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Alteration of pk a of the bacteriorhodopsin protonated schiff base a study with model compounds

, : Alteration of pk a of the bacteriorhodopsin protonated schiff base a study with model compounds. Biochemistry 25(18): 5249-5258

Factors influencing the pKa value of retinal protonated Schiff base (RSBH+) are examined by using fluorinated alcohols and series of retinals bearing nonconjugated positive charges along the polyene. It is shown that the effective pKa of RSBH+ is increased by a solvent forming a strong hydrogen bond, that stabilizes the anion but weakly interacts with the Schiff-base proton. A positive charge in the vicinity of the Schiff-base linkage markedly reduces the effective pKa. The effect is significantly enhanced in fluorinated alcohols in which positive charges are weakly solvated. It is suggested that drastic pKa reduction might take place during bacteriorhodopsin (bR) photocycle either by elimination of hydrogen-bonding stabilization or by a positive charge approaching the Schiff-base linkage. Weak solvation of the positively charged Schiff-base nitrogen (relative to ethanol solution) and strong solvation with its counterion lead to a red shift in the absorption maximum of retinal protonated Schiff base up to ca. 2400 cm-1 in hexafluoro-2-propanol relative to ethanol. This mechanism of introducing red shift in the absorption maximum of RSBH+ might play a role in determining part of the opsin shift found in bR and the red shift observed in the transformation from the bR570 to K610 intermediate following light absorption. Nonconjugated positive charges shift the absorption maximum of RSBH+. Their influence is further enhanced with fluorinated alcohols as solvents.


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