Non-isomerizable artificial pigments: implications for the primary light-induced events in bacteriorhodopsin.

The primary events in the photosynthetic retinal protein bacteriorhodopsin (bR) are reviewed in light of photophysical and photochemical experiments with artificial bR in which the native retinal polyene is replaced by a variety of chromophores. Focus is on retinals in which the "critical" C13=C14 bond is locked with respect to isomerization by a rigid ring structure. Other systems include retinal oxime and non-isomerizable dyes noncovalently residing in the binding site. The early photophysical events are analyzed in view of recent pump-probe experiments with sub-picosecond time resolution comparing the behavior of bR pigments with those of model protonated Schiff bases in solution. An additional approach is based on the light-induced cleavage of the protonated Schiff base bond that links retinal to the protein by reacting with hydroxylamine. Also described are EPR experiments monitoring reduction and oxidation reactions of a spin label covalently attached to various protein sites. It is concluded that in bR the initial relaxation out of the Franck-Condon (FC) state does not involve substantial C13=C14 torsional motion and is considerably catalyzed by the protein matrix. Prior to the decay of the relaxed fluorescent state (FS or I state), the protein is activated via a mechanism that does not require double bond isomerization. Most plausibly, it is a result of charge delocalization in the excited state of the polyene (or other) chromophores. More generally, it is concluded that proteins and other macromolecules may undergo structural changes (that may affect their chemical reactivity) following optical excitation of an appropriately (covalently or non-covalently) bound chromophore. Possible relations between the light-induced changes due to charge delocalization, and those associated with C13=C14 isomerization (that are at the basis of the bR photocycle), are discussed. It is suggested that the two effects may couple at a certain stage of the photocycle, and it is the combination of the two that drives the cross-membrane proton pump mechanism.