Vibrational spectra of room-temperature rhodopsin: concentration dependence in picosecond resonance coherent anti-Stokes Raman scattering

The vibrational degrees of freedom of room-temperature rhodopsin (Rh RT), the central trans-membrane protein in vision, are measured at room temperature by picosecond resonance coherent anti-Stokes Raman scattering (PR/CARS). High signal-to-noise PR/CARS data for the ethylenic stretching, Schiff base, and hydrogen-out-of-plane modes of the retinal chromophore are quantitatively analyzed via third-order susceptibility relationships. The accurate determination of spectral features permit the PR/CARS bandshapes to be analyzed as a function of Rh RT concentration, an essential factor in using picosecond time-resolved CARS techniques to measure the vibrational spectroscopy of picosecond intermediates in the Rh RT photosequence. Of particular importance is the recognition that PR/CARS bandshapes are sensitive functions of both the chromophore concentration and the excitation wavelength, as measured relative to the absorption spectra of specific chromophores (static and transient).