Ground‐ and Excited‐State Vibrational Coherence Dynamics in Bacteriorhodopsin Probed With Degenerate Four‐Wave‐Mixing Experiments

Abstract

Vibrational coherence dynamics in the all-trans retinal chromophore in Bacteriorhodopsin (BR) are investigated by means of temporally and spectrally resolved degenerate four-wave-mixing experiments. The coherence dynamics depend on the excitation wavelength when BR samples are excited at different wavelengths in a spectral range between 520 nm to above 620 nm. The trends in the dynamics observed by tuning of the excitation wavelength allow an assignment of the wave packet dynamics to ground- and excited-state potential energy surfaces. Specifically, the intensity of so-called "out-of-plane" modes of polyene-chain substituents increases for excitation wavelengths near 500 nm. It is shown that this is consistent with the assignment of out-of-plane modes to excited-state coherence dynamics. Moreover, intense low-frequency coherence dynamics around 200 cm(-1) are observed for signal detection in two different spectral regions of excited-state absorption. These modulations are assigned to excited-state dynamics due to the observed dependence on the excitation wavelength. In addition, we show that generally high-frequency modes (>1010 cm(-1)) originate from wave packet motion in the electronic ground state of all-trans retinal.

Effect of point mutations on the ultrafast photo-isomerization of Anabaena sensory rhodopsin

“…For the transient vibrational spectroscopy, DFWM and Pump-DFWM experiments were employed as described in detail in references [52][53][54] . In short, two home-built non-collinear optical parametric amplifiers (1 kHz) were used to generate ultrashort excitation and probing pulses with typical durations below 15fs.…”

“…Understanding the ultrafast vibrational evolution is a central aspect in the investigation of the reaction dynamics of the retinal chromophore [58][59][60][61][62][63] Time resolved vibrational spectroscopies, like degenerate four wave mixing (DFWM) 53 75,76 , allow to resolve such structural changes in the ground and electronically excited states up to the formation of photoproducts. A detailed study of the evolution and origin of vibrational modes in the ground and excited state has been done, for example, in the case of bacteriorhodopsin (bR) 53,64,74 . Excitation of vibrational coherences in bR has been shown to be highly dependent on the excitation wavelength in the spectral range from 520 up to 620nm 53 .…”

“…A detailed study of the evolution and origin of vibrational modes in the ground and excited state has been done, for example, in the case of bacteriorhodopsin (bR) 53,64,74 . Excitation of vibrational coherences in bR has been shown to be highly dependent on the excitation wavelength in the spectral range from 520 up to 620nm 53 . The "out-of-plane" modes of polyene-chain substituents and low frequency modes have been found to be very intense in the electronically excited state of bR.…”

“…While initial work revealed the time scales for isomerization of the PSBR, and how they depend on the protein species or on point mutations, more attention was recently devoted to the mechanism, by which the protein steers this ultrafast reaction. New femtosecond spectroscopic [8][9][10][11][12][13][14][15][16][17][18][19] and quantum chemistry tools were devised [20][21][22][23][24][25][26] with the aim of drawing a very detailed picture of the excited state multidimensional potential energy surface (PES) including the conical intersection (CI) seam, of the time-resolved motion of PSBR at the atomic level, the reaction-driving vibrational modes and the dynamic changes in the charge distribution on PSBR and in the protein electrostatics. Additional, though partial, insight on these aspects comes from comparative studies of PSBR in solution [27][28][29][30][31][32][33][34] and of proteins reconstituted with non-isomerizing PSBR derivatives [35][36][37] .…”

Vibrationally Coherent Crossing and Coupling of Electronic States during Internal Conversion inβ-Carotene

“…In addition, spectroscopic signatures of VCs in the time-domain are orders of magnitude smaller than their electronic counterparts demanding high spectroscopic sensitivity. Even under optimized experimental conditions [17], however, the isolation of excited state signatures is challenging because of the unavoidable and often dominant presence of ground state and solvent VCs [18]. As a result, no clear observation of multiple excited state nuclear wavepackets generated directly by the excitation pulse has been reported to date.…”