KINETICS OF PHOTOINDUCED ELECTRON-TRANSFER REACTIONS WITHIN SOL-GEL SILICA GLASS DOPED WITH ZINC CYTOCHROME-C - STUDY OF ELECTROSTATIC EFFECTS IN CONFINED LIQUIDS

Silica hydrogel (glass) was doped with native (iron-containing) cytoch rome c and with its zinc derivative. Ultraviolet-visible, circular dic hroism, and resonance Raman spectra of both proteins and the lifetime of the triplet stale of the zinc protein show that encapsulation in th e sol-gel glass only slightly perturbs the polypeptide backbone and do es not detectably perturb the heme group. Because thermal (ground-stat e) redox reactions of the encapsulated native cytochrome c are Very sl ow, we take advantage of the transparency of the silica to study, by l aser flash spectrometry, photoinduced (excited-state) redox reactions of zinc cytochrome c, which occur in milliseconds. The triplet state, (3)Zncyt, is oxidatively quenched by [Fe(CN)(6)](3-), dioxygen, and p- benzoquinone. These reactions are monophasic in bulk solutions but bip hasic in solutions confined in glass. Changes in ionic strength and pH differently affect the kinetics in these two environments. Adsorption of cytochrome c, which is positively charged, to the pore walls, whic h are negatively charged at pH 7.0, affects the kinetics in the doped lass. Exclusion of the [Fe(CN)(6)](3-) anions from the glass interior also affects the kinetics. Even at equilibrium the anion concentration is lower inside the glass than in the external solution. This exclusi on can be lessened or eliminated by raising ionic strength and lowerin g the pH value. The electroneutral quenchers are not excluded from the glass, Diffusion of all three quenchers is slower in the confined sol ution than in the bulk solution, as expected. The smaller the molecule , the lesser this hindrance by the glass matrix. In light of these fin dings, the assumption that porosity of sol-gel glasses ensures uniform penetration of relatively small. molecules into the pores must be tak en skeptically and tested for each solute (Or analyte) of interest, es pecially for the charged ones. These considerations are important in t he design of sensors.