The potential span of photoredox reactions of porphyrins and chlorophyll at the lipid bilayer-water interface

Abstract

Lipid bilayers containing chlorophyll (Chl) or magnesium octaethylporphyrin (MgOEP) and separating solutions containing varying amounts of differing acceptors are illuminated by a dye laser pulse (FWHM 0.3 microseconds) at 590 mm. Interfacial charge transfer is measured at the first current peak in a voltage clamp circuit. The constants describing the hyperbolic saturations of the charge transferred by differing acceptors are only weakly related to the redox potential of the acceptors. An assymetric molecule, anthraquinone-2-sulfonate, is over 20 times as effective in accepting the electron as is the symmetrical anthraquinone-2,6-disulfonate. In contrast to this variable effectiveness, the maximum amount of charge transferred as a function of acceptor redox potential is constant up to a cut-off value: -0.6 V (vs. standard hydrogen electrode) for MgOEP and -0.5 V for Chl. The reversible redox potential of MgOEP in the bilayer was determined by following both the decrease in photoactivity and the transmembrane potential as a function of aqueous redox potential. It is +0.77 V for MgOEP and approximately 0.7 V for Chl (limited by stability). Thus, a total of 1.4 V of reversible redox potential (free energy) is obtained from 1.8 eV (internal energy) of the triplet excited state of MgOEP.