Primary photochemistry and photoisomerization of retinal at 77 degrees K in cattle and squid rhodopsins.

The relative quantum yields of the photoreactions Rhodopsin in equilibrium Bathorhodopsin in equilibrium Isorhodopsin over an extended wavelength region have been determined in cattle and squid rhodopsins at 77 degrees K. The quantum yields were found to be wavelength independent and unchanged for samples suspended in D2O. The rhodopsin-bathorhodopsin forward and backward quantum yields sum to larger than one. These results are consistent with the previous suggestion that the excited singlet potential of rhodopsin has a single minimum along the 11-12 torsional coordinate. The values of the quantum yields are important for evaluating dynamic models of the rhodopsin-bathorhodopsin transition. We conclude that equilibration in the common excited state afer excitation of rhodopsin, as previously suggested, does not occur. Models involving molecular excitation trajectories conserving torsional momenta and excited state to ground state surface crossings better fit the data, and a semiquantitative analysis is presented. Probabilities of surface crossings are calculated.