The relaxation dynamics of the excited electronic states of retinal in bacteriorhodopsin by two-pump-probe femtosecond studies

Abstract

We present the results of two-pump and probe femtosecond experiments designed to follow the relaxation dynamics of the lowest excited state (S 1) populated by different modes. In the first mode, a direct (S0 3 S1) radiative excitation of the ground state is used. In the second mode, an indirect excitation is used where the S 1 state is populated by the use of two femtosecond laser pulses with different colors and delay times between them. The first pulse excites the S 0 3 S1 transition whereas the second pulse excites the S 1 3 Sn transition. The nonradiative relaxation from the Sn state populates the lowest excited state. Our results suggest that the S 1 state relaxes faster when populated nonradiatively from the Sn state than when pumped directly by the S0 3 S1 excitation. Additionally, the S n 3 S1 nonradiative relaxation time is found to change by varying the delay time between the two pump pulses. The observed dependence of the lowest excited state population as well as its dependence on the delay between the two pump pulses are found to fit a kinetic model in which the S n state populates a different surface (called S 1) than the one being directly excited (S 1). The possible involvement of the Ag type states, the J intermediate, and the conical intersection leading to the S 0 or to the isomerization product (K intermediate) are discussed in the framework of the proposed model. Halobacterium salinarum (Halobium salinarium) is a member of Halobacteria, which is part of the domain Archaea. In an anaerobic condition under light illumination, H. salinarum demonstrates a pH decrease in cell suspension and ATP synthesis (1, 2), the main features of photosynthesis. The photosynthetic activity in H. salinarum was attributed to bacteriorhodopsin (bR), a 26-kDa protein that is hexagonally packed within the purple membrane (1-4). Since the discovery of the purple membrane, its enigmatically efficient photosynthesis is in the center of the most active research. To our surprise, the archaic organism effectively utilizes more than 50% of the absorbed light energy (5, 6).Retinal, a light-absorbing polyene, is the bR chromophore and is linked to its Lys-216 by a protonated Schiff base. The chromophore undergoes light-induced isomerization and masters proton transfer across the membrane (7-11). The photoisomerization of retinal initiates a series of retinal protein structural reconformations of the bacteriorhodopsin that lead to the formation of different intermediates and finally return it to its initial state with all-trans-retinal. Overall, this photocycle consists of at least seven intermediates of bR with different visible absorption spectra and lifetimes bR 568 3 S 1 3 J 3 K 630 3 L 550 3 M 412 3 N 520 3 O 640 3 bR 568 (4).Retinal in solution shows relatively slow and nonselective photoisomerization around several double bonds (12-14) with low yield. The specific ultrafast, all-trans, 13-cis photoisomerization of the retinal in bR is the purple membrane photosynthesis key event which has a quantum yield of 55%. The p...

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