Potentiometric and Thermodynamic Studies of Some Metal–Cysteine Complexes
Faezeh Sheikhani and Alireza Farrokhi
Chemistry Department, University of Birjand, P.O. Box 414, Birjand, Iran
(Received: January 17, 2017; Accepted: September 19, 2017; Published Online: November 29, 2017; DOI: 10.1002/jccs.201700022)
A new and simple potentiometric method is developed to determine the stability constants of cyste-
ine complexes with Mn(II), Cu(II), Zn(II), Cd(II), and Ce(III) metal ions using a modiﬁed Bjerrum
method. Potentiometric titrations were performed in water and water/dioxane mixtures at ﬁve dif-
ferent temperatures. The least and highest stable complexes were those of Cu(II) and Zn(II),
respectively. Furthermore, by conducting the experiments at temperatures of 15, 20, 25, 35, and 45
C, the thermodynamic parameters ΔH
, and ΔG
for the pertinent complexes were calcu-
lated. The results show that the ΔH
values of the complexes are positive except for Zn(II). Nega-
tive values of ΔG
are evidence for the spontaneity of the reactions.
Keywords: Stability constant; Amino acid complex; Potentiometric titration; Thermodynamic
Metal ions play a vital role on the structure and
function of many biomacromolecules.
Toxic metal ions
could speciﬁcally bind to biologically important mole-
cules during their uptake, transportation, and storage.
Interaction between metal ions and amino acids that
mimics the metal–enzyme mechanism has attracted the
attention of many biochemists. Amino acids are biden-
tate ligands that chelate with the majority of transition
metals through the amino nitrogen atom and the car-
boxyl group, yielding very stable complexes.
transition metal–amino acid complexes are biologically
active, increasing the diffusivity of the metal complexes
and consequently their biological activity inside the cell.
Some show antiviral activities against the tobacco
mosaic virus in host plants.
Others have been tested
for antitumor activity
and as transfer agents of biologi-
cal alkylating compounds.
Cysteine is one of the two
sulfur-containing amino acids and plays a key role in
stabilizing extracellular proteins. The explanation of
these phenomena in biological systems requires the
determination of the stability constants as a measure of
how well the complexes of the amino acids with various
metal ions form in a medium similar to those of biologi-
was the ﬁrst to measure the stability of
metal amines using the concentration of metal free
ligand and the total ligand concentration. Later, Calvin
modiﬁed his method to make it more
practical and applicable to any metal complex chelating
system. Complexes of metal ions with amino acids
in aqueous and non-aqueous solutions have been inves-
tigated extensively by numerous workers using
Their results show that the estima-
tion of the equilibrium concentration of metal(II) com-
plexes as a function of pH provides a useful picture of
the metal ion binding in biological systems. The behav-
ior of metal–cysteine complexes at different tempera-
tures was probed by Masoud et al.
They came to the
conclusion that in all complexes the log k
higher than log k
values. The thermodynamic parame-
ters for the formation of glycine with metal ions were
investigated by Casale
explaining the salt effects.
The formation of binary and ternary complexes of
Zn(II) with cysteine was studied by Shoukry et al.
found that sulfhydryl-containing amino acids ligate
partly like mercaptoethylamine and partly like mercap-
topropionic acid. The stability of Cu(II) complexes with
several amino acids, which are useful antibacterial
agents, were investigated in aqueous solutions by Bastug
They showed that the –COO
group of amino
*Corresponding author. Email: firstname.lastname@example.org
J. Chin. Chem. Soc. 2018, 65, 217–224
© 2017 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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