Direct determination by 2H-NMR of the ionization state of phospholipids and of a local anaesthetic at the membrane surface.

Protonation and deprotonation of the primary amino group of phosphatidylethanolamine, and of the lipid phosphate groups of phosphatidylethanolamine with phosphatidylcholine, have been observed directly and isothermally in equimolar mixed fluid bilayers of the two phospholipids. In addition, the acid-base titration of the secondary amino group of the local anaesthetic, tetracaine, whilst partitioned into the bilayers, has also been determined. Here we show how studies by deuterium nuclear magnetic resonance of non-perturbing deuterons, specifically placed at the membrane polar-apolar interface, can give direct information about the electrostatics at a membrane surface.