Pigments found in the membranes of certain bacteria are of a chemical nature surprisingly like that of pigments employed in the chemistry of vision in animals. The bacterial pigments clearly have a much different function than do the corresponding pigments found in the human eye. However, some of the fundemental reactions in both systems are very similar in nature. This project in the Organic and Macromolecular Chemistry Program is designed to investigate the basic chemistry of the biological function of these bacterial pigments. At least three membrane proteins possessing retinal as the chromophore have been isolated from the ancient bacterium Halobacterium halobium: bacteriorhodopsin (bR) which pumps protons across the membrane, halorhodopsin (hR) which translocates chloride ions across the membrane, and sensory rhodopsin (sR) which is responsible for the phototactic behavior of H. halobium. Because the bacteria are grown in concentrated sodium chloride solution without much fear of contamination, and because the above-mentioned pigments are among the simplest of membrane proteins, they are ideally suited for studying membrane related phenomena. The pigments are thus attracting wide interest from the various scientific disciplines. The objective is to investigate the mechanism of the pigment functions on a molecular structural basis, the approach being studies on pigment analogs that have been incorporated with various synthetic retinals and/or labeled amino acids. The main targets are: (i) determination of the tertiary structure of bR by photoaffinity labeling and other means; (ii) delineation of structural factors of the chromophoric moiety required for proton pumping in bR; and (iii) clarification of the sequence of events occurring within bR leading to the transfer of protons. Related studies with sR and hR will also be performed. Fourier transform infrared (FTIR) difference spectroscopy as well as other spectroscopic techniques will be used. Such studies will contribute to our understanding of membrane transport, a central but little understood theme.
