Water concentration profiles in membranes measured by ESEEM of spin-labeled lipids

Electron spin echo envelope modulation (ESEEM) spectroscopy of phospholipids spin-labelled systematically down the sn-2 chain was used to detect the penetration of water (D2O) into bilayer membranes of dipalmitoyl phosphatidylcholine with and without 50 mol% cholesterol. Three-pulse stimulated echoes allow the resolution of two superimposed 2H-ESEEM spectral components of different widths, for spin labels located in the upper part of the lipid chains. Quantum-chemical calculations (DFT) and ESEEM simulations assign the broad spectral component to one or two D2O molecules that are directly hydrogen-bonded to the N-O group of the spin label. Classical ESEEM simulations establish that the narrow spectral component arises from non-bonded water (D2O) molecules that are free in the hydrocarbon-chain region of the bilayer membrane. The amplitudes of the broad 2H-ESEEM spectral component correlate directly with those of the narrow component, for spin labels at different positions down the lipid chain, reflecting the local H-bonding equilibria. The D2O-ESEEM amplitudes decrease with position down the chain towards the bilayer center, displaying a sigmoidal dependence on position that is characteristic of transmembrane polarity profiles established by other, less direct, spin-label methods. The midpoint of the sigmoidal profile is shifted towards the membrane center for membranes without cholesterol, relative to those with cholesterol, and the D2O-ESEEM amplitude in the outer regions of the chain is greater in the presence of cholesterol than in its absence. For both membrane types, the D2O amplitude is almost vanishingly small at the bilayer center. The water-penetration profiles reverse-correlate with the lipid-chain packing density, as reflected by 1H-ESEEM intensities from protons of the membrane matrix. Analysis of the H-bonding equilibria provides essential information on the binding of water molecules to H-bond acceptors within the hydrophobic interior of membranes. For membranes containing cholesterol, approximately 40% of the nitroxides in the region adjacent to the lipid headgroups are H-bonded to water, of which ca. 15% are doubly H-bonded. Corresponding H-bonded populations in membranes without cholesterol are ca. 20%, of which ca. 6% are doubly bonded.