Deuterium substitution effect on the excited-state dynamics of rhodopsin

We investigated the excited-state dynamics of the cis-trans photoisomerization of rhodopsin by analyzing deuterium substitution effects for hydrogen atoms bonded to C(11) and C(12) of the retinal chromophore by the method of Fourier transform of optical absorption spectra (FTOA). Plotting the absolute value of the time correlation function of modified vibrational wave packet, we found that the deuterium substitution effects do not appear in the excited-state dynamics until about 20 fs after photon absorption, weakly appear in the time range 20-60 fs, significantly appear in the time range 70-110 fs, and complicatedly appear in the time range 110-170 fs. By analyzing those deuterium substitution effects, we obtained a result that the concerted motions of hydrogen out-of-plane (HOOP) waggings at C(11) and C(12), which are found to exist in native rhodopsin in the time range 20-60 fs, do not contribute to the excited-state dynamics in its time range appreciably and that the coupled motions of hydrogen atoms at C(11) and C(12), which are significantly coupled with the skeletal twisting motion of the chromophore in the time range 70-110 fs, contribute to the excited dynamics in its time range substantially. The hydrogen motions after 110 fs contribute to the excited-state dynamics in a complicate way. This cis-trans photoisomerization process of rhodopsin is basically similar to that of bacteriorhodopsin, which was obtained by the comparative analysis of the FTOA of 13-trans-locked-bacteriorhodopsin with native bacteriorhodopsin.