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

We have prepared phenylphosphorylcholine and methylphosphorylcholine as small, zwitterionic, H2O soluble analogs of the head groups of phosphatidylcholine molecules, and carried out IR and equilibrium vapor pressure studies on them. Phenylphosphorylcholine forms hydrates containing 2.5, 1.5 and 0.5 mol H2O, and methylphosphorylcholine forms hydrates with 2.5 and 0.5 mol H2O. There is some IR evidence for hydrogen bonding from H2O to the phenyl or methyl substituted oxygens of the phosphate groups, but the effect is slight. Hydrogen bonds in these hydrates do not differ appreciably from values for H2O or ice. Of the five hydrates, only methylphosphorylcholine hemihydrate bonds H2O tightly; however, there is no indication of strong hydrogen bonding in this compound. We conclude that the tenacious retention of the last molecule of H2O by phosphatidylcholine membranes does not arise from hydrogen bonding between H2O and phosphate oxygen.