CATALYSIS OF THE RETINAL SUBPICOSECOND PHOTOISOMERIZATION PROCESS IREACID PURPLE BACTERIORHODOPSIN AND SOME BACTERIORHODOPSIN MUTANTS BY CHLORIDE-IONS

The dynamics and the spectra of the excited state of the retinal in ba cteriorhodopsin (bR) and its K-intermediate at pH 0 was compared with that of bR and halorhodopsin at pH 6.5. The quantum yield of photoisom erization in acid purple bR was estimated to be at least 0.5. The chan ge of pH from 6.5 to 2 causes a shift of the absorption maximum from 5 68 to 600 nm (acid blue bR) and decreases the rate of photoisomerizati on. A further decrease in pH from 2 to 0 shifts the absorption maximum back to 575 nm when HCl is used (acid purple bR). We found that the r ate of photoisomerization increases when the pH decreases from 2 to 0. The effect of chloride anions on the dynamics of the retinal photoiso merization of acid bR (pH 2 and 0) and some mutants (D85N, D212N, and R82Q) was also studied. The addition of 1 M HCl (to make acid purple b R, pH 0) or 1 M NaCl to acid blue bR (pH 2) was found to catalyze the rate of the retinal photoisomerization process. Similarly, the additio n of 1 M NaCl to the solution of some bR mutants that have a reduced r ate of retinal photoisomerization (D85N, D212N, and R82Q) was found to catalyze the rate of their retinal photoisomerization process up to t he value observed in wild-type bR. These results are explained by prop osing that the bound Cl- compensates for the loss of the negative char ges of the COO- groups of Asp85 and/or Asp212 either by neutralization at low pH or by residue replacement in D85N and D212N mutants.