Picosecond time-resolved resonance Raman spectroscopy of the primary products in the bacteriorhodopsin photocycle.

Abstract

The initial photochemical and photophysical events of the Bacteriorhodopsin (BR) photocycle are investigated using resonance Raman spectroscopy. The salt water bacterium, Halobacterium halobium, converts light into chemical energy via this cycle. Light induced isomerization of the all-trans retinal chromophore causes proton translocation across the lipid membrane containing the protein. Absorption experiments reveal red shifts in BR absorption on a picosecond time scale. Picosecond time-resolved resonance Raman spectroscopy (PTR³) provides a vibrational probe of these changes. PTR³ utilizes two tunable dye lasers in a pump-probe configuration. One initiates photochemistry while a second probes the chromophore. The vibrational spectrum of the K-590 intermediate present 50 ps after the initiation of the photocycle is obtained by PTR³ spectroscopy. The ability to separate photolytic excitation from the Raman probe facilitates the application of a quantitative model of the optical excitation process to time resolved vibrational measurements of K-590. These spectra are analyzed to find the isomerization state of retinal in K-590 by comparison with the resonance Raman spectra of model compounds. These resonance Raman results are compared to earlier measurements of the K intermediate. PTR³ spectra of K-590 present later in the photocycle are also obtained. These spectra remain unchanged over the period investigated (40 ps-26 ns). These results confirm that isomerization of the chromophore is one of the primary events following initiation of the photocycle. Changes in relative Raman intensities observed earlier than 40 ps are discussed with reference to the photophysics of the optical excitation process. PTR³ techniques are applied to antistokes Raman measurements of BR. The existence of a significant vibrationally excited population is revealed. Differences in the Raman band positions in the stokes and antistokes spectra demonstrate that several quanta of the higher frequency modes in the BR Raman spectrum are excited. These modes decay with a time constant of ≈7 ps. These observations suggest the retinal chromophore does not experience rapid uniform internal vibrational redistribution following the internal conversion producing the vibrationally excited species.