An intramolecular hydrogen bond with large proton polarizability within the head group of phosphatidylserine. An infrared investigation.

Films of O-phospho-L-serine-P-ethylester (PSE) were studied by infrared spectroscopy. PSE films were studied pure and as 1:1 mixture with LiOH, NaOH, KOH, and Ca(OH)2 as a function of the degree of hydration. The same investigations were performed if (L-glu)n was added to the system (ratio 1:1, PSE/glu residue). In the PSE molecules an intramolecular (I) COOH...-OP in equilibrium with COO-...HOP (II) hydrogen bond is present. In this bond a double minimum proton potential occurs and it shows large proton polarizability. This hydrogen bond is relatively stable as shown by the neutralization experiments. At low degree of hydration the cations are present at the phosphate groups. The Li ions polarize the intramolecular hydrogen bonds much more than the other cations, i.e., the weight of the proton-limiting structure COOH...-OP is increased by Li ions. Regarding these results one has to assume that such a hydrogen bond is also present in the phosphatidylserine head groups. It is discussed that such hydrogen bonds could be part of a lateral charge-conducting system in the polar surfaces of biological membranes. Such systems could connect proton-creating and proton-consuming centers at the membrane surface and conduct positive charge at an extremely high rate.