Structure of bacteriorhodopsin in the acidified membrane and at high ionic strength: resonance Raman study

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Abstract
Resonance Raman (RR) spectra of the retinylidene Schiff''s base (SB) chromophore of bacteriorhodopsin (BR) were obtained from purple membrane (PM) suspensions of Halobacterium halobium in 3 different equilibrium states: (1) the native state at neutral pH; (2) BR-605, the acidified membrane at pH 2; (3) BR-565(Cl-) or BR-545 (F-), at low pH and high ionic strength. In the native state the retinyl chromophore is stabilized by an ion-pair complex of the protonated SB group with a negative carboxylate counterion (A-) of the protein, giving SBH+---A-. In the acidic form, BR-605, it is inferred from the RR spectra that the chromophore is immobilized. This is ascribed to a breakdown of the ionic interaction by protonation of A-. In the two (3) forms the RR spectra show that the original structure of the chromophore in the native state is reestablished, which is attributed to the formation of SBH+---Cl- (F-) complexes with the anions. Thus, the retinyl chromophore in BR is largely governed by the SBH+---A- ion-pair formation, which is stabilized by surrounding water molecules.