ChemInform Abstract: A Quantum Chemical Study on the Mechanism of cis‐trans Isomerization in Retinal‐Like Protonated Schiff Bases

Abstract

The dynamics of the photochemical cis-trans isomerization in retinal-like protonated Schiff bases is studied by means of MNDO/CI calculations. The aim of these calculations is a better understanding of the mechanism which accounts for the highly regioselective and efficient photoisomerization of rhodopsin and bacteriorhodopsin in the primary step after light absorption. Calculations on the model compound protonated l-imino-2,4-pentadiene show that the regioselectivity and efficiency of this reaction can be explained from the intrinsic properties of this molecule. Whereas it is found that the protonated Schiff bases have a lowest 'B,+-like excited state, the second 'A,:like excited state is particularly photochemical labile. This latter state serves in diminishing (or even removing) the barrier on the potential energy surface of the initially excited state, thus enhancing the rate for photoisomerization. The transition probability for a radiationless return of the excited molecule to its ground state was evaluated explicitly for the photoisomerization around the various double bonds in protonated 1 -imino- 2,4-pentadiene by means of semiclassical trajectory calculations. The transition probability depends on the energy gap between the ground and excited state and the nonadiabatic coupling between these states for the 90° twisted molecule. The extent of the energy gap is related to the distance from the twisted bond to the nitrogen atom. The role of this electron-deficient nitrogen atom is to stabilize the polarized resonance structure which describes the 90° twisted molecule in the excited state. When this stabilization is too strong, the polarized resonance structure drops below the diradicalar ground state which results in an increased energy gap and a reduced efficiency of photoisomerization. The possibility for a concerted bicycle pedal isomerization around two double bonds is investigated by a calculation of the two-dimensional energy surfaces and nonadiabatic couplings for a combined rotation around these two bonds. A strictly bicycle pedal motion is found to be unfavorable, but a mechanism which involves a complete rotation around one double bond assisted by a partial rotation of the second double bond might provide a route for the photoisomerization of the retinylidene chromophore in the confined environment of a protein. Calculations on a model compound of the protonated Schiff base of retinal show that the extent of the stability of the 90° twisted molecule in the excited state can be directed by locating external point-charges around the molecule. In nature, these point-charges are provided by the protein opsin, and their presence has been used to explain the opsin shift of the various intermediates in the photocycles of rhodopsin and bacteriorhodopsin. Our calculations show that these external point-charges also have an important impact on the energy gap between the ground and excited state and, therefore, on the regioselectivity and efficiency of photoisomerization in the retinylidene chromophore. The primary step in the photoisomerization in bac- teriorhodopsin can be best understood from an external point-charge model with a negative counterion near the protonated nitrogen atom and an ion pair near the cyclohexene ring.