Probing ultrafast photochemistry of retinal proteins in the near-IR: Bacteriorhodopsin and anabaena sensory rhodopsin vs retinal protonated Schiff base in solution

Photochemistry of bacteriorhodopsin (bR), anabaena sensory rhodopsin (ASR), and all-trans retinal protonated Schiff base (RPSB) in ethanol is followed with femtosecond pump-hyperspectral near-IR (NIR) probe spectroscopy. This is the first systematic probing of retinal protein photochemistry in this spectral range. Stimulated emission of the proteins is demonstrated to extend deep into the NIR, and to decay on the same characteristic time scales previously determined by visible probing. No signs of a transient NIR absorption band above λ_pr > 1.3 μm, which was recently reported and is verified here for the RPSB in solution, is observed in either protein. This discrepancy demonstrates that the protein surroundings change photochemical traits of the chromophore significantly, inducing changes either in the energies or couplings of photochemically relevant electronic excited states. In addition, low-frequency and heavily damped spectral modulations are observed in the NIR signals of all three systems up to 1.4 μm. By background subtraction and Fourier analysis they are shown to resemble wave packet signatures in the visible, stemming from multiple vibrational modes and by analogy are assigned to torsional wave packets in the excited state of the retinal chromophore. Differences in the vibrational frequencies between the three samples and the said discrepancy in transient spectra are discussed in terms of opsin effects on the RPSB electronic structure.