Component analysis of the fast photoelectric signal from model bacteriorhodopsin membranes part 4. A method for isolating the B2 component and the evidence for its polarity reversal at low pH

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

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.

Effect of metal ion exchange on the photocurrent response from bacteriorhodopsin on tin oxide electrodes

The transient photocurrent response from bacteriorhodopsin (bR) on tin oxide electrodes was strongly influenced by metal ions bound to bR molecules. The photocurrent polarity reversal pH, which corresponded to the pH value for the reversal of the proton release/uptake sequence in the bR photocycle, of cation-substituted purple membrane (PM) was shifted to lower pH with the increase in the cation affinities to carboxyl groups and a close correlation was noted between the two values. This suggests that the metal ion present in the extracellular region of a bR molecule modulates the pK(a) of proton release groups of bR by stabilizing the ionized state of the proton-releasing glutamic acids. The behavior of photocurrents at light-off in alkaline media, reflecting the proton uptake by bR, was unchanged by binding monovalent (Na(+) and K(+)) or divalent cations (Mg(2+) and Ca(2+)), but was drastically changed by binding La(3+) ions. This can be explained by invoking a substantial slowing of the proton uptake process in the presence of La(3+).

Photocurrent from oriented membrane films containing acid-blue and acid-purple bacteriorhodopsin and its mutants

This paper investigates the fast photocurrent components, B1 and B2, from oriented bacteriorhodopsin (BR) membrane films at low pH, under pulsed laser excitation. Adding chloride ion changes the acid-blue BR to its acid-purple form. In the presence of chloride, the acid-purple BR shows a positive B2 component in the same direction as that of BR at neutral pH, indicating a rapid intramolecular charge transfer. In the absence of chloride, the acid-blue BR shows only a negative B1 with multi-components, indicating a rapid charge separation process associated with retinal photoisomerization. The multi-components in B1 are possibly formed due to the heterogeneity of the acid-blue BR. In addition, BR mutants, D85N and D115N, at low pH and in the presence of chloride, generate the B2 component as well. The observation of chloride-dependent B2 component in various cases at low pH, is in favor of a possible transient chloride ion transfer, although the nature of the charge being transferred cannot be identified so far.