Membrane photopotential generation by interfacial differences in the turnover of a photodynamic reaction

The adsorption of a membrane-impermeable photosensitizer to only one membra ne leaflet is found to trigger a localized photodynamic reaction; i.e., the amount of carbonyl cyanide m-chlorophenylhydrazone (CCCP) molecules damage d in the leaflet facing the photosensitizer is roughly identical to the tot al amount of CCCP inactivated. Whereas the latter quantity is assessed from the drop in membrane conductivity G, the former is evaluated from the phot opotential phi that is proportional to the interfacial concentration differ ence of the uncoupler. Localized photodestruction is encountered by CCCP di ffusion to the site of photodamage. A simple model that accounts for both p hotoinhibition and diffusion predicts the dependence of the photopotential on light intensity, buffer capacity, and pH of the medium. It is concluded that only a limited amount of the reactive oxygen species responsible for C CCP photodamage diffuses across the membrane. If the concentration of react ive oxygen species is decreased by addition of NaN3 or by substituting aque ous oxygen for argon, cp is inhibited. If, in contrast, their life time is increased by substitution of H2O for D2O, phi increases.