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

Abstract

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.

Buried water molecules in helical transmembrane proteins

“…If water randomly occupies polar cavities in TM proteins, then the profile of water binding sites projected on the membrane normal would simply parallel the water activity in the bilayer (assuming that cavities occur randomly). This would result in a symmetric bilobed profile, with the highest water content near the two membrane surfaces and the lowest in the center (Zaccai et al 1975;Gawrisch et al 2007). The transmembrane positions of the water molecules in 30 TM proteins are shown in Figure 6A.…”

Membrane Transport in Primitive Cells

“…The distribution of membrane lipids is asymmetric, with certain classes of lipids, e.g., glycolipids, predominating in one leaflet. Water molecules extensively interact with the hydrophilic exterior of the membrane (Gawrisch et al 2007). The depth of water penetration extends past the head group and glycerol to the carbonyl groups, yielding a hydrophobic membrane interior of approximately 3 nm (Wiener and White 1992;Gawrisch et al 2007).…”

“…Water molecules extensively interact with the hydrophilic exterior of the membrane (Gawrisch et al 2007). The depth of water penetration extends past the head group and glycerol to the carbonyl groups, yielding a hydrophobic membrane interior of approximately 3 nm (Wiener and White 1992;Gawrisch et al 2007). It should be emphasized that biological membranes also contain nonlipid components, with the mass of protein components often exceeding that of lipids (Singer 2004).…”

Heteronuclear chemical shift correlation and J‐resolved MAS NMR spectroscopy of lipid membranes

“…[14] Homonuclear two-dimensional (2D) NOESY-MAS NMR experiments have offered important insights into the organisation of membrane lipids [15 -18] and have been particularly successful when performed in combination with pulsed field gradients. [19,20] Heteronuclear spectroscopy has been used to obtain correlations between carbon chemical shifts and proton dipolar order [21,22] by combining symmetry-based coherence selection [10] with INEPT J-coupling-mediated magnetisation transfer. Carbon proton chemical shift correlations have been obtained under free or frequency-switched LG (FSLG) decoupled proton evolution with CP or INEPT carbon excitation to investigate mixed lipid bilayers of phosphatidylcholine (PC), cholesterol and sphingomyelin.…”