Unfolding study of native bacteriorhodopsin under acidic condition

In this study, we measured the structural properties of “blue membrane”, which is specific formation of a purple membrane (PM) upon acid titration or removal of cations, by the force curve measurement mode of an atomic force microscopy. The PM fragments were immobilized on a glass substrate and force curve measurements were carried out on the fragments under neutral (pH 7.2) and acidic (pH 2.4) condition. The results revealed that peak positions of the unfolding spectra obtained under acidic condition were shifted to the shorter extension region than those at neutral pH and that the relative position of only the first peak was changed by about 5 nm. These results suggest the possibility that the specific secondary structure is formed at the site from its C-terminus to helices F in acidified PM.