Ultrafast photochemistry of light-adapted and dark-adapted bacteriorhodopsin: effects of the initial retinal configuration.

Femtosecond spectroscopy is used to compare photochemical dynamics in light-adapted and dark-adapted bacteriorhodopsin (BR). The retinal prosthetic group is initially all-trans in the former, while it is nearly a 1:1 mixture with 13-cis in the latter. Comparing photochemistry in both serves to assess how the initial retinal configuration influences internal conversion and photoisomerization dynamics. Contrary to an earlier study, our results show that after excitation of the 13-cis form it crosses back to the ground state much more rapidly than the biologically active all-trans reactant. A similar result was recently obtained for another microbial retinal protein, Anabaena Sensory Rhodospin (ASR), which can be toggled by light between two analogous ground state configurations. Together, these studies suggest that this disparity in rates may be a general trend in the photochemistry of microbial retinal proteins. This may bear as well on the well-known enhancement in photoisomerization rates going from microbial retinal proteins to the visual pigments, as the latter also start the course of photoreception in a cis retinal configuration, in that case 11-cis. In lieu of indications for pretwisting or straining of the 13-cis retinal forms of BR and ASR, akin to those reported for rhodopsin, current results challenge many of the mechanisms held responsible for the ballistic photochemical dynamics observed in visual pigment.