Synthesis of different pyrazine-bridged platinum(II) complexes and 1H NMR study of their catalytic abilities in the hydrolysis of the N-acetylated l-methionylglycine

Article abstract of DOI:10.1016/j.poly.2013.08.016

Four binuclear {[Pt(L)Cl]₂(μ-pz)}Cl₂-type complexes have been synthesized and characterized by elemental microanalyses and NMR (¹H and ¹³C) spectroscopy (L is ethylenediamine, en; (±)-1,2-propylenediamine, 1,2-pn; isobutylenediamine, ibn; trans-(±)-1,2-diaminocyclohexane, dach and pz is bridging pyrazine ligand). The chlorido complexes were converted into the corresponding aqua species, {[Pt(L)(H₂O)]₂(μ-pz)}⁴⁺, and ¹H NMR spectroscopy was applied to study their reactions with the N-acetylated l-methionylglycine, Ac-L-Met-Gly. The {[Pt(L)(H₂O)] ₂(μ-pz)}⁴⁺ complex and dipeptide were reacted in 1:1 and 1:2 M ratios, respectively, and all reactions were performed in the pH range 2.0-2.5 and at 37 C. In the reactions with equimolar amounts of the reactants all Pt(II) aqua complexes bind to the methionine side chain of Ac-L-Met-Gly dipeptide and promote the cleavage of the amide bond involving the carboxylic group of methionine. It was found that the amount of hydrolyzed dipeptide strongly depends from the steric bulk of bidentate coordinated diamine ligand L in {[Pt(L)(H₂O)]₂(μ-pz)}⁴⁺ complex (en > 1,2-pn > ibn > dach). However, in the reaction with an excess of dipeptide the influence of the nature of diamine ligand L on this hydrolytic process could not be observed due to the fact that slow decomposition of {[Pt(L)(H ₂O)]₂(μ-pz)}⁴⁺ complex was occured.

Full text of DOI:10.1016/j.poly.2013.08.016

Polyhedron 65 (2013) 42–47
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis of different pyrazine-bridged platinum(II) complexes
1
and H NMR study of their catalytic abilities in the hydrolysis of the
N-acetylated L-methionylglycine
⇑
ˇ
ˇ
´
´
Snezana Rajkovic , Darko P. Ašanin, Marija D. Zivkovic, Miloš I. Djuran
Department of Chemistry, Faculty of Science, University of Kragujevac, R. Domanovi´ca 12, PO Box 60, 34000 Kragujevac, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history:
Four binuclear {[Pt(L)Cl]2(l-pz)}Cl2-type complexes have been synthesized and characterized by elemen-
tal microanalyses and NMR (1H and 13C) spectroscopy (L is ethylenediamine, en; ( )-1,2-propylenedi-
amine, 1,2-pn; isobutylenediamine, ibn; trans-( )-1,2-diaminocyclohexane, dach and pz is bridging
pyrazine ligand). The chlorido complexes were converted into the corresponding aqua species, {[Pt(L)
Received 29 May 2013
Accepted 4 August 2013
Available online 14 August 2013
(H2O)]2(
l
-pz)}4+, and 1H NMR spectroscopy was applied to study their reactions with the N-acetylated
L
-methionylglycine, Ac-L-Met-Gly. The {[Pt(L)(H2O)]2(l
-pz)}4+ complex and dipeptide were reacted in
Keywords:
Binuclear platinum(II) complexes
1:1 and 1:2 M ratios, respectively, and all reactions were performed in the pH range 2.0–2.5 and at
37 °C. In the reactions with equimolar amounts of the reactants all Pt(II) aqua complexes bind to the
methionine side chain of Ac-L-Met-Gly dipeptide and promote the cleavage of the amide bond involving
the carboxylic group of methionine. It was found that the amount of hydrolyzed dipeptide strongly
L
-Methionylglycine
1H NMR spectroscopy
Hydrolysis
depends from the steric bulk of bidentate coordinated diamine ligand L in {[Pt(L)(H2O)]2(l
-pz)}4+ com-
plex (en > 1,2-pn > ibn > dach). However, in the reaction with an excess of dipeptide the influence of
the nature of diamine ligand L on this hydrolytic process could not be observed due to the fact that slow
decomposition of {[Pt(L)(H2O)]2(l
-pz)}4+ complex was occured.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
[PdCl4]2À, the active form was a mononuclear palladium(II)–pep-
tide complex, while with [Pd(H2O)4]2+ and [Pd(en)(H2O)2]2+ com-
plexes, binuclear hydrolytically active palladium(II)–peptide
complexes bridged with two methionine side chains were formed.
Moreover, it was shown that these binuclear palladium(II)–peptide
complexes are more efficient than the corresponding mononuclear
complex in promoting the hydrolysis of the scissile amide bond in
methionine-containing peptides. In accordance to this, the reac-
Hydrolytic reactions of uncatalyzed peptide bonds are extre-
mely slow. Enzymes are commonly used as cleavage agents for this
reactions but investigation of the hydrolytic reactions promoted by
metal complexes suggested that they can be applied for cleavage of
unactivated amide bonds very efficiently. Recent years have wit-
nessed an increasing interest in the study of the interactions of
platinum(II) [1–3] and palladium(II) [2–16] complexes with sulfur-
and histidine-containing peptides and proteins [17–20] as effective
catalyst for the hydrolytic cleavage of the above mentioned
peptides.
In general, it was shown that aqua complexes of these metal
ions spontaneously bind to the heteroatom in the side chain of
methionine [1–8] or histidine [2,3,9–16,21] and promote cleavage
of the amide bond involving the carboxylic group of the anchoring
amino acid. The hydrolytic reactions of methionine-containing
peptides with different palladium(II) complexes were investigated
[3,8,9,21–23] and it was shown that different promoters produce
different hydrolytically active palladium(II)–peptide complexes.
Thus, in the reactions of methionine-containing peptides with
tions of the thiolate-bridged (Me4N)2[Pd2(l-SPh)2Cl4] complex
with different methionine-containing peptides of the type Ac-L-
Met-X (X is Gly, Val, Phe or Ala) showed that this complex was
an effective catalyst for the rapid cleavage of methionine-contain-
ing dipeptides in non-aqueous solvents [24]. An important advan-
tage of dimerization is the possibility of cooperation between the
metals, as was shown for hydrolysis of DNA, RNA, and their models
catalyzed by polynuclear metal complexes and metalloenzymes
[25–30]. Very recently in one of our laboratories we compared
hydrolytic abilities of two Pt(II) complexes, mononuclear
[Pt(en)(H2O)2]2+ and binuclear {[Pt(en)(H2O)]2( -pz)}4+, in the
l
reaction with Ac-L-Met-Gly dipeptide. It was shown that two Pt(II)
ions bridged with one aromatic pyrazine ligand in the {[Pt(en)
(H2O)]2(l
-pz)}4+ complex are more efficient in the hydrolysis of
the Ac-L-Met-Gly dipeptide, even when the hydrolytic reaction
was performed with an excess of the mononuclear complex. Our
⇑
Corresponding author. Tel.: +381 34 300 251; fax: +381 34 335 040.
E-mail address: snezana@kg.ac.rs (S. Rajkovic´).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.08.016

´
43
S. Rajkovic et al. / Polyhedron 65 (2013) 42–47
latest findings for {[Pt(en)(H2O)]2(
l
-pz)}4+ complex [31] together
evaporated and the residue washed with ether. The crude product
was dissolved in a minimal amount of 0.5 mol/dm3 LiCl aqueous
solution. The obtained solution was left overnight in the dark.
The pale-yellow precipitate of {[Pt(L)Cl]2(l-pz)}Cl2 was removed
by filtration, washed with methanol and then ether, and air-dried.
with those for different binuclear Pd(II) complexes [24] showed
that polynuclear Pt(II) and Pd(II) complexes can be perspective cat-
alytic reagents for amide bond hydrolysis in the reactions with
methionine-containing peptides.
In this work, an attempt was made to gain further insight into
selective hydrolysis of the methionine-containing peptides in the
presence of different binuclear platinum(II) complexes. For these
purposes, the 1H NMR spectroscopy was applied to study the
influence of the chelating diamine ligand L in {[Pt(L)(H2O)]2
Depending of the type diamine ligand L the yield of {[Pt(L)Cl]2
(
l
-pz)}Cl2 complex was between 30–40%. Anal. Calc. for {[Pt
(1,2-pn)Cl]2( -pz)}Cl2 = C10H24N6Cl4Pt2 (FW = 760.31): C, 15.80;
H, 3.18; N, 11.05. Found: C, 15.45; H, 3.19; N, 10.82%. Anal.
Calc. for {[Pt(ibn)Cl]2( -pz)}Cl2 = C12H28N6Cl4Pt2 (FW = 788.36):
C, 18.28; H, 3.58; N, 10.66. Found: C, 17.84; H, 3.63; N, 10.47%. Anal.
Calc. for {[Pt(dach)Cl]2( -pz)}Cl2 = C16H32N6Cl4Pt2 (FW = 840.43):
C, 22.87; H, 3.84; N, 10.00. Found: C, 22.56; H, 3.94; N, 9.56%.
l
l
(l
-pz)}4+-type complexes (L is ethylenediamine, en; ( )-1,2-pro-
pylenediamine, 1,2-pn; isobutylenediamine, ibn; trans-( )-1,
2-diaminocyclohexane, dach and pz is bridging pyrazine ligand)
on the hydrolytic cleavage of the Ac-L-Met-Gly dipeptide.
l
2.3. Preparation of {[Pt(L)(H2O)]2(
l
-pz)}4+
2. Experimental
The {[Pt(L)Cl]2( -pz)}Cl2 complexes were converted into the
l
corresponding aqua complexes by treatment with 3.98 equivalents
of AgNO3, according to a previously published method [34]. In each
case, the formed solid AgCl was removed by filtration in the dark,
and the fresh solutions of the aqua complexes were kept in a refrig-
erator and used in the further experiments.
2.1. Materials
Distilled water was demineralized and purified to a resistance
greater than 10 M
ylenediamine (en), ( )-1,2-propylenediamine (1,2-pn), isobutylen-
ediamine (ibn), trans-( )-1,2-diaminocyclohexane (dach), pyrazine
(or 1,4-diazine), pz and K2[PtCl4] were obtained from the Aldrich
Chemical Co. All common chemicals were of reagent grade. The
X
cmÀ1. The compounds D2O, DNO3, NaOD, eth-
2.4. Measurements
All pH measurements were realized at ambient temperature
using an Iskra MA 5704 pH meter calibrated with Fischer certified
buffer solutions of pH 4.00 and 7.00. The results were not corrected
for the deuterium isotope effect.
dipeptide L-methionylglycine (L-Met-Gly) was obtained from the
Sigma Chemical Co. The terminal amino group in this dipeptide
was acetylated by a standard method [4].
The NMR spectra of D2O solution containing TSP (sodium
trimethylsilylpropane-3-sulfonate) as the internal reference
2.2. Preparation of {[Pt(L)Cl]2(
pn, ibn or dach)
l-pz)}Cl2-type complexes (L is en, 1,2-
were recorded with
a Varian Gemini 2000 spectrometer
(200 MHz). Fresh solutions of aqua complexes and dipeptide
were prepared separately and then mixed in 1:1 and 1:2 M ra-
tios, respectively. The initial concentration of dipeptide and
aqua complexes solutions were 40 mM. All reactions were per-
formed in the pH range 2.0–2.5 and at 37 °C. Elemental micro-
analyses for carbon, hydrogen and nitrogen were performed by
the Microanalytical Laboratory, Faculty of Chemistry, University
of Belgrade.
The binuclear platinum(II) complexes of the type {[Pt(L)Cl]2
l-pz)}Cl2 were synthesized from the corresponding mononuclear
[Pt(L)Cl2] complexes by modification of the procedure published in
(
the literature [31,32].
Preparation of [Pt(L)Cl2]: All mononuclear Pt(II) complexes were
prepared with minor modification of a method previously used in
our laboratory for the preparation of a series of [M(L)Cl2] com-
plexes (M is Pt(II) or Pd(II); L is bidentate coordinated diamine or
amino acid) [15,23,33]. K2PtCl4 was dissolved in water and mixed
with an equimolar amount of diamine ligand (L). The pH of the
solution was adjusted to ca. 3 by addition of 1 M HCl and mixture
was stirred at 80 °C for 2 h. All complexes were crystallized from
water at room temperature. The pure complexes were obtained
by recrystallization from warm water and than cooling at room
temperature. The experimental results of the elemental analyses
for C, H and N parameters for all investigated Pt(II) complexes
are in accordance with theoretical values calculated for [Pt(L)Cl2]
complexes.
Preparation of [Pt(L)(dmf)Cl]NO3: The solid [Pt(L)Cl2] complex
was converted into the corresponding monodimethylformamide
(dmf) [Pt(L)(dmf)Cl]NO3 complex by treatment with 0.98 equiva-
lents of AgNO3 on the following manner. To a solution of 55.3 mg
(0.325 mmol) of AgNO3 in 5 cm3 of dmf was added a suspension
of 0.332 mmol of [Pt(L)Cl2] in 10 cm3 of dmf. The mixture was stir-
red overnight at room temperature in the dark. The precipitated
AgCl was removed by filtration and resulting pale yellow dmf solu-
tion of [Pt(L)(dmf)Cl]NO3 was used as the starting material for the
preparation of the required pyrazine-bridged platinum(II) com-
3. Results and discussion
Four binuclear {[Pt(L)Cl]2(l-pz)}Cl2-type complexes have been
synthesized and characterized by elemental microanalyses and
NMR (1H and 13C) spectroscopy (L is ( )-1,2-propylenediamine,
1,2-pn; isobutylenediamine, ibn and trans-( )-1,2-diaminocyclo-
hexane, dach; see Fig. 1). The schematic presentation of the reac-
tion for the syntheses of these complexes is given in Fig. 2. The
spectroscopic results of these complexes are in accordance with
those for similar complexes in literature [15,23,32] and with pro-
posed formula {[Pt(L)Cl]2(l-pz)}Cl2 (Table 1). As it was shown in
Fig. 1, all investigated Pt(II) complexes have the same bridging pyr-
azine ligand but different chelating diamine ligand L. In the present
study we investigated the influence of this chelating ligand L on
the hydrolysis of the amide bond in the N-acetylated L-methionyl-
glycine dipeptide (Ac-L-Met-Gly). The terminal amino group in this
peptide was acetylated to protect its binding to the Pt(II) atom. In
order to investigate the influence of structural changes in the che-
lating diamine ligand L on the hydrolysis of Ac-L-Met-Gly dipep-
tide, all chlorido Pt(II) complexes were converted into the
plexes, {[Pt(L)Cl]2(l-pz)}Cl2.
Preparation of {[Pt(L)Cl]2(l-pz)}Cl2: The dmf solution of the pyr-
corresponding aqua species, {[Pt(L)(H2O)]2(l
-pz)}4+. The plati-
azine ligand (pz) (13.29 mg, 0.166 mmol) was added dropwise to
the solution of [Pt(L)(dmf)Cl]NO3. The mixture was stirred at room
temperature in the dark for 12 h. The solvent was then rotary
num(II) aqua complex and dipeptide were reacted in 1:1 and
1:2 M ratios, respectively, and all reactions were performed in
the pH range 2.0–2.5 and at 37 °C. As was shown in our previous

´
S. Rajkovic et al. / Polyhedron 65 (2013) 42–47
44
2+
2+
H2
N
H2
N
H2
N
H2
N
CH3
Cl
H3C
Cl
N
Cl
Cl
N
Pt
Pt
Pt
Pt
N
N
N
N
N
N
H2
H2
H2
H2
{[Pt(en)Cl]2(µ-pz)}2+
{[Pt(1,2-pn)Cl]2(µ-pz)}2+
2+
2+
H2
H2
H2
N
H2
N
N
N
CH3
CH3
Cl
Cl
H3C
H3C
Cl
Cl
N
Pt
Pt
Pt
Pt
N
N
N
N
N
N
N
H2
H2
H2
H2
{[Pt(dach)Cl]2(µ-pz)}2+
{[Pt(ibn)Cl]2(µ-pz)}2+
Fig. 1. Schematic drawing of the binuclear {[Pt(L)Cl]2(l-pz)}Cl2 complexes. These complexes were converted into the corresponding aqua species, {[Pt(L)(H2O)]2(l ,
-pz)}4+
and used in the reactions with the N-acetylated
L-methionylglycine.
H
N
H
N
2
2
Cl
DMF
+ AgNO3
in DMF
N
N
+
L
Pt
L
Pt
- AgCl
Cl
Cl
+
N
H
N
H
0.5 mol
pz
2
2
[Pt(L)Cl(DMF)]
[Pt(L)Cl ]
2
in DMF
12 h
2
2
H
N
H
N
H
2
H
N
2
2
2
N
Cl
Cl
N
Cl
Cl
in excess of LiCl
- DMF
L
L
L
Pt
Pt
L
Pt
2NO3
Pt
2Cl
N
N
N
N
H
N
H
N
H
N
H
2
2
2
2
{[Pt(L)Cl] (µ-pz)}(NO )
{[Pt(L)Cl] (µ-pz)}Cl
2
3 2
2
2
Fig. 2. The schematic presentation of the reaction for the synthesis of binuclear {[Pt(L)Cl]2(
l-pz)}Cl2-type complexes (L is ethylenediamine, en; ( )-1,2-propylenediamine,
1,2-pn; isobutylenediamine, ibn; trans-( )-1,2-diaminocyclohexane, dach and pz is bridging pyrazine ligand).
Table 1
Characteristic NMR (1H and 13C) chemical shifts for the {[Pt(L)Cl]2(
complexes [15,23,32].
l-pz)}Cl2-type complexes. These chemical shifts are in accordance with those previously reported for similar
Platinum(II) complex
{[Pt(en)Cl]2( -pz)}Cl2
{[Pt(1,2-pn)Cl]2( -pz)}Cl2
Characteristic 1H NMR resonances (d, ppm)
2.64 (s, enCH2)
Characteristic 13C NMR resonances (d, ppm)
l
9.03 (s, pzCH)
9.01 (s, pzCH)
52.34 (enCH2)
153.46 (pz)
153.44 (pz)
l
1.34 (d, 1,2-pnCH3),
2.45–2.98 (m, 1,2-pnCH2),
3.11–3.32 (m, 1,2-pnCH),
17.83 (1,2-pnCH3),
54.52 (1,2-pnCH2),
59.86 (1,2-pnCH),
{[Pt(ibn)Cl]2(
l
-pz)}Cl2
1.42–1.48 (m, ibnCH3),
2.66 (s, ibnCH2),
9.13 (s, pzCH)
9.00 (s, pzCH)
25.96 (ibnCH3),
60.09 (ibnCH2),
63.61 (ibnCH),
153.36 (pz)
153.31(pz)
{[Pt(dach)Cl]2(
l
-pz)}Cl2
1.27–1.62 (m, dachCH2, C4,C5),
1.76–2.08 (m, dachCH2,C3,C6),
2.45–2.61 (m, dachCH,C1,C2),
26.55 (dach, C4,C5),
34.56 (dach, C3,C6),
65.15 and 65.30 (dach, C1,C2),
studies [11,21,22], acidic solutions are needed to suppress
the formation of hydroxo-bridged oligomeric Pt(II) complexes,
which are catalytically inactive. All chelate diamine ligands L in
expected to remain bound to the platinum(II) atom during the
reactions with Ac-L-Met-Gly dipeptide. The reactions of {[Pt(L)
(H2O)]2(
l
-pz)}4+ complexes with Ac-L-Met-Gly were followed by
{[Pt(L)(H2O)]2(l
-pz)}4+ complexes are inert to substitution and
applying 1H NMR spectroscopy.

´
45
S. Rajkovic et al. / Polyhedron 65 (2013) 42–47
3.1. The reactions of {[Pt(L)(H2O)]2(
equimolar amount of Ac-L-Met-Gly dipeptide
l
-pz)}4+ complexes with an
chain was registered from the simultaneous decline of the reso-
nance at 2.11 ppm due to the S-methyl protons of the free dipep-
tide and the growth of a resonance at 2.54 ppm corresponding to
these protons for the dipeptide coordinated to platinum(II)
through the sulfur atom. These chemical shifts are in accordance
with those previously reported for the reactions of platinum(II)
complexes with different methionine-containing molecules
[1–3,35]. In all investigated reactions the intermediate {[Pt(L)(Ac-
Recently we investigated the reactions of binuclear {[Pt(en)
(H2O)]2(l
-pz)}4+ complex with methionine-containing peptides
[31]. This pyrazine-bridged Pt(II) complex was shown as very
effective hydrolytic reagent in the cleavage of these peptides. As
continuation of this study, here we compared the rate of hydrolysis
of the Ac-L-Met-Gly dipeptide in the presence of {[Pt(en)(H2O)]2
L-Met-Gly-S)](l
-pz)[Pt(L)(H2O)]}4+ complex is hydrolytically
(
l
-pz)}4+ complex with those for three new binuclear {[Pt(L)(H2-
l
active species and promotes the regioselective cleavage of the
Met-Gly amide bond in the Ac-L-Met-Gly dipeptide. These hydro-
lytic reactions can be followed successfully using 1H NMR
spectroscopy by observing the methylene glycine protons in
O)]2(
-pz)}4+ complexes having different bidentate diamine ligand
-pz)}4+ complex
L. When an equimolar amount of {[Pt(L)(H2O)]2(
l
was incubated with Ac-L-Met-Gly, under the above mentioned
experimental conditions, all reactions resulted in formation of
{[Pt(L)(Ac-L-Met-Gly-S)](l
-pz)[Pt(L)(H2O)]}4+ complex and these
platinum(II)–dipeptide complex {[Pt(L)(Ac-L-Met-Gly-S)](
l
-pz)
protons for the free glycine, see Fig. 4. As can be seen from this
figure, the resonance at 3.99 ppm corresponding to the methylene
glycine protons of the dipeptide attached to platinum(II) in
[Pt(L)(H2O)]}4+ in a yield of 95% for less than 30 min (Fig. 3). The
monodentate binding of the platinum(II) to the methionine side
O
H
C
H C
3
OH
N
N
H
O
O
{[Pt(L)(H2O)]2(µ-pz)}4+
+
(L = en, 1,2-pn, ibn or dach)
S
H C
3
2.0 < pH < 2.5
t = 37 oC
Ac-L-Met-Gly
H C
3
H C
3
C
O
C
O
HN
H O
2
HN
O
O
NH
N
NH
N
OH
OH
O
O
S
OH
S
OH
2
2
H C
3
H C
3
Pt
NH
2
H N
2
Pt
N
Pt
H N
NH
H N
Pt
NH
N
2
2
L
L
L
L
H N
2
NH
2
2
2
external attack
{[Pt(L)(Ac-L-Met-Gly-S)](µ-pz)[Pt(L)(H2O)]}4+
internal delivery
intermediate hydrolytically active platinum(II)-peptide complex
H C
3
C
O
HN
+
Gly
OH
O
S
OH
Pt
2
H C
3
N
NH
H N
2
Pt
N
2
L
L
H N
2
NH
2
{[Pt(L)(Ac-L-Met-S)](µ-pz)[Pt(L)(H2O)]}4+
Fig. 3. The reaction scheme of the hydrolytic reaction of the Ac-L-Met-Gly dipeptide with binuclear {[Pt(L)(H2O)]2(
complex and dipeptide were mixed in a 1:1 M ratio and all reactions were performed at 2.0 < pH < 2.5 and at 37 °C.
l
-pz)}4+-type complexes. The corresponding Pt(II)-aqua

´
S. Rajkovic et al. / Polyhedron 65 (2013) 42–47
46
Fig. 4. Parts of the 1H NMR spectra recorded during the reaction of {[Pt(en)(H2O)]2( -pz)}4+ (a) and {[Pt(dach)(H2O)]2( -pz)}4+ (b) complexes with an equimolar amount of
l l
Ac-L-Met-Gly dipeptide as a function of time in the pH range 2.0–2.5 and at 37 °C. The resonances assigned as (N) and (d) correspond to the methylene glycine protons of Ac-
L-Met-Gly dipeptide monodentate bound to the binuclear platinum(II) complex and these protons for the free glycine, respectively.
{[Pt(L)(Ac-L-Met-Gly-S)](
l
-pz)[Pt(L)(H2O)]}4+ complex decreased,
cleavage of the Met-Gly amide bond in the reactions between dif-
ferent binuclear {[Pt(L)(H2O)]2(
-pz)}4+ complexes and Ac-L-Met-
while that at 3.77 ppm for these protons in the free glycine in-
creased. Upon addition of amino acid glycine to the reaction mix-
ture, the resonance at 3.77 ppm was enhanced. The amounts of
the non-hydrolyzed dipeptide coordinated to platinum(II) complex
and hydrolysis products were determined from the known initial
concentration of Ac-L-Met-Gly and from the integrated resonance
for the methylene protons of the free glycine. The changes in con-
centrations of the free glycine and non-hydrolyzed dipeptide in
l
Gly dipeptide is given in Fig. 5. From this figure it can be concluded
that the amount of the hydrolyzed Ac-L-Met-Gly dipeptide de-
creased in the following order: en > 1,2-pn > ibn > dach. Therefore,
the amount of hydrolyzed Ac-L-Met-Gly dipeptide in the reaction
with {[Pt(en)(H2O)]2(
or one and a half times larger than with {[Pt(ibn)(H2O)]2(
and {[Pt(1,2-pn)(H2O)]2(
-pz)}4+ complex, respectively. However,
during the same time the amount of hydrolyzed Ac-L-Met-Gly
dipeptide in the presence of {[Pt(dach)(H2O)]2(
-pz)}4+ was eight
times smaller than with {[Pt(en)(H2O)]2(
-pz)}4+ complex. Finally,
after 24 h in the presence of the {[Pt(dach)(H2O)]2(
-pz)}4+ com-
plex only 30% of the Met-Gly amide bond hydrolyzed, while with
the {[Pt(en)(H2O)]2(
-pz)}4+ complex more than 80% of this bond
had cleaved. Difference in the hydrolytic abilities of the
investigated binuclear {[Pt(L)(H2O)]2(
-pz)}4+ complexes can be
l
-pz)}4+ complex after 30 min is almost two
l
-pz)}4+
l
{[Pt(L)(Ac-L-Met-Gly-S)](
mined every 30 min during 24 h. During this time the total
amounts of {[Pt(L)(Ac-L-Met-Gly-S)](
-pz)[Pt(L)(H2O)]}4+ complex
l
-pz)[Pt(L)(H2O)]}4+ complex were deter-
l
l
l
and free glycine was always equal to the initial concentration of
Ac-L-Met-Gly dipeptide. The time dependence of the hydrolytic
l
l
l
100
attributed to the steric bulk of their bidentate coordinated diamine
ligand L. In Fig. 6 we compared the catalytic abilities of en, 1,2-pn,
Series1
en
80
60
40
20
0
Series2
1,2-pn
Series3
ibn
Series4
dach
1
0.5
2
3
6
24 h
Fig. 6. Dependence of the amount of hydrolyzed Ac-L-Met-Gly dipeptide on the
Fig. 5. Time dependence of the hydrolytic cleavage of the Met-Gly amide bond in
number of carbon atom in the structural skeleton of ligand L of {[Pt(L)(H2O)]2(l-
the Ac-L-Met-Gly dipeptide with an equimolar amount of {[Pt(L)(H2O)]2(
l
-pz)}4+
-
pz)}4+ complex. The amount of the hydrolyzed dipeptide was determined after 2 h
of reaction.
type complex.

´
47
S. Rajkovic et al. / Polyhedron 65 (2013) 42–47
ibn and dach Pt(II) binuclear complexes in the cleavage of the Met-
Gly amide bond of Ac-L-Met-Gly dipeptide after 2 h of reaction. In
spite of the fact that all diamine ligands L in the investigated
with the nature of the chelating diamine ligand L in {[Pt(L)(H2-
O)]2(
-pz)}4+ complex.
l
{[Pt(L)(H2O)]2(l
-pz)}4+ complexes form five-membered chelate
4. Conclusions
ring, it is obvious that the amount of hydrolyzed peptide was de-
creased by increasing the number of carbon atom in the structural
skeleton of ligand L. Our latest findings that inhibition of the
hydrolytic reaction can be achieved by structural modification of
the catalyst are in accordance with those for different mononuclear
Pd(II) complexes and histidine-containing peptides [15]. Summing
up together our results related to the hydrolytic cleavage of methi-
onine-containing peptides in the presence of different binuclear
In this paper, it was demonstrated that by modification of the
binuclear {[Pt(L)(H2O)]2(l
-pz)}4+ complex by the introduction of
a sterically hindered diamine ligand L, inhibition of the hydrolytic
cleavage of the amide bond involving the carboxylic group of
methionine can be achieved. These results should be taken into
consideration when designing new polynuclear platinum(II) com-
plexes as effective agents in the hydrolysis of methionine-contain-
ing peptides. Studies aimed at investigating these new possible
synthetic metallopeptidases are in progress.
{[Pt(L)(H2O)]2(l
-pz)}4+ complexes with previous results with histi-
dine-containing peptides and different mononuclear [Pd(L)
(H2O)2]2+ complexes (L is bidentate coordinated diamine ligand)
[15], we can conclude that inhibition of the hydrolytic reaction
can be attributed to the steric bulk of the catalyst. These findings
can be explained in terms of two possible limiting mechanisms
for hydrolytic cleavage of the peptide bond promoted by Pd(II)
and Pt(II) complexes [10–12], see Fig 3. First possibility is that
the platinum(II) atom coordinated to the methionine side chain
polarizes the carbonyl group in the scissile peptide bond and acti-
vates its carbon atom toward attack by water molecule from the
solvent (external attack). For the reaction to occur by this mecha-
nism the Pt(II) and carbonyl oxygen atoms should be proximate.
Another possibility is that an aqua ligand from the second Pt(II)
Acknowledgements
This work was funded in part by the Ministry of Education, Sci-
ence and Technological Development of the Republic of Serbia
(Project No. 172036).
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