A NEW PHOTOLYSIS INTERMEDIATE IN ARTIFICIAL AND NATIVE VISUAL PIGMENTS

Nanosecond time-resolved and continuous illumination, low-temperature, spectroscopic studies reveal a new photolysis intermediate in a wide variety of artificial visual pigments as well as in native rhodopsin. This new intermediate, BSI, has a blue-shifted spectrum relative to the pigments as well as to their batho and lumi intermediates. At room temperature BSI is formed subsequent to batho and approaches an equilibrium with batho before decaying to the lumi intermediate. Chromophore modifications, which modify the beta-ionone ring, eliminate conjugation between the ring and the polyene chain, add bulky groups to the C4 position on the ring, or remove the 13-methyl group all yield time-resolved spectra which lead to the general scheme. [GRAPHICS] The same mechanism is shown to be valid in isorhodopsin and in the native bovine pigment rhodopsin. Chromophore modifications described above affect the batho reversible BSI equilibrium as well as the kinetics of approach to equilibrium but have little effect on the spectra of the intermediates or on the rate of the BSI to lumi transition. Implications for the nature of the BSI intermediate are discussed. Though BSI has a spectrum blue-shifted from that of batho, BSI is higher in enthalpy. It is proposed that this apparent conflict may be due to the fact that the photon energy, initially stored in chromophore-protein interactions, is transmitted to the protein during the batho-to-BSI transition. If energy at the BSI stage is still stored in the chromophore, models simply relating energy storage to bathochromic shifts must be ruled out.