Molecular Mechanism for the Initial Process of Visual Excitation. I. Model of Photoisomerization in Rhodopsin and Its Theoretical Basis by a Quantum Mechanical Calculation of Adiabatic Potential

A model for explaining the photoisomerization of the chromophore in rhodopsin molecule with a high quantum yield is proposed on the basis of the strain effect. Necessary conditions assumed in the model are shown to be satisfied by the calculation using a self-consistent HMO theory which is recently developed by one of the authors. It is also shown that when a double-bond is twisted by about 40°, a steep change occurs in the molecular geometry, and that calculated adiabatic potentials, absorption wavelengths and transition dipole moments show some anomalies, corresponding to the change. Calculated energy barriers for isomerization around single-bonds of Schiff-bases of retinal in the ground state are very small.