Effect of photoselection upon saturation and the dichroic ratio in flash experiments upon effectively immobilized systems

Abstract

Flash spectroscopy of photochemical or photobiological systems, such as bacteriorhodopsin in purple membrane, for which the chromophore transition dipole moment does not undergo complete reorientation during the time of the flash, is considered as a function of light intensity. Due to photoselection, saturation proceeds very slowly with increasing flash intensity and the linear dichroic ratio decays rapidly from a maximum of three at zero flash intensity. Simple formulae are derived to describe these effects under stringent assumptions. Calculations are also performed which relax the assumptions by taking into account (i) non-zero optical density, (ii) thermal decay of the photoproduct during the flash and (iii) non-zero angle y between the initial chromophore and its photoproduct. Agreement with experiments on bacteriorhodopsin in purple membranes is excellent.