Nature of the primary photochemical events in rhodopsin and bacteriorhodopsin

Abstract

M-AM-Sym-1 The Nature of the Primary Photochemical Events in Rhodopsin and Bacteriorhodopsin R. R. Birge, T. M. Cooper, C. M. Einterz, L. A. Findsen, M. B. Masthay, L. P. Murray, G. A. Schick, and C. F. Zhang. Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213 Recent experimental and theoretical studies of the binding site geometries and the primary photochemical events in rhodopsin and bacteriorhodopsin will be presented. Our experimental studies included two-photon spectroscopy, pulsed laser photocalorimetry and quantum yield measurements. Our theoretical studies included semiempirical molecular orbital theory and molecular dynamics calculations. Spectroscopic studies indicate that the counterion in rhodopsin is not intimately associated with the imine proton in rhodopsin, but lies underneath the plane of the chromophore. Energy storage associated with the rhodopsin e-+ bathorhodopsin phototransformation is measured to be -32 kcal mol[1 and is partitioned theoretically to involve -60% conformational distortion and -40%o charge separation. Energy storage associated with the isorhodopsin -* bathorhodopsin phototransformation is measured to be -27 kcal mol-1. Absolute quantum yield measurements of the isorhodopsin to bathorhodopsin phototransformation at 77K indicate a wavelength dependence that ranges from 0.089±0.021 at 565nm to 0.168±0.012 at 440nm. We attribute this result to the presence of a small activation barrier (-0.2 kcal mol-1) in the 9-cis -+ 9-tans excited state surface. The absence of a wavelength dependence in the rhodopsin -* bathorhodopsin photoreaction indicates a barrier-less photoprocess. Two-photon studies indicate that the binding site of bacteriorhodopsin is more "ionic" than that of rhodopsin. Theoretical calculations suggest the possibility that the J intermediate may be an excited state species, which is generated due to the trapping of photoactivated molecules in a 13-transoid minimum in the excited state surface. Photocalorimetry and photostationary state investigations of light adapted bacteriorhodopsin at 77K indicate that the ratio of the forward (01) to reverse (I2) quantum yields is I1/I2 = 0.445 ± 0.031. We conclude that the forward quantum yield must be less than 0.5.