Resonance Raman excitation profiles of bacteriorhodopsin

Abstract

We have measured resonance Raman excitation profiles for 12 vibrational modes of light-adapted bacteriorhodopsin using 27 excitation wavelengths ranging from 472 to 752 nm. The excitation profiles are quite similar and have approximately the shape and width of the absorption spectrum. Calculations using a 29-mode harmonic sum over states with large amounts of inhomogeneous broadening (Gaussian standard deviation 750 cm−1) yield good fits to the absorption and excitation profile band shapes and to the absolute absorption and Raman cross sections. These calculations also give nearly correct intensities for most of the observed overtone and combination bands. The absolute Raman cross sections allow us to differentiate between the homogeneous and inhomogeneous contributions to the spectral breadth. Our upper limit of 250 cm−1 for the homogeneous linewidth, as well as the absence of strong low-frequency Raman lines, implies that the lack of resolution in the bacteriorhodopsin absorption is due primarily to inhomogeneous effects. Finally, the permissible contribution of torsional vibrations to the spectral diffuseness and the consistency of these results with an excited state isomerization are explored using wave packet propagation on a linear excited state torsional potential.