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 similar to100 fs a single vibration below 200 fs dominates the detected spectral modulations. Its frequency of similar to120 in retinal protonated Schiff base is virtually unchanged by locking the C-13=C-14 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. (C) 2003 Published by Elsevier B.V.