Photoreduction of Methylviologen with Ascorbate Incorporated in Inner Solution of Lipid Vesicles

Abstract

Methylviologen (MV²⁺) was \ educed with ascorbate (Asc^-) in inner solution of egg-phosphatidylcholine vesicle in the course of reaction photosensitized by Acridine Orange (AOH⁺). Electron was transported against thermodynamical potential gradient. Accumulation of MV^+. slowed down as the reaction proceeded and the concentration of MV^+. reached a constant value after an hour (Fig.1). Accumulated MV^+. was reoxidized slowly in the dark. This reoxidation must be the effect of small residual amount of oxygen which cannot be purged by our deaeration procedure and/or some other oxidizing agents.
In homogeneous aqueous solution containing Asc^-, MV²⁺ and AOH⁺, accumulation of MV^+. was not observed. In the homogeneous system, electron is transferred rapidly from MV^+. to the oxidized dye (AOH^2.+), and from MVt to the oxidized form of. Asc^- (sAsc^-). But, in the vesicle solution, these backward electron-transfer reactions were intercepted by adsorption of the oxidized dye onto the vesicle surface and by immediate reduction of the oxidized dye by ascorbate in the vesicle interior. Ascorbate radical (sAsc^-.) generated by one-electron process disproportionates into ascorbate and dehydroascorbic acid, the latter can hardly be reduced by one-electron donor such as MV^+..
Continuous illumination was performed under vari ous conditions. A reaction under a specific condition may be characterized by two parameters, initial rate of MV^+. formation (ν₀) and MV^+. concentration at the steady state (cs). Dependence of these param eters on the reaction conditions is shown in Figs.4, 7 and 8.
These results are explained by the kinetic model for the steady state shown in Fig.10. On the basis of this model, one obtains eqs. (8) and (14). Partitioning of the charge-separation products between bulk and membraneous phase is essential for efficient accumulation of MV^+., because it affects the value of k′s in these equations.
In conclusion, although photoreduction of MV^+. in the vesicle solution is irreversible, the existence of bilayers is essential for effective charge separation.