INTERACTION OF ASPARTATE-85 WITH A WATER MOLECULE AND THE PROTONATED SCHIFF-BASE IN THE L-INTERMEDIATE OF BACTERIORHODOPSIN - A FOURIER-TRANSFORM INFRARED SPECTROSCOPIC STUDY

Fourier-transform infrared spectra were recorded at 170 K before and after irradiating the Asp(85)--> Asn mutant of bacteriorhodopsin. The difference spectrum exhibits protein bands such as those due to the perturbations of Asp(96) and Asp(115) and the N-H stretching vibration of tryptophan, characteristic of the L minus all-trans-bacteriorhodopsin spectrum of the wild-type protein. However, some vibrational bands of the peptide backbone and the chromophore are different from L and more characteristic of N of the wild-type protein. Remarkably, the shift observed for the vibrational band due to an internal water molecule upon L formation [Maeda, Sasaki, Shichida, and Yoshizawa (1992) Biochemistry 31, 462-467] is absent. These changes in the spectrum of the mutant could originate from the destruction of a hydrogen-bonding system consisting of Asp(85), the water molecule, and the Schiff base, upon replacement of Asp(85) with asparagine. These observations constitute direct evidence for the interaction of water with Asp(85) at the time when it is protonated by the Schiff base.