Insights into excited-state and isomerization dynamics of bacteiorhodospin from ultrafast transient UV absorption

Abstract

A visible-pump/UV-probe transient absorption is used to characterize the ultrafast dynamics of bacteriorhodopsin with 80-fs time resolution. We identify three spectral components in the 265- to 310-nm region, related to the all-trans retinal, tryptophan (Trp)-86 and the isomerized photoproduct, allowing us to map the dynamics from reactants to products, along with the response of Trp amino acids. The signal of the photoproduct appears with a time delay of ≈250 fs and is characterized by a steep rise (≈150 fs), followed by additional rise and decay components, with time scales characteristic of the J intermediate. The delayed onset and the steep rise point to an impulsive formation of a transition state on the way to isomerization. We argue that this impulsive formation results from a splitting of a wave packet of torsional modes on the potential surface at the branching between the all-trans and the cis forms. Parallel to these dynamics, the signal caused by Trp response rises in ≈200 fs, because of the translocation of charge along the conjugate chain, and possible mechanisms are presented, which trigger isomerization. © 2006 by The National Academy of Sciences of the USA.