ACID–BASE PROPERTIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 69

stoichiometric molar ratio of 1 : 2 ([Zn(II)] : [G]). The spectral measurements
provided evidence of the formation of dinuclear complexes. The species,
[ZnG], [ZnG 2 H 2 ] 2− , and [ZnG 2 H] 3− , contributed to the total Zn at pH 7.4. In
the proposed structure of [ZnG], [ZnG 2 H 2 ] 2− , Zn(II) coordinated to the thio-
late of cysteine in the HG 2− ligand. Recently, Zn(II) complexation with GSSG
and its nine analogues with C-terminal modifications has been studied using
potentiometric and NMR spectroscopic techniques [120]. Four different
complex species, three monomeric (ZnHL − , ZnL 2− , and ZnH -1 L 3− ) and a bime-
tallic (Zn 2 L), were shown at different pH values (Fig. 2.6b). The major species
present at the physiological pH is ZnL Zn GSSG
2

− −

([ ] ) (Fig. 2.6b). Also, the
GlyH 1 proton was sensitive to the formation of the ZnL 2− complex. The role
of the Glu residue in the complex formation was observed when comparing
the chemical shift profiles with the speciation of the complexes (Fig. 2.6b).
The removal equivalence of β-Glu protons in the complex formation increased
the signal separation of β-Cys protons. In summary, binding of Zn 2+ with thiol
ligands may be important in intracellular Zn(II) transport and storage.
In summary, protonation of amino acid side chains of proteins vary with
pH, temperature, ionic strength, and polarity of the system. Understanding the
influence of these variables on equilibrium provides distribution of protonated/
unprotonated species under biological environment, which can then be used
to evaluate species-specific reaction rates of redox reactions in order to learn
the mechanism of damage to proteins caused by reactive intermediates (Chap-
ters 3-6). Furthermore, information on metal binding to bioligands may give
an insight on the role of metals in generating reactive species and in affecting
rates of redox reactions.

−

2

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