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

Picosecond resonance coherent anti-Stokes Raman scattering (PR/CARS) from a dark-adapted bacteriorhodopsin (DA/BR) sample containing comparable amounts of two ground-state species having different retinal structures, BR-570 (13-trans, 15-anti) and BR-548 (13-cis, 15 syn) are reported. Two picosecond (6 ps FWHM) laser pulses (578 nm (Ω1) and 604–643 nm (Ωs)) are used to record PR/CARS data via electronically-resonant excitation in both BR-570 and BR-548. PR/CARS spectra from the isomerie mixture in DA/BR requires analysis in terms of third-order susceptibility (x(3)) relationships which permit a quantitative separation of the contributions from each species. PR/CARS spectra for BR-570 (light-adapted BR or LA/BR) in protonated and deuterated water samples together with their associated x(3) analyses (published earlier) are used in the x(3) analysis of the DA/BR data to quantitatively separate the PR/CARS spectrum of BR-548. The x(3) analysis of the PR/CARS data provides a detailed characterization of the vibrational modes in BR-548 thought to contain a 13-cis, 15-syn retinal. As importantly, the experimental and analytical methodologies developed for analyzing the DA/BR mixture are directly applicable to picosecond time-resolved CARS experiments designed to examine BR mixtures containing intermediates with distinct retinal structures formed during the BR photocycle.