Molecular mechanism for the initial process of visual excitation. IV. Energy surfaces of visual pigments and photoisomerization mechanism.

Using the twisted conformations of the chromophores for visual pigments and intermediates which were theoretically determined in the previous paper, energy surfaces of the pigment at - 190 degrees C were obtained as functions of the torsional angles theta 9-10 and theta 11-12 or of the torsional angles theta 9-10 and theta 13-14. In these calculations, the existence of specific reaction paths between rhodopsin (R) and bathorhodopsin (B), between isorhodopsin I (I) and bathorhodopsin, and between isorhodopsin II (I') and bathorhodopsin were assumed. It was shown that the total energy surfaces of the excited states had minima C1 at theta 9-10 approximately -10 degrees and theta 11-12 approximately -80 degrees, C2 at theta 9-10 approximately -85 degrees and theta 11-12 approximately -5 degrees, and C3 at theta 9-10 approximately -0 degree and theta 13-14 approximately -90 degrees. These minima are considered to correspond to the thermally barrierless common states as denoted by Rosenfeld et al. Using the total energy surfaces in the ground and excited states, the molecular mechanism of the photoisomerization reaction was suggested. Quantum yields for the photoconversions among R, I, I' and B were related to the rates of vibrational relaxations, radiationless transitions and thermal excitations. Some discussion was made of the temperature effect on the quantum yield. Similar calculations of the energy surfaces were also made at other temperatures where lumirhodopsin or metarhodopsin I is stable. Relative energy levels of the pigments and the intermediates were discussed.