Retinal isomerization in bacteriorhodopsin is controlled by specific chromophore-protein interactions. A study with noncovalent artificial pigments

It has previously been shown that, in mutants lacking the Lys-216 residue. protonated Schiff bases of retinal occupy noncovalently the bacteriorhodops in (bR) binding site. Moreover, the retinal-Lys-216 covalent bond is not a prerequisite for initiating the photochemical and proton pump activity of t he pigment. In the present work, various Schiff bases of aromatic polyene c hromophores were incubated with bacterioopsin to give noncovalent pigments that retain the Lys-216 residue in the binding site. It was observed that t he pigment's absorption was considerably red-shifted relative to the corres ponding protonated Schiff bases (PSB) in solution and was sensitive to Schi ff base linkage substitution. Their PSB pK(a) is considerably elevated, sim ilarly to those of related covalently bound pigments. However, the characte ristic low-pH purple to blue transition is not observed, but rather a chrom ophore release from the binding site takes place that is characterized by a pK(a) of similar to6 (sensitive to the specific complex). It is suggested that, in variance with native bR, in these complexes Asp-85 is protonated a nd Asp-212 serves as the sole negatively charged counterion. In contrast to the bound analogues, no photocycle could be detected. It is suggested that a specific retinal-protein geometrical arrangement in the binding site is a prerequisite for achieving the selective retinal photoisomerization.