Hydration of the Counterion of the Schiff Base in the Chloride-Transporting Mutant of Bacteriorhodopsin: FTIR and FT-Raman Studies of the Effects of Anion Binding When Asp85 Is Replaced with a Neutral Residue,

Abstract

The chromophores of the D85T and D85N mutants of bacteriorhodopsin are blue but become purple like the wild type when chloride or bromide binds near the Schiff base. In D85T this occurs near neutral pH, but in D85N only at pH 2O and ²H₂O was detected by Fourier transform Raman spectroscopy. The H-bonding strength of the Schiff base in the unphotolyzed state was weaker when chloride or bromide was bound to the mutants than with Asp85 as the counterion in the wild type. Thus, although the geometry of the environment is different, there is at least one water molecule coordinated to the bound halide in these mutants, in a way similar to water bound to Asp85 in the wild type.