Photochemical dynamics of all-trans retinal protonated Schiff-base in solution: Excitation wavelength dependence

Photochemistry of protonated all-trans retinal Schiff-base (RPSB), the active chromophore in bacteriorhodopsin (BR) and sensory rhodopsins has been investigated with femtosecond multichannel pump probe spectroscopy at two excitation wavelengths. In a recent study of an RPSB analogue which mimics the opsin shift in BR, significant excitation wavelength dependence of the transient spectra was observed and assigned to structural inhomogeneity in the ground state. Our aim is to determine if similar inhomogeneity is manifest also in the native RPSB in solution which is the archtypical model for appreciating the apoproteins effect on retinal protein photochemistry. Significant differences in transient spectra collected after 390 and 480 nm excitation are observed and are likewise assigned to ground state structural inhomogeneity. For both excitation wavelengths the stimulated emission band extends well beyond 900 nm, much deeper than previously reported in the near IR. The shallowness of this feature and a newly revealed dip in its intensity near 760 nm are attributed to an overlapping excited state absorption, as reported for BR. This assignment identifies the documented RPSB excited state absorption band which peaks at 500 nm as the counterpart of the 460 nm absorption feature reported for the reactive excited state of BR coined I460. Implications of this assignment, and possible mechanisms for inhomogeneous broadening of the electronic absorption spectrum of RPSB in solution are discussed.