Importance of bound divalent cations to the tyrosine deprotonation during the photocycle of bacteriorhodopsin.

The transient absorption changes occurring at 297 nm during the photocycles of the deionized and acidified bacteriorhodopsins (blue membranes) were studied. As opposed to what happens during the photocycle of the purple membrane, for the blue membranes only the fast absorption increase corresponding to trans-cis isomerization of the retinal chromophore is present; the slow rise attributed to the tyrosine deprotonation is not observed. The addition of different salts to the deionized membrane restores the original color and causes a tyrosine deprotonation during the photocycle. This suggests that the presence of cations is required for the deprotonation of tyrosine as it is for the deprotonation of the retinylidene Schiff base. These results are discussed in terms of the recently proposed cation model for the observed deprotonation processes in the photocycle of bacteriorhodopsin.