Voltanunetric studies of diffusional and migrational transport of ferrocene derivative of tripeptide glutathione

The transport of mono- and divalent anions of S-ferrocenylmethyl-L-glutathione (FcCH(2)SG) was investigated voltammetrically at a microelectrode in solutions of low ionic strength. The electrooxidation reactions of the ferrocenyl group attached to biologically active glutathione neutralized with a strong base in two consecutive steps can be represented as FcCHSG- -> Fc(+)CH(2)SG(-) + e and FcCH2SG(2-) -> Fc(+)CH(2)SG(2-) + e for the mono- and divalent anions of FcCH(2)SG, respectively. The limiting currents due to these electrode processes were investigated under the conditions of varying content of supporting ions. The results obtained for the electrooxidation of the monovalent anion of FcCH(2)SG deviate significantly from the theoretical predictions derived for the charge cancellation electrode processes (i.e., processes producing uncharged species upon electron transfer). The differences observed are attributed to the specific migrational behavior of the generated dipole-like product which formally bears no net charge but in fact contains two oppositely charged moieties within a molecule. To interpret the data obtained for the divalent anion of FcCH(2)SG, a recent model of the theory of migrational voltammetry has been adapted and extended. The agreement between the experimental data and the theory is obtained only for the ratio of diffusion coefficients of the electrode process product (Fc(+)CH(2)SG(2-)) and the substrate (FcCH(2)SG(2-)) smaller than 1. This leads to a conclusion that upon oxidation this molecule undergoes a conformation change and winds up because of the formation of a coordination bond. The conclusion is supported by molecular-mechanics calculations. The presented methodology allows one to study quantitatively the changes in the concentration distribution of biologically active molecules driven by migration and diffusion and to diagnose their possible structural changes upon reduction/oxidation. Both factors are essential for the proper understanding of the functionality of biologically active systems.