Orientational changes of the transition dipole moment of retinal chromophore on the disk membrane due to the conversion of rhodopsin to bathorhodopsin and to isorhodopsin

Transition dipole orientations in the early photolysis intermediates of rhodopsin

The linear dichroism spectrum of rhodopsin in sonicated bovine disk membranes was measured 30, 60, 170, and 600 ns after room temperature photolysis with a linearly polarized, 7-ns laser pulse (lambda = 355 or 477 nm). A global exponential fitting procedure based on singular value decomposition was used to fit the linear dichroism data to two exponential processes which differed spectrally from one another and whose lifetimes were 42 +/- 7 ns and 225 +/- 40 ns. These results are interpreted in terms of a sequential model where bathorhodopsin (BATHO, lambda max = 543 nm) decays toward equilibrium with a blue shifted intermediate (BSI, lambda max = 478 nm). BSI then decays to lumirhodopsin (LUMI, lambda max = 492 nm). It has been suggested that two bathorhodopsins decay in parallel to their products. However, a Monte Carlo simulation of partial photolysis of solid-state visual pigment samples shows that one mechanism which creates populations of BATHO having different photolysis rates at 77 K may not be responsible for the two decay rates reported here at room temperature. The angle between the cis band and 498-nm band transition dipoles of rhodopsin is determined to be 38 degrees. The angles between both these transition dipoles and those of the long-wave-length bands of BATHO, BSI, and LUMI are also determined. It is shown that when BATHO is formed its transition dipole moves away from the original cis band transition dipole direction. The transition dipole then moves roughly twice as much towards the original cis band direction when BSI appears. Production of LUMI is associated with return of the transition dipole almost to the original orientation relative to the cis band, but with some displacement normal to the plane which contains the previous motions. The correlation between the lambda max of an intermediate and its transition dipole direction is discussed.

Retinal Schiff base position relative to the surfaces of photoreceptor disk

Surface-enhanced Raman spectra of native and photobleached bovine rod outer segment disks as well as inside-out (inverted) photoreceptor disks adsorbed on silver hydrosol have been analyzed. Surface-enhanced spectra of inverted disks and disk-monoclonal antibody complexes reveal the short-range mechanism of enhancement. The distance between retinal Schiff base and the cytoplasmic side of native disk has been shown to be 5-10 A.

Evidence for a common BATHO-intermediate in the bleaching of rhodopsin and isorhodopsin

Nanosecond transient spectroscopic measurements of the BATHO products formed from photolysis of bovine rhodopsin (RHO) and isorhodopsin (ISO) are discussed. BATHO absorption spectra of RHO and ISO differ slightly, but both pigments exhibit wavelength maxima near 560 nm. An additional transient absorption at 440 nm is observed immediately following excitation of RHO but not ISO. The decay times and Arrhenius activation energies of the two 560 nm absorbing transients are the same. In addition to spectral differences in the photolysis of RHO and ISO, the bleaching yield as a function of 532 nm laser power is different, with the yield in RHO saturating at lower laser power than ISO. The bleaching yield of the two pigments has been modeled using the known extinction coefficients and quantum yields for the interconversion of RHO, ISO, and a single BATHO species. Agreement between experiment and the model is found if the effects of the laser polarization are considered. The data are consistent with a common BATHO in the photolysis of RHO and ISO.

Primary intermediates of rhodopsin studied by low temperature spectrophotometry and laser photolysis. Bathorhodopsin, hypsorhodopsin and photorhodopsin

The primary photochemical processes of rhodopsin studied by low temperature spectrophotometry and picosecond laser spectroscopy in our group was summarized. Low temperature spectroscopic experiments demonstrated that the retinylidene chromophores of hypso- and bathorhodopsins are in a twisted all-trans forms. Excitation of rhodopsin with 532 nm laser pulse (width: 25 psec) yielded a new bathochromic photoproduct "photorhodopsin"; its spectrum was located at longer wavelengths than that of bathorhodopsin. Photorhodopsin decays to bathorhodopsin with time constants of about 200 psec in squid and 40 psec in cattle. Squid and octopus hypsorhodopsins were produced within 25 psec by high energy pulse, but not by low energy pulse. Thus hypsorhodopsin is produced by two photon reactions (sequential two photochemical reactions) and decayed to bathorhodopsin with time constant of 125 psec.