A deuterium NMR study of labelled n-alkanol anesthetics in a model membrane

The 2H-NMR spectra of 50 wt.% aqueous multilamellar dispersions of dipalmitoylphosphatidylcholine (DPPC) containing either selectively deuterated 1-decanol (25 mol%) or [2H17]-1-octanol (25 mol%) have been measured as a function of temperature. Both alkanols are potent anesthetics. A detailed carbon-deuterium bond order parameter profile of 1-decanol in liquid crystalline phospholipid dispersions at 50 degrees C was determined from the quadrupolar splittings of 1-decanols deuterated at eight different positions. A maximum order parameter SCD = 0.20 was obtained for [5,5-2H2]-1-decanol, with labels at both ends of the 1-decanol exhibiting reduced order parameters. Explanations for the reduced order towards the hydroxyl group of 1-decanol are discussed in terms of either increased amplitudes of motion or geometric effects due to hydrogen bonding. By comparing the order parameter profile of sn-2 chain deuterated phosphatidylcholine dispersions containing 25 mol% 1-decanol (J.L. Thewalt, S.R. Wassall, H. Gorrissen and R.J. Cushley, Biochim. Biophys. Acta, 817 (1985) 355) with the profile of deuterated 1-decanol in DPPC, we estimate that decanol is approximately parallel to the C-3 to C-13 region of the phosphatidylcholine's sn-2 chain. Variation of the spectral moments M1 with temperature indicates that both 1-decanol and 1-octanol are sensitive to the packing of the lipid in which they are dissolved. Below the phase transition temperature, the 2H-NMR spectra of either 1-decanol (selectively deuterated) or 1-octanol (perdeuterated) are broad powder patterns, characteristic of axially symmetric rotation about the alcohol's long axis. This is in contrast to the 2H-NMR spectra obtained from deuterated phosphatidylcholine under similar conditions, which implies that the phospholipid acyl chain conformations are more restricted than those of the alcohol at these temperatures. From the M1 behavior of the various alkanol chain segments with temperature, the gel to liquid crystalline phase transition is seen to initiate in the middle of the DPPC/1-alkanol bilayer.