Chlorophyll Photosensitized Vectorial Electron Transport Across Phospholipid Vesicle Bilayers: Kinetics and Mechanism*

Abstract

Abstract— The characterization and kinetic analysis by laser Rash photolysis of an improved model system for observing chlorophyll a photosensitized electron transfer across a lipid bilayer membrane is described. In this system, the electron acceptor is a water‐soluble naphthoquinone, S‐(2‐methyl‐l,4‐naphthoquinonyl‐3)‐glutathione (MGNQ) which is dissolved in the inner aqueous compartments of phospholipid bilayer vesicles, and the electron donor is glutathione (GSH) which is dissolved in the outer aqueous phase. Chlorophyll (Chl) is dissolved in the membrane. Oxidative quenching of the triplet state of Chl by the quinone at the inner surface of the vesicle produces the Chl+ and MGNQ‐ radicals. Chi+ is reduced by GSH at the outer surface of the vesicle (k= 2.6 × 106M‐1 s‐1) in competition with the recombination between Chl+. and MGNO‐ (k= 2.5 × 103 S‐1). It is shown that a kinetic mechanism involving competition between recombination, electron transfer across the bilayer, and reduction by donor at the opposite surface can quantitatively account for the decay of Chl+. Electron transport across the bilayer is postulated to occur by a two‐step mechanism involving electron exchange between Chl and Chl+ within the lipid monolayer (k= 3.2 × 106 M‐1 s‐1) and across the bilayer. The rate constant for the latter exchange process approaches 104 s‐1 as the concentration of Chl in the bilayer increases. Under appropriate conditions, approximately 20% of all photons absorbed by the vesicle system result in electron transfer across the mcmbrane from GSH to MGNQ.

Photoinduced electron transfer across membranes

“…The presence of [3-carotene and Chl dimers in the membrane was assumed to be necessary for transmembrane electron transfer. However in later works [43,44,48,[51][52][53] it was suggested that in fact the presence of neither ~-carotene nor Chl dimers is crucial for PET. It is also worth noting that the high quantum yield of electron phototransfer, reported by Mangel [41] also has not yet been confirmed by other investigations.…”

Long Range Electron Tunneling in Biological and Model Systems

“…Some other models of metalloporphyrins photosensitized electron transfer across the membranes of vesicles such as transmembrane electron tunneling have been given (Ford and Tollin, 1983;Yusupov et al, 1985). There is not enough data to estimate the role of any mechanism of transmembrane electron transfer in vesicles.…”

“…Electron transfer across the membranes of vesicles by the long range electron tunneling mechanism has been postulated in many papers (Tunuli and Fendler, 1981;Lyimar et al, 1982;Kuznetsov et Some other models of metalloporphyrins photosensitized electron transfer across the membranes of vesicles such as transmembrane electron tunneling have been given (Ford and Tollin, 1983;Yusupov et al, 1985). There is not enough data to estimate the role of any mechanism of transmembrane electron transfer in vesicles.…”

Action of Heavy Metals on Hill Activity and O2 Evolution in Anacystis nidulans

“…GSH is an electron donor (10). Its biological importance is presumed to be due to its ability to bind with metals (24).…”