Role of the Protein Environment in Photoisomerization of Type I and Type II Rhodopsins: a Theoretical Perspective

Abstract Primary photoisomerization reactions of the retinal protonated Schiff base (RPSB) inside type I and type II rhodopsins are ultrafast and exhibit high quantum yields. Specific protein environments are thought to facilitate photoisomerization of RPSB; however, the detailed mechanisms of tuning the reaction timescales and specificity are far from being understood. Here, by using molecular dynamics simulations and large-scale XMCQDPT2-based QM/MM modeling, we gain insight into the role played by the protein environment in specific photoisomerization of RPSB from all- trans to 13- cis in microbial rhodopsin KR2 and from 11- cis to all- trans in bovine visual rhodopsin. By analyzing the calculated vibronic band shapes, we explore the early-time excited-state dynamics of RPSB in both types of rhodopsins. We show that the protein environment changes vibrational modes, which become excited following the S 0 –S 1 transition, by pre-twisting the chromophore about a certain double bond in the ground electronic state. This reduces a barrier that hinders intramolecular rotation in the excited state, thus facilitating photoisomerization across the specific double bond. Moreover, pre-twisting of RPSB also provides conditions for initial in-phase excitation of the fundamental vibrational modes involved in the reaction coordinate, enabling vibrationally coherent barrierless excited-state decay in the photochemistry of vision.