COMPREHENSIVE STUDY OF MOLECULAR-CONFORMATION AND ADIABATIC POTENTIAL SURFACE OF ALL-TRANS RETINAL BY THE CNDO/2 METHOD

Extensive calculations of the adiabatic potential surface (APS) of all-trans retinal as a function of the twisting angle around the 6-7 bond, φ-7, have been performed by means of the CNDO/2 molecular orbital method. In such calculations, complete geometry optimization is made for each value of φ-7. Our calculated APS has a maximum around φ6-7=-170° and has no minimum around φ6-7=-190°. This result is in contradiction to that calculated by the consistent force field method, where a deep minimum was obtained around φ6-7=-190°. Analyzing the molecular conformations at φ6-7=-190° in detail, we have found that there is no space for the 38-H to be inserted between 17-CH3 and 18-CH3 at the conformation around φ6-7=-190° but severe steric hindrances take place, giving rise to a large potential barrier. We have also analyzed each contribution of the three methyl groups in the ionone ring to the potential barrier by calculating the APS of retinal analogs whose methyl groups are successively replaced by hydrogen atoms. © 1990.