Fourier-transform infrared studies on cation binding to native and modified purple membranes

Abstract

Fourier-transform infrared spectroscopy has been used to examine the structural differences in the protein moiety between the native purple and the deionized blue membranes, both at pH 5.0. The spectra demonstrate that deionization of purple membrane decreases the content of the distorted .alpha.II-helices in favor of the more common .alpha.I-helices. Changes in the signals from .beta.-turns are also observed. The changes corresponding to the carboyxl groups suggest that deionization leads to a decrease in the strength of the hydrogen bonds involving carboxyl groups. Most of these effects are reversed progressively upon binding of one to five Mn2+ per bacteriorhodopsin to the deionized membrane. Binding of Hg2+ to the deionized membranes does not restore the purple color but induces global changes similar to, but less intense than, those brought about by Mn2+ binding. However, the effects attributed to the carboxyl groups are opposite to those found for Mn2+. Schiff base reduction or bleaching induces a decrease of the content of the .alpha.II-helix in favor of the .alpha.I-helix and a decrease in the strength of hydrogen bonds to carboyxl groups. Deionization of these modified membranes leads to a further loss in the .alpha.II content. These results indicate a conformational rearrangement of the protein structure between the native purple membrane and the deionized membrane, which could arise from surface potential changes elicited by bound cations. The changes observed in the carboxyl groups suggest that some of them are located structurally close to the retinal environment and may be involved in cation binding.