Abstract

Bacteriorhodopsin (BR) is a membrane protein which converts light energy to chemical energy by pumping protons unidirectionally across the membrane. The driving force for pumping protons is derived from the photoisomerization of all-trans retinal to 13-cis retinal, the quantum yield of which is reported to be approximately 0.6¹. The quantum yield appears to be independent of temperature and is minimally affected by mutations to numerous neighboring residues in the protein. Previous studies have suggested that photoexcited BR relaxes in 0.5 ps to J which in turn relaxes in 3 ps to K². The long-lived intermediate K has been identified as the 13-cis isomer of retinal³. The relative constancy of the quantum yield for the formation of K is not well understood, nor are the differences between J and K. To investigate the primary photoprocesses of BR as well as the role of the protein in mediating the photoisomerization of retinal, we measured time-resolved absorbance spectra over a broad spectral range with high sensitivity.

© 1996 Optical Society of America