Light-dependent pH gradients are generated in liposomes containing ferrocyanide.

Ferrocyanide solutions absorb light energy in a photochemical reaction that causes cyanide radicals to be lost from the iron complex. Because cyanide is a weak acid (pKa = 9.2), at pH ranges between 6 and 9 it associates with protons, thereby causing the pH of its solutions to increase. The pH change is proportional to the photon flux and concentration of ferrocyanide. For instance, the pH of an unbuffered 1.0 mM potassium ferrocyanide solution increases from 6 to 9 within seconds after illumination. We have encapsulated ferrocyanide in liposomes, and demonstrated that light-dependent pH gradients were readily produced, alkaline inside. This preparation offers a convenient system for generating pH gradients across lipid bilayer membranes. It also provides a useful model for simulating primitive bioenergic systems relevant to light energy transduction by early forms of life.