EFFECT OF EXTERNAL-PRESSURE ON THE SPECTROSCOPIC AND CONFORMATIONAL PROPERTIES OF THE VISUAL CHROMOPHORES

The effect of external pressure (3900-23,700 psi) on the spectroscopic properties of all-trans- and 13-cis-retinal and the conformational equilibrium between the 12-s-cis and 12-s-trans conformers of 11-cis-retinal is analyzed in methylcyclohexane solvent. A pressure and solvent effect formalism whcih explicitly includes the contributions of dispersive and electrostatic interactions, and the effect of external pressure on those interactions, is used in the calculation of the total free energy change with applied external pressure. The 12-s-trans conformer apparently is preferentially stabilized at high external pressure despite a predicted larger molecular volume in solution. A significant preferential electrostatic stabilization of the 12-s-trans species with increased pressure due to the larger dipole moment of this conformer and the increase in the solvent reaction field with external pressure may explain this stability. The 12-s-trans conformer is calculated to have a dipole moment at least 1.6 D larger and a cavity radius at least 0.012 .ANG. larger than the 12-s-cis conformer. The 12-s-trans conformer is calculated to have a vacuum free energy approximately 0.5 kcal/mol lower than the 12-s-trans conformer.