Kinetics of Photoinduced Electron-Transfer Reactions within Sol-Gel Silica Glass Doped with Zinc Cytochrome c. Study of Electrostatic Effects in Confined Liquids

Abstract

Silica hydrogel (glass) was doped with native (iron-containing) cytochrome c and with its zinc derivative. Ultraviolet−visible, circular dichroism, and resonance Raman spectra of both proteins and the lifetime of the triplet state of the zinc protein show that encapsulation in the sol-gel glass only slightly perturbs the polypeptide backbone and does not detectably perturb the heme group. Because thermal (ground-state) redox reactions of the encapsulated native cytochrome c are very slow, we take advantage of the transparency of the silica to study, by laser flash spectrometry, photoinduced (excited-state) redox reactions of zinc cytochrome c, which occur in milliseconds. The triplet state, 3Zncyt, is oxidatively quenched by [Fe(CN)6]3-, dioxygen, and p-benzoquinone. These reactions are monophasic in bulk solutions but biphasic 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,...