to N1 and
the phosphate groups, exist at pH * 2.8 or lower. The unprotonated N7 site in the
diprotonated 112 species is the potential coordination site at low pH. In the (111) complex
species, the single proton binds to the phosphate group.
The pKa values of the protonated species of the IMP complex are 3.01 (log10
b112 - log10 b111) and 6.05 (log10 b111 - log10 b110). i.e., the stability constant difference
(3.01) between the (112) and (111) complexes is due to the N1 deprotonation and the
stability constant difference (6.05) between the (111) and (110) complexes is due to
the –PO (OH) group. The N1H group was acidified upon complex formation by
2
6
.14 (9.15 - 3.01) pK units for IMP. Acidification of the N1H group upon complex
formation is consistent with previous reports for IMP and GMP complexes [40]. The
phosphate group was not acidified upon complex formation since it is far away from the
2?
0
coordination center [35]. A proposed coordination process of Pd(MAMP) with 5 -IMP is
reported in Scheme 4.
The IMP complexes are more stable than those of the pyrimidines. The extra stabi-
lization may be due to either the involvement of the phosphate group in hydrogen bonding
with the exocyclic amine [43, 44], which will favor complex formation, or the different
0
coulombic forces involved in the interaction of the trinegatively charged 5 -IMP ion and
2
?
the mononegatively charged pyrimidinic ions with Pd(MAMP)
.
3
.5 Ring Opening of [Pd(MAMP)(CBDCA)] and the Formation
of [Pd(MAMP)(CBDCA-O)(DNA)]
1
The presence of ring-opened species was confirmed [45] using H-NMR for cis-
0
[Pt(NH ) (CBDCA-O)(5 -GMP-N7)]. These studies indicate that CBDCA is attached
through one carboxylate oxygen and 5 -GMP through N7 of the purine base. The ring-
3
2
0
opened species was further confirmed by kinetic studies of the [Pd(diammine)(CBDCA)]
0
complex and 5 -IMP. These studies conclude that CBDCA is attached through one car-
0
boxylate oxygen and 5 -GMP through N7 of the purine base. Also, such a reaction for
0
[Pd(en)(CBDCA)] with thiourea, inosine 5 -monophosphate or iodide was mechanistically
investigated [36]. It seems therefore to be of considerable interest to conduct an investi-
gation in solution to prove the mechanism of the ring opening reaction of chelated CBDCA
by DNA constituents. The potentiometric data for the system consisting of
2
?
[
Pd(MAMP)(H O) ] , CBDCA and some DNA constituents were fitted by assuming
2 2
different models. The accepted model for pyrimidines is consistent with the formation of
the 1110 species. The accepted model for IMP was found to consist of 1110 and 1111
species (Table 4). The proton on the N1H corresponds to pK of 5.90 (acidification of
a
9
.15 – 5.90 = 3.25 pK units).
It is interesting to note that the quaternary complex of IMP is more stable than those of
pyrimidines. This may be explained on the premise that the cyclobutane ring forms a close
hydrophobic contact with the purine ring of IMP. Such hydrophobic contacts may con-
tribute to the stabilization of the quaternary complexes. The same finding was obtained
0
from an NMR investigation of the ring opening reaction of carboplatin with the 5 -GMP
complex [36]. These studies conclude that CBDCA is attached through one carboxylate
0
oxygen and 5 -IMP through N7 of the purine base. It should be recognized that further
studies are necessary to elucidate the ring opening of chelated CBDCA by DNA con-
stituents, especially multinuclear NMR measurements. These aspects will be considered in
future investigations. Estimation of the concentration distribution of the various species in
1
23

J Solution Chem (2017) 46:1024–1047
1039
2?
Table 4 Formation constants for mixed ligand complexes of [Pd(MAMP)(H
2
O)
2
]
with cyclobutanedi-
-3
carboxylic acid and some DNA units at 25 °C and 0.1 molÁdm ionic strength
a
r
b
c
System
l
p
q
log10
b
S
-
-
-
-
6
7
7
6
Uracil
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
1
17.70 (0.02)
17.97 (0.02)
13.01 (0.02)
17.71 (0.03)
18.65 (0.08)
24.55 (0.007)
2?
1.1 9 10
8.0 9 10
7.7 9 10
1.9 9 10
1.1 9 10
Thymine
Cytosine
Inosine
0
-9
Inosine-5 -
monophosphate
a
l, p, q and r are the stoichiometric coefficients corresponding to Pd(MAMP) , cyclobutanedicarboxylic
?
acid, DNA subunits and H , respectively
b
Standard deviations are given in parentheses
c
Sum of square of residuals
solution provides a useful picture of metal ion binding. To illustrate the main features
observed in the species distribution plots in these systems, the speciation diagram obtained
for the Pd(MAMP)–CBDCA–IMP system is shown in Fig. 3. The quaternary species
Pd(MAMP)–CBDCA–IMP species (1110) attains a maximum of 98% in the pH range
7.2–10.0. The quaternary protonated 1111 species reaches a maximum of 97% near
pH = 3.9. This reveals that in the physiological pH range the ring opening of chelated
CBDCA by DNA is quite feasible.
3
.6 Displacement Reaction of Coordinated Inosine
It was shown above that N-donor ligands such as DNA constituents have an affinity for
[Pd(MAMP)(H O) ] , which may have important biological implications since the
2?
2
2
100
90
80
70
60
50
40
30
20
10
0
1110
1
111
1
112
100-2
1
020
1012
1
011
3
1
2
4
5
6
7
8
9
10
11
12
13
14
pH
Fig. 3 Concentration distribution of various species as a function of pH in the Pd(MAMP)–CBDCA–IMP
system
123

1040
J Solution Chem (2017) 46:1024–1047
interaction with DNA is thought to be responsible for the antitumor activity of related
(II)
complexes. However, the preference of Pd to coordinate to S-donor ligands was pre-
viously documented [46, 47]. These results suggest that Pd(II)–N adducts can easily be
converted into Pd–S adducts. Consequently, the equilibrium constant for such conversion
(
II)
is of biological significance, especially after the discovery of both Pt –methionine
(
II)
complexes in the urine of patients receiving cisplatin therapy and Pt –methionine com-
plexes in blood plasma upon injection of cisplatin into rats [48].
If we consider inosine as a typical DNA constituent (presented by HL) and SMC as a
typical S-ligand (presented by H B), the equilibria involved in the complex-formation and
2
displacement reactions are:
þ
À
HL ꢀ H þ L
H2B ꢀ 2Hþ + B2À
PdðMAMPފ þþLÀ ꢀ ½PdðMAMPÞLŠþ
2
ð8aÞ
½
100
110
À
Á
b110 ½PdðMAMPÞLŠþ ¼ ½PdðMAMPÞLŠþ
.½PdðMAMPފ2þ
½LŠ
ð8bÞ
ð9aÞ
PdðMAMPފ þþB2À ꢀ ½PdðMAMPÞBŠ
2
½
1
00
110
.
þÂ 2ÀÃ
2
b110ð½PdðMAMPÞBŠÞ ¼ ½PdðMAMPÞBŠ ½PdðMAMPފ
B
ð9bÞ
ð10Þ
Keq
þ
100 110
À
À
½
PdðMAMPÞLŠ þB ꢀ ½PdðMAMPÞBŠ þ L
The equilibrium constant for the displacement reaction given in Eq. 10 is given by:
ꢀ
þÂ 2ÀÃ
À
Keq ¼ ½PdðMAMPÞBŠ½L Š ½PdðMAMPÞLŠ
Substitution from Eqs. 8b and 9b in Eq. 11 results in:
B
ð11Þ
ð12Þ
ꢀ
À
Á
Keq ¼ b110ð½PdðMAMPÞBŠÞ b
½PdðMAMPÞLŠþ
110
?
log10 b110 values for the [Pd(MAMP)L] and [Pd(MAMP)B] complexes are 7.46 and 9.63,
respectively (Tables 1 and 3), and by substitution in Eq. 12 results in log10 Keq = 2.17. In
case of using cytosine as a DNA constituent, log10 Keq = 5.04. Similarly, in the case of
using methionine as a typical S-ligand and inosine as a typical DNA constituent, the
log b values for the [Pd(MAMP)inosine] and [Pd(MAMP)methionine] complexes are
10 110
7
.46 and 9.42, respectively (Tables 1 and 3), and hence the log10 Keq = 1.96. These values
clearly indicate how sulfur ligands such as SMC and methionine are effective in displacing
the DNA constituent, i.e., the main target in tumor chemotherapy. This in in agreement
with the fact that simultaneous treatment with methionine and cisplatin reduces the cell
toxicity of cisplatin [49].
Chelated cyclobutanedicarboxylate (log10 K = 7.20, see Table 3) may undergo a dis-
placement reaction with inosine. Log10 Keq for such a reaction was calculated as described
above and amounts to 0.24. In the case of the displacement reaction of coordinated
cyclobutanedicarboxylate with cytosine, log10 Keq = -2.59. The low and negative values
of the equilibrium constant for the displacement reaction of coordinated
1
23

J Solution Chem (2017) 46:1024–1047
1041
5
4
3
.5
5
.5
4
.5
3
0
0.05
0.1
0.15
0.2
0.25
0.3
Chloride ion concentration
Fig. 4 Effect of chloride ion concentration on the stability of the Pd(BMAP–CBDCA) system where log10
K refers to the 110 species
cyclobutanedicarboxylate by a DNA constituent is of biological significance since it is in
line with the finding that carboplatin interacts with DNA through ring opening of chelated
CBDCA and not through displacement of CBDCA [36].
3
.7 Effect of Chloride Ion Concentration on the Equilibrium Constants
of [Pd(MAMP)(H O) ] and Pd(MAMP)-CBDCA Complexes
2
1
2
2
(II)
Antitumor Pt –amine complexes are usually administrated as cis-dichloro-complexes. In
-
3
human blood plasma the chloride content is high (100 mmolÁdm ). Within many cells, the
-3
chloride ion concentration is much lower (only ca. 4 mmolÁdm ). A realistic extrapolation
of the present study to biologically relevant conditions will require information on the
effect of the chloride concentration on the reported stability constants. The dichloro forms
of antitumor Pt(II)–amine complexes persists in human blood plasma with its high
-
3
-
1
00 mmolÁdm Cl ion content [4]. Therefore, a realistic extrapolation of the present
study to biologically relevant conditions will require information on the effect of the
chloride concentration on the reported stability constants. The zero net charge on the
-
complex hastens its passage through cell walls. Within many cells the Cl ion concen-
-
3
tration is much lower, only 4 mmolÁdm . Under this low chloride ion concentration, the
-
reactivity of the Pt(II)–amine complex increases. Chloride ion competes with OH for
2
reaction with the [Pd(MAMP)(H O) ] complex, so the pK values of the first and second
?
2
2
a
coordinated water molecules increase with increasing chloride ion concentration as shown
in the results given in Table 5. This means that hydrolysis is suppressed by increasing the
chloride ion concentration. The data on the effect of chloride ion concentration on the
2?
hydrolysis constants of [Pd(MAMP)(H O) ] and the stability of [Pd(MAMP)(CBDCA)]
2
2
-
3
complex, keeping the ionic strength constant at 0.30 molÁdm , are given in Tables 5 and
, respectively. The stability constants of the Pd(MAMP)–CBDCA complexes, tend to
decrease with increasing chloride ion concentration (Table 6). This can be accounted for
6
123

1042
J Solution Chem (2017) 46:1024–1047
Table 5 Hydrolysis constants of
?
2 2
-
Cl ]
a
b
c
[
l
p
r
log10
b
pK
a
2
[Pd(MAMP)(H O) ] at differ-
ent concentrations of chlorides at
-
3
-
3
0.0 molÁdm
1
1
2
1
1
2
1
1
2
1
1
2
1
1
2
1
1
2
1
1
2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
-1
-2
-1
-1
-2
-1
-1
-2
-1
-1
-2
-1
-1
-2
-1
-1
-2
-1
-1
-2
-1
-4.81 (0.02)
-14.96 (0.04)
-0.64 (0.08)
-5.05 (0.03)
-15.28 (0.06)
-0.51 (0.09)
-5.26 (0.06)
-15.56 (0.08)
-0.43 (0.1)
4.81
2
5 °C and 0.3 molÁdm ionic
strength
10.15
-
3
0
0
0
0
0
.05 molÁdm
5.05
10.23
-3
.1 molÁdm
.15 molÁdm
.20 molÁdm
.25 molÁdm
5.26
10.30
-
3
3
-5.40 (0.07)
-15.85 (0.09)
-0.32 (0.1)
5.40
10.45
a
l, p and r are the stoichiometric
coefficients corresponding to
-
-5.55 (0.03)
-16.14 (0.05)
-0.23 (0.09)
-5.75 (0.03)
-16.49 (0.06)
-0.11 (0.08)
-5.96 (0.03)
-16.87 (0.04)
-0.03 (0.07)
5.55
2
?
1
Pd(MAMP) , ligand and H ,
respectively
10.59
b
2?
log10 b of Pd(MAMP)
-3
5.75
Standard deviations are given in
parenthesis; sum of square of
residuals are in the range
10.74
-7
-3
1–8 9 10
0.3 molÁdm
5.96
c
pKa
water molecules in
1
and pKa
2
of coordinated
10.91
?2
2 2
[Pd(MAMP)(H O) ]
Table 6 Effect of chloride ion
concentration on the formation
constants of Pd(MAMP)-
CBDCA at 25 °C and
-
Cl ]
a
b
c
a
[
l
p
r
log10
b
pK
-
3
)
(
mol.dm
-3
0.0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
7.88 (0.09)
9.81 (0.06)
5.15 (0.08)
8.86 (0.06)
4.71 (0.07)
8.78 (0.05)
4.32 (0.08)
8.65 (0.06)
3.97 (0.09)
8.48 (0.05)
3.55 (0.07)
8.23 (0.09)
3.12 (0.05)
7.93 (0.08)
1.93
3.71
4.07
4.33
4.51
4.68
4.81
0
.3 molÁdm (KCl ? NaNO3)
ionic strength
0
0
.05
.1
a
l,p and r are the stoichiometric
coefficients corresponding to
0.15
2
?
1
Pd(MAMP) , ligand and H ,
respectively
0
0
0
.20
.25
.3
b
2?
log10 b of Pd(MAMP)
Standard deviations are given in
parenthesis; sum of square of
residuals are in the range
-7
3–8 9 10
c
pK
a
of the protonated species
(
111) = log10 b111 - log10 b
110
1
23

J Solution Chem (2017) 46:1024–1047
1043
on the basis that the concentrations of the active species, the mono- and the diaqua
-
complexes, decrease with increasing [Cl ]; this will in turn affect the stability of the
complexes formed. A similar trend was observed for the reaction of Pd(Me en)Cl
4
2
´ ´
0
Me en = N,N,N,N-tetramethylethylenediamine) with inosine and 5 -inosine monophos-
4
(
-
phate as a function of the chloride concentration, where addition of Cl to the reaction
mixture caused a significant decrease in the observed rate constants [42]. The variation in
2
stability constant of the Pd(MAMP) complex with CBDCA, as a function of Cl ion
?
-
concentration, is shown in Fig. 4. The pK of the protonated species 111 increases with
a
-
increase of [Cl ]. This may be explained on the premise that the chloride ion coordinated
(
II)
to Pd
bonding. This will lead to an increase of the pK value of the uncoordinated carboxylic
ion stabilizes the uncoordinated carboxylic group proton through hydrogen
a
group.
4
Quantum Chemical Calculations and Equilibrium Studies
Energy minimization studies were carried out on the basis of the semi-empirical PM3 level
(
II)
provided by HyperChem 7.5 software. The most stable structures for the metal com-
plexes obtained were subsequently optimized to the closest local minimum at the
semiempirical level using PM3 parameterizations. The values of the following parameters:
the highest occupied molecular orbital energy (EHOMO), the lowest unoccupied molecular
orbital energy (ELUMO), the difference between HOMO and LUMO energy levels (DE),
Mulliken electronegativity (v), chemical potential (Pi), global hardness (g), global softness
(
S) and global electrophilicity (x) [50–53] have been calculated [54] using this semi-
empirical PM3 method as implemented in HyperChem [31]. In the first step, the molecular
geometries of all compounds were fully optimized in the gas phase to gradients of
-
1
-1
˚
0
.04 kJÁmol ÁA and afterwards the molecular descriptors were determined.
The concepts of the parameters v and Pi are related to each other. The inverse of the
global hardness is designated as the absolute softness r.
To understand the mode of chelation and stability of these complexes in solution, we
have optimized first the structure of metal complexes and then we have calculated and
discussed the highest occupied molecular orbital–lowest unoccupied molecular orbital
(HOMO–LUMO) gap for all these complexes (see Table 2S) in order to correlate the
experimental results of stability constants with the calculated molecular parameters of the
complexes (Table 7). The optimized structure for the [Pd–MAMP–a-Ala] complex as a
representative example of the investigated compounds with the atomic numbering
´
˚
scheme is shown in the Fig. 5. The bond angles and bond lengths (A) obtained from the
energy minimum optimized structure of the [Pd–MAMP–a-Ala] complex are given. In
H L
Table 7 Calculated EHOMO (E ), ELUMO (E ), energy band gap (DE), chemical potential (Pi), elec-
tronegativity (v), electron affinity (EA), global hardness (g), absolute softness (r), global softness (S) and
a
electrophilicity index (x) for the Pd-MAMP-L complexes
b
b
b
b
EA
Compound
HOMO
LUMO
DE
Pi
v
g
r
S
x
Pd-MAMP-a-Ala -3.325
Pd-MAMP-b-Ala -3.090
0.437
0.550
3.762 3.325 -0.44 1.444 1.88 0.532 0.266 0.554
3.640 3.090 -0.55 1.27
1.82 0.549 0.275 0.443
a
L a-alanine, b-alanine
b
-19
The unit of HOMO, LUMO, Pi and EA is eV; 1ev = 1.6022 9 10
J
123

1044
J Solution Chem (2017) 46:1024–1047
Fig. 5 The molecular structure of the [Pd(MAMP)–a-Ala] complex along with the atom numbering scheme
most of the cases, the actual bond lengths and bond angles are close to the optimal values,
and thus the proposed structure of the compound is acceptable (See Table 3S).
The calculations for the molecular parameters support the experimental formation
constant. The HOMO–LUMO gap is used as a direct indicator of stability [55, 56]. A large
HOMO–LUMO gap indicates greater stability and decreased chemical reactivity. The
results show the following trend in HOMO–LUMO gap for the complexes: Pd–MAMP–a-
Ala [ Pd–MAMP–b-Ala. The Pd–MAMP–a-Ala complex has an energy gap
(
DE = 3.762) greater than that of the Pd–MAMP–b-Ala complex (DE = 3.640). A large
HOMO–LUMO gap is in accord with the experimental formation constant values.
According to the maximum hardness principle, greater hardness (g) causes more sta-
bility in the molecule [57]. Absolute hardness is half of the HOMO–LUMO energy gap.
The hardness (g) calculated for the Pd(II) complexes is shown in Table 7. The hardness
parameter of the complexes follows this sequence Pd–MAMP–a-Ala (g = 1.88) [ Pd–
MAMP–b-Ala (g = 1.82) which is also in accord with the energy gap and formation
constants of the Pd(II) complexes.
5
Conclusions
2
The present investigation describes the formation equilibria of [Pd(MAMP)(H O) ] with
?
2
2
ligands of biological significance. In combination of stability constants data of such dia-
qua-complexes with amino acids, peptides and DNA constituents, it would be possible to
1
23

J Solution Chem (2017) 46:1024–1047
1045
calculate the equilibrium distribution of the metal species in biological fluids where all
types of ligands are present simultaneously. This would form a clear basis for under-
standing the mode of action of such metal species under physiological conditions. Amino
acids form highly stable complexes, the substitutent on the a-carbon atom has a significant
effect on the stability of the formed complex. Complex formation promotes deprotonation
2?
of the hydroxo group. The present study shows clearly that the [Pd(MAMP)(H O) ]
2
2
complex can form strong bonds with peptides and promote facile deprotonation of the
peptide. In this work, it was possible to distinguish between the reactivity of nucleic bases,
0
nucleosides and 5 -nucleotides. It is hoped that the obtained data from complex formation
equilibria of [Pd(MAMP)(H O) ] with bioactive ligands in aqueous solution will be a
2
2
significant contribution to workers carrying out mechanistic studies in biological media.
The difference values between log10 K1 and log10 K2 for the interaction of
[Pd(MAMP)(H O) ] with DNA constituents are positive, indicating that the coordination
2 2
of the first ligand molecule to the metal ion is more favorable than the bonding to the
second one. The molecular properties of the structures such as hardness and HOMO–
LUMO energy gap supported the experimental formation constants of these complexes.
References
1. Rosenberg, B., Van Camp, L., Trasko, J.E., Mansour, V.H.: Platinum compounds: a new class of potent
antitumour agent. Nature 222, 385–386 (1969)
2. Wong, E., Giandomenico, C.M.: Current status of platinum-based antitumor drugs. Chem. Rev. 99,
2451–2466 (1999)
3
4
. Guo, Z., Sadler, P.J.: Medicinal inorganic chemistry. Adv. Inorg. Chem. 49, 183–306 (2000)
. Rosenberg, B.: In: Sigel, H. (ed.) Metal Ions in Biological Systems, vol. 11, pp. 127–196. Marcel
Dekker, New York (1980)
5
6
7
8
. Roberts, J.J.: In: Eichhorn, G.L., Marzilli, L.G. (eds.) Metal Ions in Genetic Information Transfer, vol.
11, pp. 273–332. Elsevier, Amsterdam (1981)
. Gill, D.S.: In: Hacker, M.P., Douple, E.B. (eds.) Platinum Coordination Complexes in Cancer Che-
motherapy, vol. 11, pp. 267–278. Nijhoff, Boston (1984)
. Heim, M.E.: In: Keppler, B.K. (ed.) Metal Complexes in Cancer Chemotherapy, vol. 11, p. 9. VCH,
Weinheim (1993)
. Summa, N., Soldatovi c´ , T., Dahlenburg, L., Bugar cˇ i c´ , Z.D., van Eldik, R.: Kinetics and mechanism of
ˇ
?
the substitution reactions of [PtCl(bpma)] , [PtCl(gly-met-S,N,N)] and their aqua analogues with
L-methionine, glutathione and 5‘-GMP. J. Biol. Inorg. Chem. 12, 1141–1150 (2007)
ˇ
9
. Bugar cˇ i c´ , Z.D., Nandibewoor, S.T., Hamza, M.S.A., Heinemann, F., Van Eldik, R.: Kinetics and
mechanism of the reactions of Pd(II) complexes with azoles and diazines. Crystal structure of
[Pd(bpma)(H
0. Lippert, B. (ed.): Cisplatin Chemistry and Biochemistry of Leading Anticancer Drugs. Wiley-VCH,
Z u¨ rich (1999)
1. Wang, D., Lippard, S.J.: Cellular processing of platinum anticancer drugs. Nat. Rev. Drug Discov. 4,
2
O)](ClO
4
)
2
Á2H
2
O. Dalton Trans. 24, 2984–2990 (2006)
1
1
1
307–320 (2005)
2. Abd El-Karim, A.T., El-Sherif, A.A.: Potentiometric, thermodynamics and coordination properties for
binary and mixed ligand complexes of copper(II) with imidazole-4-acetic acid and tryptophan or
phenylalanine aromatic amino acids. J. Solution Chem. 45, 712–731 (2016)
1
3. Abdelkarim, A.T., Al-Shomrani, M.M., Rayan, A.M., El-Sherif, A.A.: Mixed ligand complex formation
of cetirizine drug with bivalent transition metal(II) ions in the presence of 2-aminomethylbenzimida-
zole: synthesis, structural, biological, pH-metric and thermodynamic studies. J. Solution Chem. 44,
1673–1704 (2015)
1
4. Rayan, A.M., Abdlkarim, A.T., Ahmed, M.M., El-Sherif, A.A., Barakat, M.H.: Complex formation of
cetirizine drug with bivalent transition metal(II) ions in the presence of alanine: synthesis, characteri-
zation, equilibrium studies, and biological activity studies. J. Coord. Chem. 68, 678–703 (2015)
123

1046
J Solution Chem (2017) 46:1024–1047
1
1
1
1
1
5. Abdelkarim, A.T., El-Sherif, A.A.: Physicochemical studies and biological activity of mixed ligand
complexes involving bivalent transition metals with a novel Schiff base and glycine as a representative
amino acid. J. Eur. Chem. 5, 328–333 (2014)
6. Aljahdali, M.S., Abdelkarim, A.T., El-Sherif, A.A., Ahmed, M.M.: Synthesis, characterization, equi-
librium studies, and biological activity of complexes involving copper(II), 2-aminomethylthiophenyl-4-
bromosalicylaldehyde Schiff base, and selected amino acids. J. Coord. Chem. 67, 870–890 (2014)
7. Aljahdali, M., El-Sherif, A.A., Shoukry, M.M., Hosny, W.M., Abd-Elmoghny, M.G.: Complex for-
mation equilibria of unusual seven coordinate Fe(III) complexes with DNA constituents. J. Solution
Chem. 42, 1663–1679 (2013)
8. El-Sherif, A.A., Shoukry, M.M., Abd-Elgawad, M.M.A.: Protonation equilibria of some selected a-
amino acids in DMSO–water mixture and their Cu(II)-complexes. J. Solution Chem. 42, 412–427
(2013)
9. El-Sherif, A.A., Shoukry, M.M.: The palladium(II)-picolylamine promoted hydrolysis of amino acid
esters; kinetic evidence for inter- and intramolecular mechanisms. J. Progr. React. Kinet. Mech 36,
2
15–226 (2011)
0. El-Sherif, A.A.: Synthesis and characterization of some potential antitumor palladium(II) complexes of
-aminomethylbenzimidazole and amino acids. J. Coord. Chem. 64, 2035–2055 (2011)
2
2
2
1. El-Sherif, A.A., Shoukry, M.M., El-Bahnasawy, R.M., Ahmed, D.M.: Complex formation reactions of
palladium(II)-1,3-diaminopropane with various biologically relevant ligands. Kinetics of hydrolysis of
glycine methyl ester through complex formation. Cent. Eur. J. Chem. 8, 919–927 (2010)
2. Rau, T., Van Eldik, R.: In: Sigel, A., Sigel, H. (eds.) Metal Ions in Biological Systems, p. 340. Marcel
Dekker, New York (1996)
2
2
3. Shoukry, M.M., Shehata, M.R., Abdel-Razik, A., Abdel-Karim, A.T.: Equilibrium studies of mixed
ligand complexes involving (1,2-diaminopropane)-palladium(II) and some bioligands. Monatsh. Chem.
130, 409–423 (1999)
2
2
2
4. El-Sherif, A.A., Shoukry, M.M., van Eldik, R.: Complex-formation reactions and stability constants for
mixed–ligand complexes of diaqua(2-picolylamine)palladium(II) with some bio-relevant ligands.
J. Chem. Soc., Dalton Trans. 7, 1425–1432 (2003)
5. Rau, T., Shoukry, M.M., van Eldik, R.: Complex formation and ligand substitution reactions of (2-
picolylamine)palladium(II) with various biologically relevant ligands. Characterization of (2-picoly-
lamine)(1,1-cyclobutanedicarboxylato)palladium(II). Inorg. Chem. 36, 1454–1463 (1997)
0
6. Mohamed, M.M.A., Shoukry, M.M.: Complex formation reactions of (N,N -dimethylethylenediamine)
palladium(II) with various biologically relevant ligands. Polyhedron 20, 343–352 (2001)
7. Bates, R.G.: Determination of pH-Theory and Practice, 2nd edn. Wiley, New York (1975)
8. Stark, J.G., Wallace, H.G. (eds.): Chemistry Data Book, p. 75. Murray, London (1975)
9. Gans, P., Sabatini, A., Vacca, A.: An improved computer program for the computation of formation
constants from potentiometric data. Inorg. Chim. Acta 18, 237–239 (1976)
0. Pettit, L.: University of Leeds, Personal Communication
2
2
2
3
3
3
1. HyperChem version 7.5. Hypercube, Inc. (2003)
2. Perrin, D.D.: Stability Constants of Metal-Ion Complexes: Part B Organic Ligands. Pergamon Press,
Oxford (1979)
0
3
3
3. Shehata, M.R.: Mixed ligand complexes of diaquo(2,2 -bipyridine)palladium(II) with cyclobutane-1,1-
dicarboxylic acid and DNA constituents. Trans. Met. Chem. 26, 198–204 (2001)
4. Lim, M.C.: Mixed–ligand complexes of palladium. Diaqua(ethylenediamine)palladium(II) complexes
of ethanolamine, L-serine, L-threonine, L-homoserine, and L-hydroxyproline. Inorg. Chem. 20,
1377–1379 (1981)
3
3
3
5. El-Sherif, A.A.: Coordination chemistry of palladium(II) ternary complexes with relevant biomolecules.
In: Stoichiometry and Research, The Importance of Quantity in Biomedicine, pp. 79–120. In-Tech
Publisher, Rijeka (2012)
6. Shehata, M.R., Shoukry, M.M., Nasr, F.M.H., van Eldik, R.: Complex-formation reactions of dicho-
loro(S-methyl-L-cysteine) palladium(II) with bio-relevant ligands. Labilization induced by S-donor
chelates. Dalton Trans. 6, 779–786 (2008)
7. Lim, M.C.: Mixed-ligand complexes of palladium(II). Part 1. Diaqua(ethylenediamine)palladium(II)
complexes of glycylglycine and glycinamide. J. Chem. Soc., Dalton Trans. 1, 15–17 (1977)
8. Martin, R.B.: Nucleoside sites for transition metal ion binding. Acc. Chem Res. 18, 32–38 (1985)
9. Maskos, K.: Spectroscopic studies on the copper(II)–inosine system. J. Inorg Biochem. 25, 1–14 (1985)
0. Sigel, H., Massoud, S.S., Corfu, N.A.: Comparison of the extent of macrochelate formation in com-
3
3
4
2-
2-
plexes of divalent metal ions with guanosine (GMP ), inosine (IMP ), and adenosine 5’-
2
monophosphate (AMP ). The crucial role of N-7 basicity in metal ion–nucleic base recognition. J. Am.
-
Chem. Soc. 116, 2958–2971 (1994)
1
23

J Solution Chem (2017) 46:1024–1047
1047
4
4
4
4
4
4
4
4
1. Shoukry, M.M., van Eldik, R.: Correlation between kinetic and thermodynamic complex-formation
0
constants for the interaction of bis(amine)palladium(II) with inosine, inosine 5 -monophosphate and
0
guanosine 5 -monophosphate. J. Chem. Soc., Dalton Trans. 13, 2673–2678 (1996)
2. Shoukry, M.M., Hohmann, H., van Eldik, R.: Mechanistic information on the reaction of model pal-
0
ladium(II) complexes with purine nucleosides and 5 -nucleotides in reference to the antitumor activity
of related platinum complexes. Inorg. Chim. Acta 198, 187–192 (1992)
3. Ano, S.O., Intini, F.P., Natile, G., Marzilli, L.G.: Viewing early stages of guanine nucleotide attack on
Pt(II) complexes designed with in-plane bulk to trap initial adducts. Relevance to cis-type Pt(II) anti-
cancer drugs. J. Am. Chem. Soc. 119, 8570–8571 (1977)
4. Kiser, D., Intini, F.P., Xu, Y., Natile, G., Marzilli, L.G.: Atropisomerization of cis-bis(5’GMP)–plat-
inum(II)–diamine complexes with non-C2-symmetrical asymmetric diamine ligands containing NH
groups directed to one side of the coordination plane. Inorg. Chem. 33, 4149–4158 (1994)
5. Frey, U., Ranford, J.D., Sadler, P.J.: Ring-opening reactions of the anticancer drug carboplatin: NMR
3 2
characterization of cis-[Pt(NH ) (CBDCA-O)(5‘-GMP-N7)] in solution. Inorg Chem. 32, 1333–1340
(1993)
6. Rau, T., Alsfasser, R., Zahl, A., Van Eldik, R.: Structural and kinetic studies on the formation of
platinum(II) and palladium(II) complexes with l-cysteine-derived ligands. Inorg. Chem. 37, 4223–4230
(
1998)
?
7. Lemma, K., Elmroth, S.K.C., Elding, L.I.: Substitution reactions of [Pt(dien)Cl] , [Pt(dien)(GSMe)]
2?
,
(GSMe = S-methylglutathione) with some sulfur-
2
?
cis-[PtCl
2
(NH
3
)
2
] and cis-[Pt(NH
3
)
2
(GSMe)
2
]
bonding chemoprotective agents. J. Chem. Soc., Dalton Trans. 7, 1281–1286 (2002)
8. Provosta, K., Bouvet-Mullera, D., Crauste-Manciet, S., Moscovicia, J., Olivid, L., Vlaicd, G.,
Michalowicza, A.: EXAFS structural study of platinum-based anticancer drugs degradation in presence
of sulfur nucleophilic species. Biochimie 91, 1301–1306 (2009)
4
5
5
9. Alden, W.W., Repta, A.J.: Exacerbation of cisplatin-induced nephrotoxicity by methionine. Chem. Biol.
Interact. 48, 121–124 (1984)
0. Pearson, R.G.: Absolute electronegativity and hardness: applications to organic chemistry. J. Org.
Chem. 54, 1423–1430 (1989)
1. Geerlings, P., De Proft, F., Langenaeker, W.: Conceptual density functional theory. Chem. Rev. 103,
1793–1874 (2003)
5
5
2. Parr, R.G.: Electrophilicity index. J. Am. Chem. Soc. 121, 1922–1924 (1999)
3. Chattaraj, P.K., Giri, S.: Stability, reactivity, and aromaticity of compounds of a multivalent superatom.
J. Phys. Chem. A 111, 11116–11121 (2007)
5
4. Speie, G., Csihony, J., Whalen, A.M., Pie-Pont, C.G.: Studies on aerobic reactions of ammonia/3,5-
ditert-butylcatechol Schiff-base condensation products with copper, copper(I), and copper(II). Strong
copper(II)-radical ferromagnetic exchange and observations on a unique N-N coupling reaction. Inorg.
Chem. 35, 3519–3535 (1996)
5
5
5
5. Aihara, J.: Reduced HOMO–LUMO gap as an index of kinetic stability for polycyclic aromatic
hydrocarbons. J. Phys. Chem. A 103, 7487–7495 (1999)
6. Haddon, R.C., Fukunaga, T.: Absolute hardness as a measure of aromaticity. Tetrahedron Lett. 29,
4843–4846 (1988)
7. Parr, R.G., Chattara, P.K.: Principle of maximum hardness. J. Am. Chem. Soc. 113, 1854–1855 (1991)
123