Retinal photoisomerization dynamics in the primary process of the other natural photosynthetic system, bacteriorhodopsin

摘要

Summary form only given. Bacteriorhodopsin (bR) is a retinal protein membrane present in bR Halobacterium salinarium that carries out its photosynthetic function. Upon absorption of a photon by its retinal chromophore, bR goes through its photocycle. Using picosecond Raman transient spectroscopy, the author was able to show that the first step is indeed a retinal photoisomerization from all-trans to 13-Cis form. The question immediately arises as to why retinal photoisomerization is so fast when placed in the protein. Using optical subpicosecond transient absorption photoacoustic spectroscopy, the author measured the rate of photoisomerization, its quantum yield, and the energy stored in this process for bR and for a number of its relevant mutants and at different pH. It is found that the presence of the negatively charged Aspartate-85 in the retinal cavity near the 13-14 bond is the most effective in the protein catalysis of this process. This led the author to propose that the positive charge stabilization on C/sub 13/ of the retinal in the excited state by Aspartate-85 reduces the barrier to rotation around the C/sub 13/-C/sub 14/ bond and leads to the observed photocatalysis. It is further observed that while the rate of photoisomerization is sensitive to the retinal environment, its quantum yield and the energy stored in this process are not. This has led to the conclusion that the ground state surface has a large barrier to thermal isomerization around C/sub 13/-C/sub l4/ while the excited state surface is barrierless, flat, and symmetric with respect to rotation around the C/sub 13/-C/sub 14/ bond in the perpendicular configuration.