Distribution of general anesthetics in phospholipid bilayers determined using 2H NMR and 1H-1H NOE spectroscopy

The effect of the general anesthetics halothane, enflurane, and isoflurane on hydrocarbon chain packing in palmitoyl(d31)oleoylphosphatidylcholine membranes in the liquid crystalline phase was investigated using 2H NMR. Upon the addition of the anesthetics, the first five methylene units near the interface generally show a very small increase in segmental order, while segments deeper within the bilayer show a small decrease in segmental order. From the 2H NMR results, the chain length for the perdeuterated palmitoyl chain in the absence of anesthetic was found to be 12.35 A. Upon the addition of halothane, enflurane, or isoflurane, the acyl chain undergoes slight contractions of 0.11, 0.20, or 0.16 A, respectively, at 50 mol % anesthetic. A simple model was used to estimate the relative amounts of anesthetic located near the interface and deeper in the bilayer hydrocarbon region, and only a slight preference for an interfacial location was observed. Intermolecular 1H-1H nuclear Overhauser effects (NOEs) were measured between phospholipid and halothane protons. These NOEs are consistent with the intramembrane location of the anesthetics suggested by the 2H NMR data. In addition, the NOE data indicate that anesthetics prefer the interfacial and hydrocarbon regions of the membrane and are not found in high concentrations in the phospholipid headgroup.