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) cytochrome c and with its zinc derivative. Ultraviolet-visible, circular dichroism, and resonance Raman spectra of both proteins and the lifetime of the triplet stale 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, (3)Zncyt, 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, to the pore walls, which 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 exclusion can be lessened or eliminated by raising ionic strength and lowering the pH value. The electroneutral quenchers are not excluded from the glass, Diffusion of all three quenchers is slower in the confined solution than in the bulk solution, as expected. The smaller the molecule, the lesser this hindrance by the glass matrix. In light of these findings, the assumption that porosity of sol-gel glasses ensures uniform penetration of relatively small. molecules into the pores must be taken skeptically and tested for each solute (Or analyte) of interest, especially for the charged ones. These considerations are important in the design of sensors.