Effect of pH buffer molecules on the light-induced currents from oriented purple membrane

Area of Science:

• Biophysics
• Photochemistry
• Membrane Protein Dynamics

Background:

• Purple membranes contain bacteriorhodopsin, a light-driven proton pump.
• Proton expulsion during the photocycle generates photocurrent components.
• The microsecond photocurrent component (B2) is sensitive to its environment.

Purpose of the Study:

• Investigate the impact of pH buffers on the B2 photocurrent component.
• Characterize buffer-induced alterations in photocurrent waveforms.
• Elucidate the mechanisms behind buffer effects on membrane charge movement.

Main Methods:

• Studied oriented purple membranes under low salt conditions (<10 mM).
• Utilized pH buffers with varying pK values and charge states.
• Measured photocurrent components using electrophysiological techniques.
• Investigated the influence of divalent cations (e.g., Ca2+) and surface potential.

Main Results:

• pH buffers dramatically alter B2 waveform via protonation by expelled protons.
• Buffers with two positive charges in protonated form induce a negative current (N component).
• Other buffers induce a positive current (P component), a mirror image of the N component.
• Buffer effects depend on buffer pK, concentration, and are inhibited by divalent cations.
• Surface potential modulates component kinetics but not amplitude.

Conclusions:

• Buffer protonation and subsequent movement toward the membrane generate counter currents.
• The observed buffer effects are explained by light-induced proton transfer and buffer molecule dynamics.
• Divalent cations and surface potential play regulatory roles in membrane charge transport.