Vibrational coherences of the protonated Schiff base of all-trans retinal in solution

Modulations in the time gated ultrafast fluorescence of the protonated Schiff base of retinal in various solvents are reported, which reflect the creation and evolution of vibrational wave packets due to the torsional mode of retinal of frequency ∼120 cm−1. The oscillations are damped in ∼400 fs. Their frequency is significantly decreased compared to the case of the protein (170 cm−1), and is insensitive to solvent properties such as the dielectric constant and the viscosity. This, along with previous results on mutants of bacteriorhodopsin, leads us to conclude that in the protein, the isomerization dynamics of retinal is governed by steric effects and by the inhomogeneous distribution of the electrostatic field due to amino-acid residues within the protein pocket. We also discuss the origin of the modulations and conclude that they are impulsively excited via the high frequency modes of retinal.