CHLOROPHYLL PHOTOCHEMISTRY IN LIPOSOMES: TRIPLET STATE QUENCHING AND ELECTRON TRANSFER TO QUINONE.

Abstract

Liposomes incorporating chlorophyll (Chl) have been used as a model system to study various aspects of photosynthesis (such as Chl photooxidation and acceptor reduction). Laser flash photolysis studies of this system have demonstrated that the Chl triplet state (Chl(t)) can transfer an electron to acceptors such as quinones, resulting in the formation of the Chl cation radical (Chl⁺.) and the semiquinone anion radical (Q¯.). Quenching of Chl(t) by quinones in liposomes is diffusion-controlled. The quenching rate is dependent upon bilayer viscosity. Chl(t) lifetimes in the absence of quinones also reflect bilayer viscosity. Radical decay occurs by reverse electron transfer. Although the decay is non-exponential, the decay rate is independent of laser intensity. This is presumably because radical pairs once formed do not become independent of one another and back react in a manner which can be likened to geminate recombination. The non-exponentiality is due to electron exchange between quinone molecules and the heterogeneity in the distribution of molecules among the vesicles. This electron exchange is also manifested in the radical formation process. At high quinone concentration the radical yield increases with quinone concentration in non-linear fashion with respect to the amount of triplet quenched. This positive cooperative effect is interpreted in terms of high quinone concentrations increasing the efficiency of radical production by providing a pathway (via electron hopping) for removal of the electron from the site of initial electron transfer. When ubiquinone is used, only a single fast decay is observed. However, when quinones which can partition between the aqueous and lipid phases are used, radical decay occurs via a fast and a slow process. This is interpreted in terms of electron transfers from Q¯. within the bilayer to Q at the bilayer-water interface which results in a stabilization of the electron transfer products and a slowly-decaying radical. The rate of this slow decay process is also quinone concentration dependent, which is a consequence of a facilitation of electron return to Chl⁺. by Q molecules within the bilayer via an electron hopping mechanism. That such a mechanism is, in fact, operative in radical production is shown also by the observation of electron transfer from UQ¯. to BQ molecules.