Lipid hydration: headgroup CH moieties are involved in water binding

To explore the interaction potential of phospholipids, we have studied the hydration of diacyl phosphatidylcholine (PC) and methylphosphocholine (MePC), a pertinent model compound, by ir spectroscopy. Related ab initio Hartree-Fock calculations were performed for MePC. Water is considered ideal as a relevant probe molecule. Spectroscopic data for MePC reveal a strong influence of bound hydration water not only on the phosphate groups but also onto the putatively apolar CH(n) groups. The same could be demonstrated for deuterated dimyristoyl PC taken as a "complete" lipid molecule: both headgroup methyl and methylene moieties are gradually, but remarkably affected by hydration, as evidenced by strong wavenumber upshifts of C-H stretching vibration bands. These findings may originate in directed interactions of the CH(n) groups with bound water molecules, but hydration-driven conformational changes of PC headgroups could also occur. The results of the ab initio calculations rationalize the first explanation by predicting a substantial contribution of specific C-H...OH(2) interactions, mainly characterized by a dramatic loss of electron density of the sigma* antibonding molecular orbitals of C-H bonds. Hence, the propensity of the lipid headgroup methyl and methylene groups to act as donor sites in hydrogen bonding must no longer be ignored when considering the interaction potential of PCs.