Femtosecond spectroscopy of acidified bacteriorhodopsin

Bacteriorhodopsin (bR) is a light-harvesting energy-transducing pigment so far found only in Halobacterium halobium, prokaryotes that occur in natural brines. The acidic form of bR (bR₆₀₅) is an approximately 40:60 mixture of 13-cis bR₆₀₅ and all-trans bR₆₀₅. A study of the primary photoprocesses of bR₆₀₅ by femtosecond absorption spectroscopy is described. It has been found that the lowest excited singlet states of 13-cis and all-trans bR₆₀₅ have different lifetimes (1.5 ± 0.2 ps for 13-cis bR₆₀₅ and 8.6 ± 0.9 ps for all-trans bR₆₀₅) and that the red-shifted ground state intermediate so far reported in the nanosecond time scale K_acid is formed directly from the lowest excited singlet states of bR₆₀₅. It has also been found that the isomerization around C₁₃ = C₁₄ of the retinal molecule in the lowest excited singlet states in both 13-cis bR₆₀₅ and all-trans bR₆₀₅ has an overdamped nature, i.e., the isomerization angle varies only monotonously in the lowest excited singlet states of both 13-crs bR₆₀₅ and all-trans bR₆₀₅.