Comparing photoinduced vibrational coherences in bacteriorhodopsin and in native and locked retinal protonated Schiff bases

Low frequency excited state vibrational coherences induced by impulsive photoexcitation in bacteriorhodopsin are detected via femtosecond pump–probe spectroscopy, and compared with similar data in retinal protonated Schiff bases of native and locked retinals. At delays above ∼100 fs a single vibration below 200 fs dominates the detected spectral modulations. Its frequency of ∼120 in retinal protonated Schiff base is virtually unchanged by locking the C13C14 bond in the trans or cis configurations, but is increased to 170 cm−1 within the protein environment. The implications of this result on the part played by the protein in directing the reactivity of the retinal within bacteriorhodopsin is discussed.