Photoselection and circular dichroism in the purple membrane

The transient dichroic ratio D = delta A parallel/delta A perpendicular has been measured in the visible absorption region of bacteriorhodopsin in purple membrane by a flash photolysis method. D is found to be wavelength independent throughout the visible absorption band, and reaches a maximum value of 2.75 +/- 0.15 on reduction of the excitation intensity. This value is close to that expected for a single nondegenerate transition dipole moment and is incompatible with the strong exciton coupling model used to explain circular dichroism (CD) spectrum of purple membrane. A time-dependent analysis of the exciton interaction and consideration of the coupling strength suggests an explanation of these observations. It is concluded that excitation interaction between retinals in purple membrane is of the weak or very weak type defined by Förster.