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, to the pore walls, which are negatively charged at pH 7.0, affects the kinetics in the doped glass. 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.

Modulation of the Silica Sol–Gel Composition for the Promotion of Direct Electron Transfer to Encapsulated Cytochrome c

“…It is known that ormosils can provide silica structures with different degrees of hydrophobicity and the associated change in the pore polarity may favor the electron transfer with the electrode [52][53][54] . So, at this point, the chemical modification of the silica is studied in order to obtain structures able to modulate the electrostatic interactions between the silica walls and cyt c. was assessed by contact angle measurement (see Table S1 in Supporting Information).…”

“…Indeed, the silica hydrogel is often considered as a two-phase system consisting in a porous solid and a trapped aqueous phase 54 . We have found that the results obtained This behavior may be related with the structural characteristics of ormosils.…”

Dynamics of Proteins Encapsulated in Silica Sol−Gel Glasses Studied with IR Vibrational Echo Spectroscopy

“…It has been noted that the surface of the protein could also interact with or adsorb to the hydrophilic silica pore walls through electrostatic or hydrogen bonding interactions. 3,[23][24][25][26][27][28] Fluorescence anisotropy studies have demonstrated that proteins in sol-gel pores experience dramatically hindered rates of rotation due to protein adsorption to the pore walls. 23,29,30 Alternatively, the pore dimensions could influence the protein structural stability by affecting the dynamics of the surrounding solvent, which is intimately coupled to the dynamics of the protein.…”

Photochemical Coenzyme Regeneration in an Enzymatically Active Optical Material

“…Charged molecules can be partially excluded from the pore interior at low ionic strengths such as those used in these experiments. 33 The quencher plots were constructed assuming that the concentration of enzyme cofactor in the pores was equal to the concentration in bulk. It is likely that the negatively charged enzyme cofactor may not fully penetrate the sol-gel pores resulting in a lower concentration than assumed.…”