NUCLEAR-MAGNETIC-RESONANCE STUDIES OF LIPID HYDRATION IN MONOMETHYLDIOLEOYLPHOSPHATIDYLETHANOLAMINE DISPERSIONS

Solid-state proton nuclear magnetic resonance has been used to examine surface hydration in suspensions of monomethyldioleoylphosphatidyleth anolamine (MeDOPE). The magic-angle spinning (MAS) H-1 spectra for aqu eous suspensions of MeDOPE in the L(alpha) phase exhibited two resonan ces of roughly equal intensity that could be ascribed to water protons , but both their spin-lattice relaxation times and chemical shifts con verged upon conversion to the hexagonal phase. Only a single water pea k was observed for analogous samples of dioleoylphosphatidylcholine (D OPC). MAS-assisted two-dimensional nuclear Overhauser effect spectrosc opy (NOESY) was conducted for multibilayers of both MeDOPE and DOPC. T hrough-space interactions were identified between pairs of lipid proto ns, as expected from their chemical structure. For lamellar suspension s of MeDOPE, positive NOESY cross-peaks were observed between the down field-shifted water resonance (only) and both CH2N and NH2CH3+ protons of the lipid headgroup, These cross-peaks were not observed in the NO ESY spectra of MeDOPE in its hexagonal or cubic phases or for lamellar DOPC reference samples, Taken together, the observation of two water peaks, spin-lattice relaxation behavior, and NOESY connectivities in M eDOPE suspensions support the interpretation that the low-field water peak corresponds to hydrogen-bonded interlamellar water interacting st rongly with the lipid. Such a population of water molecules exists in association with MeDOPE in the lamellar phase but not for its inverted phases or for lamellar dispersions of DOPC.