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

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Abstract
The absorption polarization of the 1st intermediate (K610) formed at room temperature in the proton-pumping photochemical cycle of Halobacterium halobium bacteriorhodopsin (bR) shows a strong correlation with the polarization direction of the photolyzed parent molecule. 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.