Bacteriorhodopsin, boundary lipid and protein conformers: a spin label study.

A spin label study, as a function of temperature, has been made with the bacteriorhodopsin membrane using a stearic acid spin label. The ESR spectra show a strong variation with temperature and the presence of isosbestic points. The spectra are interpreted as indicating the presence of a two-component system with an activation energy (approx. 14 kcal/mol) corresponding to a protein conformational change. This activation energy is similar to that deduced from recent flash photolysis studies. It is concluded that the spin label is sensitive to the temperature-dependent protein conformational change in this membrane system.