The directional absorption properties of rhodopsin and its photoproducts

The experimental data on the absorption of a plane polarised light by a solution of cattle rhodopsin at −196‡ C have been theoretically analysed to model the directional absorption properties of rhodopsin and its photoproducts. It is seen that these molecules behave like planar absorbers having a ratio of about 100∶7 between the extinction coefficients along the long axis and perpendicular to it. Using this result and the experimental observations on absorption and dichroism in the retina in situ, a model for the configuration of chromophores in the disc membranes has been derived. In this model the plane of the chromophore is perpendicular to that of the disc and the long axis of the chromophore makes an angle of 6.6‡ with the plane of the disc. The solution of the problem depends on the assumption that the absorption axes are the same for the rhodopsin, prelumirhodopsin and isorhodopsin. Work partially supported by Department of Science and Technology (India)