The lateral diffusion constants of 1-palmitoyl-2-oleoyl-sn-glycero-3 phosphocholine (POPC), water, and ibuprofen were measured in multilamellar liposomes using pulsed field gradient magic-angle spinning (PFG-MAS) (1)H NMR. The analysis of diffusion data obtained in powder samples and a method for liposome curvature correction are presented. At 322 K POPC has a diffusion constant of (8.6 +/- 0.2) x 10(-12) m(2)/s when dehydrated (8.2 waters/lipid) and (1.9 +/- 0.1) x 10(-11) m(2)/s in excess water. The diffusion constant of water in dehydrated POPC was found to be (4.7 +/- 0.1) x 10(-10) m(2)/s. The radius of curvature is 21 +/- 2 microm for the dehydrated sample and 4.5 +/- 0.5 microm for POPC sample containing excess water. The activation energies of diffusion are 40.6 +/- 0.4 kJ/mole for dehydrated POPC, 30.7 +/- 0.9 kJ/mole for POPC with excess water, and 28.6 +/- 1.5 kJ/mole for water in dehydrated POPC. The diffusion constants and activation energies for a sample of POPC/ibuprofen/water (1:0.56:15) were also measured. The ibuprofen, which locates in the lipid-water interface, diffuses faster than POPC but has a slightly higher activation energy of lateral diffusion. Within certain restrictions, PFG-MAS NMR provides a useful method for characterizing membrane organization and mobility.

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 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 minutes and reduces t 1 -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.

The role of electrostatic forces in indole-lipid interactions was studied by (1)H and (2)H NMR in ether- and ester-linked phospholipid bilayers with incorporated indole. Indole-ring-current-induced (1)H NMR chemical shifts of lipid resonances in bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, 1,2-dioleoyl-sn-glycero-3-phosphocholine, 1,2-di-O-octadecenyl-sn-glycero-3-phosphocholine, and 1,2-di-O-octadecenyl-sn-glycero-3-phosphomethanol show a bimodal indole distribution, with indole residing at the upper hydrocarbon chain/glycerol region of the lipid and near the choline group, when present. (2)H NMR of indole-d(7)-incorporated lipid bilayers reveals that the former site is occupied by about two-thirds of the indole, which adopts a distinct preferred orientation with respect to the bilayer normal. The results suggest that the upper hydrocarbon chain/glycerol location is dictated by many factors, including interactions with the electric charges and dipoles, van der Waals interactions, entropic contributions, and hydrogen bonding. Indole diffusion rates are higher in lipids with ester bonds and lower in choline-containing lipids, suggesting that interactions between indole and carbonyl groups are of minor importance for lipid-indole association and that cation-pi interactions with choline drive the second indole location. Nuclear Overhauser effect spectroscopy cross-relaxation rates suggest a 30-ns lifetime for indole-lipid associations. These results may have important implications for sidedness and structural transitions in tryptophan-rich membrane proteins.