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Soft- and hard-modeling approaches for the determination of stability constants of metal-peptide systems by voltammetry.

The Zn(2+)-glutathione system is studied as a model for metal-peptide systems where some critical factors must be considered when using voltammetric techniques for the determination of stability constants. These factors are the presence of side reactions (in this case, both the protonation of glutathione and the hydrolysis of Zn(2+)), the association-dissociation rates of the complexes compared with the time scales of the measurements (which makes the complexes electrochemically labile or inert), and the electron transfer kinetics on the electrode surface (which makes the metal ion reduction reversible or irreversible). For the study of these factors, three data treatment approaches have been applied: (i) the electrochemical hard-modeling approach (modelization of both chemical equilibrium and electrochemical processes), (ii) a chemical hard-modeling approach (modelization of chemical equilibria only, based on the least-squares curve-fitting program SQUAD), and (iii) a previously developed model-free soft-modeling approach based on multivariate curve resolution with a constrained alternating least-squares optimization. By analyzing differential pulse polarographic data obtained under different experimental conditions, the influence of the mentioned factors on every approach is discussed and, if possible, the corresponding stability constants are computed. The results of this study showed the potential usefulness of voltammetry in combination with hard- and soft-modeling data analysis for the study of peptide complexation equilibria of metal ions such as Zn which have neither relevant spectroscopic properties nor proper isotopes for NMR measurements.