Molecular mechanism for the initial process of visual excitation. III. Theoretical studies of optical spectra and conformations of chromophores in visual pigments, their analogues and intermdiates based on the torsion model.

The torsion model with which we proposed to interpret the specific properties of the photoisomerization reaction of rhodopsin has been developed to apply to isorhodopsin I, isorhodopsin II and some intermediates. Based on this model, optical absorption wavelengths and oscillator strengths, as well as rotational strengths of visual pigments, analogues and intermediates at low temperatures are analyzed by varying twisted conformations of the chromophores. As a result, it was found that most of the optical data could be very well accounted for quantitatively by the torsion model. The twisting characters in the chromophore of rhodopsin are very similar to those of isorhodopsin. The obtained conformations of the chromophores are very similar in rhodopsin and its analogues, and in isorhodopsin and its analogues. Those of the chromophores of bathorhodopsin, lumirhodopsin and metarhodopsin I are similar to one another except that the conjugated chain of metarhodopsin I bends considerably when compared with the other intermediates.