Picosecond resonance coherent anti-Stokes Raman spectroscopy of bacteriorhodopsin: quantitative third-order susceptibility analysis of the dark-adapted mixture

Vibrational spectrum of the lumi intermediate in the room temperature rhodopsin photo-reaction

The vibrational spectrum (650-1750 cm(-1)) of the lumi-rhodopsin (lumi) intermediate formed in the microsecond time regime of the room-temperature rhodopsin (RhRT) photoreaction is measured for the first time using picosecond time-resolved coherent anti-Stokes Raman spectroscopy (PTR/CARS). The vibrational spectrum of lumi is recorded 2.5 micros after the 3-ps, 500-nm excitation of RhRT. Complementary to Fourier transform infrared spectra recorded at Rh sample temperatures low enough to freeze lumi, these PTR/CARS results provide the first detailed view of the vibrational degrees of freedom of room-temperature lumi (lumiRT) through the identification of 21 bands. The exceptionally low intensity (compared to those observed in bathoRT) of the hydrogen out-of-plane (HOOP) bands, the moderate intensity and absolute positions of C-C stretching bands, and the presence of high-intensity C==C stretching bands suggest that lumiRT contains an almost planar (nontwisting), all-trans retinal geometry. Independently, the 944-cm(-1) position of the most intense HOOP band implies that a resonance coupling exists between the out-of-plane retinal vibrations and at least one group among the amino acids comprising the retinal binding pocket. The formation of lumiRT, monitored via PTR/CARS spectra recorded on the nanosecond time scale, can be associated with the decay of the blue-shifted intermediate (BSI(RT)) formed in equilibrium with the bathoRT intermediate. PTR/CARS spectra measured at a 210-ns delay contain distinct vibrational features attributable to BSI(RT), which suggest that the all-trans retinal in both BSI(RT) and lumiRT is strongly coupled to part of the retinal binding pocket. With regard to the energy storage/transduction mechanism in RhRT, these results support the hypothesis that during the formation of lumiRT, the majority of the photon energy absorbed by RhRT transfers to the apoprotein opsin.

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 (RhRT), 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 RhRT concentration, an essential factor in using picosecond time-resolved CARS techniques to measure the vibrational spectroscopy of picosecond intermediates in the RhRT 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).