Polarisable Hydrogen Bond Formation and Ionic Interactions in Model Phospholipid Polar Head Molecules

The serine phosphoric acid P-methylester (SPM) and the ethanol-amine phosphoric acid P-methylester (EPM) were synthesized as water soluble models for the functional groups of the corresponding phospholipids. Investigations were made of the aqueous solutions of these molecules as a function of deprotonation and protonation. An intramolecular, easily polarisable hydrogen bond occurs in the zwitterion of the SPM. The solutions of different salts of SPM were studied as well as the influence of counter ion pairs. Counterion pairs hardly influence these bonds. At about 50% deprotonation extremely easily polarisable intermolecular bonds form. At about 100% deprotonation of the zwitterion the hydrogen bonds observed are affected by the presence of CO2. The above is indicated by changes of the bands of the carboxylic and phosphate groups, and in particular by a continuous absorption in the infrared spectrum. During protonation of the EPM easily polarisable intermolecular POH+ ... OP hydrogen bonds form at first, but as protonation increases the solutions become acidic, that is, H5O2 + groupings form.