Mechanism of nonlinear photoinduced anisotropy in bacteriorhodopsin and its derivatives.

Polymer films made with photosensitive chromophore protein bacteriorhodopsin (BR) from the extreme halophile Halobacterium salinarium as well as films made with BR derivatives exhibit a nonlinear photoinduced anisotropy. Two different methods can be used to induce anisotropy in polymer BR films. The first method is based on the anisotropic properties of the initial form of the photocycle, BR570 (B-type anisotropy). Another method is based on the anisotropic properties of the longest-lived photocycle intermediate M412 (M-type anisotropy). CW gas lasers were employed to induce a reversible anisotropy in polymer BR films. Nonlinear photoinduced anisotropy is discussed in the context of a model for the anisotropic photoselection of BR molecules under linearly polarized light. A comparison of the experimental dependencies of nonlinear photoinduced anisotropy on laser intensity with similar calculated dependencies enables one to determine the molecular dichroism of BR and its derivatives not only for the initial form of the photocycle, B but also for the longest-lived intermediate M. Here we present the data showing the correlation between the laser induced nonlinear anisotropic properties and chromophore/protein interactions in BR. The effect of polymer binder on the nonlinear photoanisotropic properties of polymer BR films is also described.