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H.H. Hammud et al. / Journal of Molecular Structure 1087 (2015) 33–40
proved to be an important group of compounds with anticancer
activity. Among the different nucleobases, adenine (6-aminopu-
rine) shows the widest range of binding possibilities because it
exhibits at least five donor sites (N9, N7, N3, N1 and N6), and a
great variety of complexes with different metal-ion binding pat-
terns have been reported [4]. It is surprising that exocyclic amino
groups of the nucleobases are not metal binding sites due to delo-
calization of the lone electron pair into the heterocyclic ring. Coor-
dination is only possible when the amino group is deprotonated or
adopts a rare imino tautomer structure. By the judicious choice of
the type of substituents that can be attached on N1, N3, N6, N7 and
N9 centers, the biological function displayed by adenine could be
tuned. This heterocyclic adenine also has an ability to coordinate
a variety of metal ions, that could be used for the stabilization of
superstructures as well as to support metal-aided catalytic trans-
formations [5]. Another important nucleobase is thymine, which
is pyrimidine like and is unique among the nucleic acid constitu-
ents because it does not have nitrogen lone pairs available for
metal complexation. Thus it can act as a bidentate ligand through
O4 and O2 [6].
recorded in KBr pellet, on a Nicolet iS 10 FT/IR spectrophotometer.
1H NMR and 13C NMR spectra were recorded on a Bruker 500 MHz
NMR spectrometer in CDCl3 and D2O, using TMS and DSS as refer-
ences; chemical shifts are reported in ppm, and signals are
expressed as s (singlet), d (doublet), t (triplet), q (quartet) and m
(multiplet).
In potentiometric titration, deionized water was boiled for two
hours to minimize atmospheric carbon dioxide contamination.
There after it was cooled to ambient room temperature, 25 °C, in
a closed vessel leaving no headspace. All pH measurements were
made using a pH-meter Eutech pH 700 in well-stirred solutions.
The temperature of the systems was maintained at 25 °C 0.1 °C
during titration, with a water bath (VWR, model 12101-10) and a
glass titration cell with a double jacket. Stock solutions of metal
ions, AA, TA and KCl for pH-metric titrations were prepared in
deionized water. Solutions of Zn(II), Co(II), Mn(II), Ni(II), and Cu(II)
were prepared from their chlorides and Cd (II) and Pb(II) from their
nitrates. NaOH solution was prepared by dissolving Analar pellets
in water, and the solution was standardized with a standard potas-
sium hydrogen phthalate solution. A total concentration of 0.3 M
KCl was used as a supporting electrolyte.
Due to biological importance of adenine and thymine, consider-
able research work has been done on the study of their metal com-
plexes in solution. The interest in such complexes continues to
increase due to the possibility of their use as models to explain
some intricate reaction in biological systems such as anticancer,
antiviral and antimalarial [7–11]. Literature survey shows that
Hamada et al. reported the potentiometric study of some divalent
metal complexes of adenine in aqueous phase at 25 °C and 1 M
NaNO3 [12]. Ammar et al. investigated the formation constants of
mixed ligand complexes Cu(II)-adenine-amino acids at 25 °C and
ionic strength (I) 0.1 M NaNO3 by pH-metric titration [13]. In
another work, the authors studied potentiometrically the forma-
tion equilibria and stability constants of binary and ternary com-
plexes of Ni(II) involving adenine and various biologically
relevant ligands [14]. Shukla et al. also used potentiometric tech-
niques in order to determine the formation constants and com-
plexation equilibria at 30 °C and at I = 0.1 M NaNO3 for
quaternary metal complexes of some divalent metal ions contain-
ing thymine [15]. The formation constants of the ternary (1:1:1)
and quaternary (1:1:1:1) complexes of some divalent metals with
Calibration of the glass electrode
Before starting pH measurements of each set, the pH meter
scale was calibrated using two standard aqueous buffer solutions.
The term pH has significance only in aqueous media. The glass
electrode potential in an aqueous solution differs from that in a
solution of mixed solvents, and a liquid-junction potential of an
uncertain value can affect the results. To overcome this difficulty,
it was necessary to calibrate the glass electrode in different solvent
mixtures.
The pH-meter readings in ethanol–water media were corrected
according to the Van Uitert and Hass relation for dioxane–water
mixtures later validated by Bates et al. [21] for use in alcohol–
water system. The pH-meter reading B in ethanol + water media
was converted into [H+] using Uitert–Hass equation:
ꢁ log½Hþꢂ ¼ B þ log UH
where log UH ¼ log UꢃH þ log cꢄ
glutamic acid/L-cysteine as primary ligand and thymine as second-
ary ligand were determined potentiometrically in aqueous med-
ium by Krishna et al. [16]. Concerning ligand synthesis, literature
reveals that carboxylic acid derivatives of adenine and thymine
had been prepared via N9 and N1 alkylation, respectively using
ethylacrylate followed by acid hydrolysis [17–19] or by reaction
with halogenated acetic acid [20].
However, to the best of our knowledge, a detailed study of com-
plexation of nucleobase-carboxylic acid derivatives with various
metal ions and the determination of their stability constants is still
lacking. The coordinating properties of ligands of this type are lar-
gely unknown at the present time. In this paper the binding ability
of functionalized 3-(thymine-1-yl)propionic acid and 3-(adenine-
9-yl)propionic acid towards some divalent heavy metals, Cd, Co,
Cu, Pb, Ni, Mn and Zn, was studied. The formation constants of
the metal complexes have been determined potentiometrically
adopting the modified Irving and Rossotti technique.
c
is the activity coefficient of the hydrogen ions in the solvent mix-
ture under consideration at the same temperature and ionic
strength, and UꢃH is a correction factor at zero ionic strength, which
depends only on the solvent composition.
In this work, the values of B were recorded in various solvent
mixtures at different temperatures containing known concentra-
tion of hydrochloric acid (1.00 ꢅ 10ꢁ3 M) and sufficient potassium
chloride to give a constant ionic strength of 0.3 M. The difference
between the logarithm of known hydrogen ion concentrations
and the corresponding values of B was used to calculate values of
the correction term log UH ¼ logðUꢃHcꢄÞ [21].
Synthesis of ligands
The ligands were synthesized according to the reaction in
Scheme 1, shown below:
Experimental part
3-(6-Aminopurine-9-yl)-propionic acid ethylester, 1(a)
Materials and methods
To a suspension of adenine (11.0 g, 81.4 mmol) in absolute eth-
anol (280 ml) and dry benzene (35 ml), sodium metal (120 mg,
5.2 mmol, 0.06 eq) was added carefully at room temperature. Upon
the disappearance of sodium, ethyl acrylate (26 ml, 244 mmol,
3 eq) was added dropwise. The resulting mixture was refluxed
overnight, and then cooled to room temperature. The solvent was
All chemicals and reagents were of analytical grade, purchased
from commercial sources. The progress of reactions was monitored
by TLC using aluminum silica gel plates 60 F254. Melting points
were measured with a Gallenkamp apparatus. IR spectra were