VOLTAMMETRY OF RETINAL IN PHOSPHOLIPID MONOLAYERS ADSORBED ON MERCURY

The redox and proton-transferring behaviour of retinal in biological membranes is of fundamental importance in physiological processes. The phospholipid-coated mercury electrode is an ideal membrane model with which to investigate these problems. All-trans, 13-cis and 9-cis retinal were incorporated into monolayers of dioleoyl phosphatidylcholine (DOPC), dimyristoyl phosphatidylcholine (DMPC), egg phosphatidylcholine (egg PC), phosphatidylserine (PS) and mixed monolayers of DOPC and egg phosphatidylethanolamine (PE) which were adsorbed onto a mercury electrode. The electrochemical technique used to measure charge transfer was cyclic voltammetry. Retinal is irreversibly reduced in phospholipid monolayers, in contrast to retinol and retinoic acid which are electro-inactive. Above solution pH 5, retinal forms an association with PS and PE in mixed retinal + PS and retinal + (4DOPC + PE) monolayers respectively. This association is electrochemically reduced at more positive potentials than the retinal in DOPC. The reduction of this association becomes more significant with increasing solution pH, and in solution pH > 9 the reduction is successive on each voltage sweep until all the retinal has formed the product. Experimental evidence indicates that retinal reduction in phospholipid layers is a two-electron process and the rate-controlling step is the second electron transfer which is preceded by protonation. The retinal + PS and retinal + PE associations are a]ready partly protonated in solutions of pH < 8. In monolayers of DOPC, proton transfer becomes limiting at high scan rates and the limiting effect is alleviated by reducing monolayer coverage which renders the monolayer more permeable to proton-transferring species.