Femtosecond Stimulated Raman Study of Excited-State Evolution in Bacteriorhodopsin

Abstract

Femtosecond time-resolved stimulated Raman spectroscopy (FSRS) is used to examine the photoisomerization dynamics in the excited state of bacteriorhodopsin. Near-IR stimulated emission is observed in the FSRS probe window that decays with a 400-600-fs time constant. Additionally, dispersive vibrational lines appear at the locations of the ground-state vibrational frequencies and decay with a 260-fs time constant. The dispersive line shapes are caused by a nonlinear effect we term Raman initiated by nonlinear emission (RINE) that generates vibrational coherence on the ground-state surface. Theoretical expressions for the RINE line shapes are developed and used to fit the spectral and temporal evolution of the spectra. The rapid 260-fs decay of the RINE peak intensity, compared to the slower evolution of the stimulated emission, indicates that the excited-state population moves in ∼260 fs to a region on the potential energy surface where the RINE signal is attenuated. This loss of RINE signal is best explained by structural evolution of the excited-state population along multiple low-frequency modes that carry the molecule out of the harmonic photochemically inactive Franck-Condon region and into the photochemically active geometry.

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

“…In ref. 18, the decay of the CϭC, C-C, and HOOP modes was determined to be 260 Ϯ 10 fs. The large mismatch in energy between these modes and the torsional modes would lead to an indirect impulsive excitation of the latter ones.…”

“…(ii) These decay to low-frequency torsional modes on a sub-250-fs time scale (11,18), exciting a wave packet of torsional modes, while translocation of charge along the retinal chain occurs (19). (iii) The transition state, which corresponds to a twisted (but not isomerized) configuration of retinal (11), is impulsively generated (Fig.…”

“…That the system is all-trans-like during the first 200 fs is in line with Zhong et al (17), who carried out transient absorption studies on wild-type bR and bR reconstituted with retinal analogs that cannot undergo isomerization and found identical dynamics during the initial 200 fs. In addition, McCamant et al (18) recently showed by fs-stimulated Raman spectroscopy that the high-frequency modes (1,000-1,500 cm Ϫ1 ) decay on a time scale of Ϸ250 fs. They argued that this observation points to the excited-state population evolving to a new, Raman-silent configuration.…”

Mode specific excited state dynamics study of bis(phenylethynyl)benzene from ultrafast Raman loss spectroscopy

“…[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Interestingly, Chen et al reported that for such systems the switching between conducting and non-conducting states appears to be related to the dihedral angles between the chromophores. 19,20 The photophysical properties of 1,4-bis(phenylethynyl) benzene (BPEB) (Fig.…”

Analysis of femtosecond stimulated Raman spectroscopy of excited-state evolution in bacteriorhodopsin

“…[7][8][9][10][11][12][13][14][15][16][17] In the time-resolved experiments, three pulses are used: a Ͻ50 fs actinic pump pulse to excite the molecule to an upper electronic state, followed by stimulated Raman spectroscopy with an overlapping pair of ps Raman pump pulse, and a ϳ15 fs continuum probe pulse. It is possible to acquire spectra with time resolution of ϳ50 fs in the time delay between the actinic pump and probe pulses, and spectral resolution of Ͻ10 cm −1 due to the picosecond Raman pump pulse.…”