Studies on the conformational state of the chromophore group (11-cis-retinal) in rhodopsin by computer molecular simulation methods

The molecular dynamics of the rhodopsin chromophore (11-cis-retinal) has been followed over a 3-ns path, whereby 3 × 106 discrete conformational states of the molecule were recorded. It is shown that within a short time, 0.3–0.4 ns from the start of simulation, the retinal β-ionone ring rotates about the C6–C7 bond through ∼60° relative to the initial configuration, and the whole chromophore becomes twisted. The results of ab initio quantum chemical calculations indicate that for the final conformation of the chromophore center (t = 3 ns) the rhodopsin absorption maximum is shifted by 10 nm toward longer wavelengths as compared with the initial state (t = 0). In other words, the energy of transition of such a system into the excited singlet state S1 upon photon capture will be lower than that for the molecule where the β-ionone ring of the chromophore is coplanar to its polyene chain.