Nuclear Overhauser Enhancement Spectroscopy Cross-Relaxation Rates and Ethanol Distribution across Membranes

Abstract

Measurement of nuclear Overhauser enhancement spectroscopy cross-relaxation rates between ethanol and palmitoyloleoylphosphatidylcholine bilayers was combined with atomic-level molecular dynamics simulations. The molecular dynamics trajectories yielded autocorrelation functions of proton dipole-dipole interactions, and, consequently, relaxation times and cross-relaxation rates. These analyses allow the measured cross-relaxation rates to be interpreted in terms of relative interaction strengths with the various segments of the lipid molecule. We determined that cross-relaxation between ethanol and specific lipid resonances is primarily determined by the sites of interaction with some modulation due to lipid disorder and to local differences in intramolecular lipid dynamics. The rates scale linearly with the lifetime of temporary ethanol-lipid associations. Ethanol interacts with palmitoyloleoylphosphatidylcholine bilayers primarily via hydrophilic interactions, in particular the formation of hydrogen bonds to the lipid phosphate group. There is a weak contribution to binding from hydrophobic interaction with lipid chain segments near the glycerol. However, the strength of hydrophobic interactions is insufficient to compensate for the energetic loss of locating ethanol in an exclusively hydrophobic environment, resulting in a probability of locating ethanol in the bilayer center that is three orders of magnitude lower than locating ethanol at the lipid/water interface. The low cross-relaxation rates between terminal methyl protons of hydrocarbon chains and ethanol are as much the result of infrequent chain upturns as of brief excursions of ethanol into the region of lipid hydrocarbon chains near the glycerol. The combination of nuclear magnetic resonance measurements and molecular dynamics simulations offers a general pathway to study the interaction of small molecules with the lipid matrix at atomic resolution.