Dynamic processes of visual transduction.

Rhodopsin is one of those rare macromolecules whose inherent chromophore, 11-cis retinaldehyde, allows one to naturally observe triggered macromolecular changes on the timescale of picoseconds to minutes. Investigations of these molecular processes have been carried out with laser monochromatic light under conditions where the photon flux used for photolysis was carefully measured. The formation of bleaching intermediates has been examined as a function of fluence. Under conditions where the formation of intermediates is unaffected by photon reversal the following observations hold: Upon the absorption of a photon, the initial photochemical event results in production of metastable bathorhodopsin within 6 psec. Artificial rhodopsin regenerated with 9-cis retinal forms a distinct bathorhodopsin which must reflect distortions at the active site differing from those generated with 11-cis retinal. Bathorhodopsin thermally decays through lumirhodopsin and meta I-rhodopsin, to meta II-rhodopsin through a series of coupled equilibria. The final meta I-meta II equilibrium is stable for seconds. The process provides a unique model for utilization of energy to drive (trigger) a biological cascade of events.