Primary step in the bacteriorhodopsin photocycle: photochemistry or excitation transfer?

The absorption polarization of the first intermediate (K610) formed at room temperature in the proton-pumping photochemical cycle of bacteriorhodopsin (bR) shows a strong correlation with the polarization direction of the photolyzed parent molecule. The results suggest that, unlike other photosynthetic systems, excitation transfer does not take place prior to the primary photochemical change in bR. These observations together with the previously observed circular dichroism and the polarization temperature dependence are discussed in terms of the exciton structure and the nature of the absorption bandwidths (i.e., homogeneous vs. inhomogeneous) of the bR monomers within the trimer structure.