Spectroelectrochemical titrations and cyclic voltammetry of methyl pheophorbide in acid: Possible role of pheophytin enol iminium in the primary process of PS II

In this paper we examine methyl pheophorbide (MP), a model of pheophytin_L, with UV-visible spectroelectrochemistry and cyclic votammetry in aprotic solvent with varying molar equivalents of acid. Our results provide evidence for the formation of the monocation of MP and its electrochemically generated radical. The spectroelectrochemical experiments furnished information about the products at equilibrium at each applied potential. In addition, when only one equivalent of acid was used, an isosbestic point was observed from the spectra taken at different potentials, thus supporting the interconversion of MP monocation and the radical. While the E°′'s for the two redox couples of MP in acid free solution were -0.66 V and 0.88 V vs. the acetonitrile/calomel reference, after one mole of nonaqueous acid was added, the first E°′, shifted from -0.66 to -0.55 V. Protonation of MP, apparently to yield the monocation, facilitates reduction by 0.11 V. It is known that the chlorophyll pair in the excited state effects electron transfer to pheophytin_L which is then followed by electron transfer to a quinone. In contrast, pheophytin_M is apparently not involved in electron transfer. One key difference between these pheophytins is that the former may convert to the iminium form during enolization. In addition to a discussion of the electrochemical results, a hypothesis is advanced for a role of pheophytin enol iminium in the photosynthetic primary process.