Hydration of POPC bilayers studied by 1H-PFG-MAS-NOESY and neutron diffraction

The stability of lipid bilayers is ultimately linked to the hydrophobic effect and the properties of water of hydration. Magic angle spinning (MAS) nuclear Overhauser enhancement spectroscopy (NOESY) with application of pulsed magnetic field gradients (PFG) was used to study the interaction of water with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayers in the fluid phase. NOESY cross-relaxation between water and polar groups of lipids, but also with methylene resonances of hydrophobic hydrocarbon chains, has been observed previously. This observation led to speculations that substantial amounts of water may reside in the hydrophobic core of bilayers. Here, the results of a quantitative analysis of cross-relaxation in a lipid 1-palmitoyl-2-oleoyl-sn-glycero-3 phosphocholine (POPC)/water mixture are reported. Coherences were selected via application of pulsed magnetic field gradients. This technique shortens acquisition times of NOESY spectra to 20 min and reduces t1-spectral noise, enabling detection of weak crosspeaks, like those between water and lipids, with higher precision than with non-gradient NOESY methods. The analysis showed that water molecules interact almost exclusively with sites of the lipid–water interface, including choline, phosphate, glycerol, and carbonyl groups. The lifetime of lipid–water associations is rather short, on the order of 100 ps, at least one order of magnitude shorter than the lifetime of lipid–lipid associations. The distribution of water molecules over the lipid bilayer was measured at identical water content by neutron diffraction. Water molecules penetrate deep into the interfacial region of bilayers but water concentration in the hydrophobic core is below the detection limit of one water molecule per lipid, in excellent agreement with the cross-relaxation data.