Hydrogen bonds with large proton polarizability in films of ( l-his) n and ( l-lys) n with a model molecule of the head group of phosphatidylserine: proton conduction in polar surfaces of biological membranes

Abstract

Films of ( l-his) n and ( l-lys) n with O-phospho- l-serine- P-ethylester (PSE) (one residue: one PSE molecule) have been investigated by IR spectroscopy. These films were studied pure and as 1:1 mixtures with LiOH, NaOH, KOH and 1:0.5 mixtures with Ca(OH) 2 as a function of the degree of hydration. In the ( l-his) n system without base the intramolecular (I) COOH⋯ −OP⇌COO −⋯HOP (II) bonds in PSE are broken and (I) COOH⋯N⇋ +NH (II) bonds between the PSE molecules and the histidine residues are formed. These hydrogen bonds show large proton polarizability. If hydroxides are present, with the exception of KOH, the formation of these bonds could not be observed. Thus, in hydrophilic surfaces of biological membranes hydrogen bonds with large proton polarizability may form between phosphatidylserine and histidine residues. The possibility is discussed that such hydrogen bonds could mediate the flux of positive charge from a protein into a charge-conducting network formed by head groups of phospholipids. In the ( l-lys) n system intramolecular hydrogen bonds in PSE are partially destroyed since protons transfer to ( l-lys) n residues. NH +⋯N⇋ +NH bonds with large proton polarizability are formed between protonated and non-pronated ( l-lys) n residues. If hydroxides are present, the ( l-lys) n residues are nevertheless partially protonated. With increasing hydration the intramolecular hydrogen bonds in PSE become progressively destroyed by deprotonation.