HYDROGEN-BONDING .55. IR AND THERMODYNAMIC STUDY OF THE LOWER HYDRATES OF PHENYLPHOSPHORYLCHOLINE AND METHYLPHOSPHORYLCHOLINE

We have prepared phenylphosphorylcholine and methylphosphorylcholine a s small, zwitterionic, H2O soluble analogs of the head groups of phosp hatidylcholine molecules, and carried out IR and equilibrium vapor pre ssure studies on them. Phenylphosphorylcholine forms hydrates containi ng 2.5, 1.5 and 0.5 mol H2O, and methylphosphorylcholine forms hydrate s with 2.5 and 0.5 mol H2O. There is some IR evidence for hydrogen bon ding from H2O to the phenyl or methyl substituted oxygens of the phosp hate groups, but the effect is slight. Hydrogen bonds in these hydrate s do not differ appreciably from values for H2O or ice. Of the five hy drates, only methylphosphorylcholine hemihydrate bonds H2O tightly; ho wever, there is no indication of strong hydrogen bonding in this compo und. We conclude that the tenacious retention of the last molecule of H2O by phosphatidylcholine membranes does not arise from hydrogen bond ing between H2O and phosphate oxygen.