Comparative Femtosecond Spectroscopy of Primary Photoreactions of Exiguobacterium sibiricum Rhodopsin and Halobacterium salinarum Bacteriorhodopsin

The primary stages of the Exiguobacterium sibiricum rhodopsin (ESR) photocycle were investigated by femtosecond absorption laser spectroscopy in the spectral range of 400-900 nm with a time resolution of 25 fs. The dynamics of the ESR photoreaction were compared with the reactions of bacterio- rhodopsin (bR) in purple membranes (bR(PM)) and in recombinant form (bR(rec)). The primary intermediates of the ESR photocycle were similar to intermediates I, J, and K in bacteriorhodopsin photoconversion. The CONTIN program was applied to analyze the characteristic times of the observed processes and to clarify the reaction scheme. A similar photoreaction pattern was observed for all studied retinal proteins, including two consecutive dynamic Stokes shift phases lasting similar to 0.05 and similar to 0.15 ps. The excited state decays through a femtosecond reactive pathway, leading to retinal isomerization and formation of product J, and a picosecond nonreactive pathway that leads only to the initial state. Retinal photoisomerization in ESR takes 0.69 ps, compared with 0.48 ps in bR(PM) and 0.74 ps in bR(rec). The nonreactive excited state decay takes 5 ps in ESR and similar to 3 ps in bR. We discuss the similarity of the primary reactions of ESR and other retinal proteins.

Automated summary

The primary stages of the Exiguobacterium sibiricum rhodopsin (ESR) photocycle were investigated using femtosecond absorption laser spectroscopy. The dynamics of the ESR photoreaction were compared with bacteriorhodopsin (bR) reactions, showing similar primary intermediates. The study found a similar photoreaction pattern for all studied retinal proteins, with different timescales for excited state decays and retinal photoisomerization.