Femtosecond formation dynamics of primary photoproducts of visual pigment rhodopsin

The coherent 11-cis-retinal photoisomerization dynamics in bovine rhodopsin was studied by femtosecond time-resolved laser absorption spectroscopy at 30-fs resolution. Femtosecond pulses of 500, 535, and 560 nm wavelength were used for rhodopsin excitation to produce different initial Franck-Condon states and relevant distinct values of the vibrational energy of the molecule in its electron excited state. Time evolution of the photoinduced rhodopsin absorption spectra was monitored after femtosecond excitation in the spectral range of 400–720 nm. Oscillations of the time-resolved absorption signals of rhodopsin photoproducts represented by photorhodopsin₅₇₀ with vibrationally-excited all-trans-retinal and rhodopsin₄₉₈ in its initial state with vibrationally-excited 11-cis-retinal were studied. These oscillations reflect the dynamics of coherent vibrational wave-packets in the ground state of photoproducts. Fourier analysis of these oscillatory components has revealed frequencies, amplitudes, and initial phases of different vibrational modes, along which the motion of wave-packets of both photoproducts occurs. The main vibrational modes established are 62, 160 cm⁻¹ and 44, 142 cm⁻¹ for photorhodopsin₅₇₀ and for rhodopsin₄₉₈, respectively. These vibrational modes are directly involved in the coherent reaction under the study, and their amplitudes in the power spectrum obtained through the Fourier transform of the kinetic curves depend on the excitation wavelength of rhodopsin.