Are Phospholipids Stabilized by Short, Strong H-Bonds?

Abstract

Short-strong H-bonds are defined as H-bonds between anions such as the carboxyls of maleate held proximal by a proton and maleate's cis-doublebond. They are stabilized by the combined resonance of the anions they connect.Bilayers in living membranes are typically stabilized by P-lipids or glycolipids (G-lipids). G-lipids, examined by Xray & NMR, show that in addition to the extensive van der Waals bonding between chains, the headgroups have multiple H-bonding between the sugars. P-lipids can be zwitterionic, PC & PE. However most P-lipids (PG, PS, PI, etc.) have anionic repulsive headgroup interactions. Yet bilayers made of P-lipids have physical properties, Tm's, etc., similar to those of the G-lipids. In chloroplasts and many bacterial membranes all of the P-lipids are either uncharged (G-lipids) or anionic! Nonetheless the Xray (& NMR) data both show that all the P-lipid headgroup conformations are identical: 3 glycerol carbons perpendicular to the membrane plane with the phosphate on top and the primary chain ester directly below them. The polar R-group is attached to the apical phosphate. This high density of surface anions must result (Guoy-Chapman) in a high density of protons at the uniformly erect phosphates at the membrane surface. Together with the high proton density this suggests short-strong H-bonds between the phosphates such as those formed in stable oleic acid bilayers (Haines, TH, PNAS 80,160 (1983)) and in cardiolipin conformed in bilayers. (Haines, BBA-Biomem-branes 1788, 1997-2002 (2002)) In the data shown in the PNAS article, the number of protons oleate needs to form bilayers varies from 20 to 80% of the total anions (carboxyls). I propose that the erect phosphates trap fleeting protons between the anions, stabilizing a phosphate ionic sheet for bilayer formation such as occurs in oleic acid bilayers.