Exploring isotropic tendency for the blue membrane containing wild-type bacteriorhodopsin

Abstract

Bacteriorhodopsin of purple membrane has wide potential applications in bioelectronics and biophotonic nanodevices. Upon acidification, it turns blue and upon further acidification by HCl, it retains its purple color. The acid-induced structural changes might be correlated to its crystalline structure, which might be mediated by lipids of purple membrane. Therefore, the present study aims at revealing the acidic pH dependence of anisotropic properties of bacteriorhodopsin. The electric impedance has been measured for parallel- and perpendicular-oriented purple membrane, in addition to the randomly-oriented one in the acidic pH range. The results have showed that the electric anisotropy is proportional to the color transitions occurred at low pH with consistent pKa values. It has found that the bacteriorhodopsin, upon turning into blue form, tends to be isotropic within narrow pH region around 2.55, whereas it preserves its anisotropy in its purple form. It is noteworthy that several mutants of bacteriorhodopsin that resemble its blue form became attractive in technical applications such as real-time holographic interferometry and optical data storage. Accordingly, such isotropic tendency might implicate bacteriorhodopsin for further potential technical applications.