Deuterium Substitution Effect on the Excited-State Dynamics of Rhodopsin ² T. Kakitani* and R. Akiyama ‡ Department of Physics, Graduate School of Science, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Y. Hatano School of Computer and CognitiVe Science, Chukyo UniVersity, Toyota, Aichi 470-03, Japan Y. Imamoto Department of Earth and Space Science, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 560, Japan Y. Shichida Department of Biophysics, Graduate School of Science, Kyoto UniVersity, Sakyo-ku, Kyoto 606, Japan P. Verdegem and J. Lugtenburg Department of Chemistry, Gorlaeus Laboratories, Leiden, The Netherlands ReceiVed: October 1, 1997; In Final Form: December 3, 1997 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. Introduction The pimary process of vision is the cis-trans photoisomer- ization of retinal chromophore in rhodopsin. This photoisomer- ization has some specific features: the isomerization takes place exclusively from 11-cis form to all-trans form of the chro- mophore, the isomerization rate is ultrafast 1,2 (in less than 200 fs) and the quantum yield is high (0.67). Furthermore, the coherent nature of the vibrational wave packet propagation in the excitation continues even after transition to the ground state, suggesting a very smooth propagation of the vibrational wavepackect on the excited potential energy surface. 3 These specific photochemical properties of rhodopsin are not found in retinal in solution. Therefore, it has been considered that the protein environment surrounding the chromophore must play a special role in the photoisomerization. 4-9 However, this photoisomerization process, namely, the excited-state dynamics, is so fast that the detailed mechanism by which the micro- environment of the protein works for realizing this specific, ultrafast, and coherent nature of photoisomerization of rhodopsin is not clarified well up to the present time. Under these situations, we have developed a new analytical method to catch the molecular reality of the excited-state dynamics in much detail. Theoretical study of the time- dependent description of molecular motions was extensively made by Gordon. 10 He related the time correlation function of the permanent dipole moment of orientation-fluctuating mol- ecules with the Fourier tranform of the infrared absorption spectrum. 10 Later in 1978, Heller formally showed that the optical absorption spectrum can be related to the Fourier transformation of the data of excited-state dynamics of the ² Abbreviations: CHAPS, 3-[(3-cholamidopropyl)dimethylammonio]-1- propanesulfonate; PC, L-R-phosphatidylcholine from fresh egg yolk; HEPES, N-(2-hydroxyethyl)piperazine-N′-2-ethanesulfonic acid. * To whom correspondence should be addressed. Tel and fax: 052-789- 3528. E-mail: kakitani@allegro.phys.nagoya-u.ac.jp. ‡ Present address: Institute for Molecular Science, Okazaki 444, Japan. 1334 J. Phys. Chem. B 1998, 102, 1334-1339 S1089-5647(97)03191-X CCC: $15.00 © 1998 American Chemical Society Published on Web 01/27/1998