Proton binding to biological membranes

Biological membranes contain proton-binding moieties. A laser-induced proton pulse was used to characterize the proton-binding properties of bacterioopsin-containing membranes and of sarcoplasmic reticulum. Different protonation and deprotonation processes occurred. The liberation of protons from pyranine dye and the protonation of the membranes were independent of temperature; the reprotonation of pyranine and proton release from the membranes were temperature dependent. In the cases of membrane-free and membrane-containing systems, the activation enthalpies and entropies were calculated from the decay rates. The activation enthalpy of 16 kJ/mol for reprotonation of pyranine in membrane-free solution is characteristic for a diffusion-controlled process. The value for the membrane-containing systems was nearly double, suggesting that the buffering moieties of the membrane surfaces strongly bind the protons, raising the activation enthalpies. This is possibly an effect of the Coulomb cages formed from closely located proton acceptor sites. The activation entropies were positive in all cases.