Interfacial photochemistry of retinal proteins

Retinal proteins are membrane-bound protein pigments that contain vitamin A aldehyde (retinal) as the chromophore. They include the visual pigment rhodopsin and four additional ones in the plasma membrane of Halobacterium salinarium (formerly Halobacterium halobium ). These proteins maintain a ®xed and asymmetric orientation in the membranes, and respond to a light stimulus by generating vectorial charge movement, which can be detected as an electric potential across the membrane or an electric current through the membrane. These phenomena are collectively called the photoelectric e€ects, which defy a rigorous quantitative treatment by means of either conventional (solution phase) photochemistry or conventional electrophysiology. As an alternative to the mainstream approach, we utilize the analytic tools of electrochemical surface science and electrophysiology to analyze two molecular models of light-induced charge separation and recombination. Being tutorial in nature, this article demands no prior knowledge about the subject. A parsimonious equivalent circuit model is developed. Data obtained from reconstituted bacteriorhodopsin membranes are used to validate the theoretical model and the analytical approach. Data generated and used by critics to refute our approach is shown to actually support it. The present analysis is suciently general to be applicable to other pigment-containing membranes, such as the visual photoreceptor membrane and the chlorophyll-based photosynthetic membranes. It provides a coherent description of a wide range of light-induced phenomena associated with various pigment-containing membranes. In contrast, the mainstream approach has been plagued with self-contradictions and paradoxes. Last, but not least, the alternative bioelectrochemical approach also exhibits a predictive power that has hitherto been generally lacking. Comparison of the photoelectric e€ects is made with regard to bacteriorhodopsin, rhodopsin, and the chlorophyll-based photosynthetic apparatus Ð in the spirit of reverse engineering (biomimetic science). The technological applications of bacteriorhodopsin as an advanced material for the 0079-6816/99/$ see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S0079 -6816 (99)00014 -3 Progress in Surface Science 62 (1999) 1±237 www.elsevier.com/locate/progsurf p Dedicated to the memory of the late Professor Alexander Mauro of The Rockefeller University. E-mail address: [email protected] (F.T. Hong) construction of molecular devices and the implication of the photoelectric behavior of bacteriorhodopsin for solar energy conversion are also discussed. # 1999 Published by Elsevier Science Ltd. All rights reserved.