Structure of a fluid dioleoylphosphatidylcholine bilayer determined by joint refinement of x-ray and neutron diffraction data. I. Scaling of neutron data and the distributions of double bonds and water

Abstract

We described in two previous papers a method for the joint refinement of the structure of fluid bilayers using neutron and x-ray diffraction data (Wiener, M. C., and S. H. White 1991a, b. Biophys. J. 59: 162-173 and 174-185). An essential part of the method is the appropriate scaling of the diffraction data. Here we describe the scaling of the neutron data and the determination of the transbilayer distribution of double bonds in liquid-crystalline (L alpha phase) phospholipid bilayers of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). The distribution was determined by neutron diffraction of oriented multilayers (66% RH) of DOPC specifically deuterated at the 9- and 10-position of both acyl chains. The double-bond distribution is described accurately by a pair of Gaussian functions each located at a position Zcc = 7.88 +/- 0.09 A from the bilayer center with 1/e-halfwidths of Acc = 4.29 +/- 0.16 A. Previously, we determined the transbilayer distribution of bromine atoms in a specifically halogenated lipid, 1-oleoyl-2-9,10-dibromostearoyl-sn-glycero-3-phosphocholine (OBPC), and showed it to be an isomorphous replacement for DOPC (Wiener, M. C., and S. H. White, 1991c. Biochemistry. In press). A comparison of the double-bond and bromine profiles indicates that the positions of the centers of the deuterated double bond and the brominated methylene Gaussian distributions are equal within experimental error and that each label undergoes similar average thermal motions with respect to the bilayer normal. The observation that the average position of a label on both acyl chains (the deuterated double bonds) is similar to the average position of a label on the 2-chain alone (the brominated methylenes) indicates that the maximum separation along the bilayer normal between the double bonds of the acyl chains is 1 A or less. The fully-resolved transbilayer water distribution, previously determined at lower resolution (Jacobs, R. E., and S. H. White. 1989. Biochemistry. 28:3421-3437), was obtained from the analysis of neutron diffraction data of DOPC hydrated with a D20/H20 mixture. The water distribution is described accurately by a pair of Gaussian functions each located at a position Zw = 22.51 +/- 0.77 A from the bilayer center with 1/e-half widths of Aw = 4.63 +/- 0.48A. We present the relative absolute neutron and x-ray structure factors of DOPC at 66% RH that will be used to solve the complete structure of DOPC which will be presented in a later paper of this series.

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

“…Lipid molecules are in tumultuous disorder (Wiener et al 1991), up to the point that terminal methyl groups of hydrophobic hydrocarbon chains occasionally reach the lipid-water interface to exchange magnetization with the choline protons of neighboring lipid headgroups . Such choline-to-chain methyl crosspeaks were also observed for the POPC sample in this study (see Fig.…”

“…5 was used to fit a single Gaussian line to the experimental difference structure factors, ΔF(n), corresponding to measurements in H 2 O and 2 H 2 O, respectively. The position of the water layer was fixed to d/2, with the only fitted parameters being the 1/e half-width of the water distribution (A w ) and the scale factors, which are needed for placing the density profiles on an absolute, per-lipid scale, as described by Wiener and White (Wiener et al 1991). The values of ΔF(n) obtained at 66%, 76%, 86%, and 93% and their standard deviations are provided in Tab.…”

“…Several laboratories have developed the experimental technology and reported data on water distribution, see e.g. (Franks and Lieb 1979;Simon et al 1982;Wiener et al 1991;Zaccai et al 1979). Although, experiments were conducted on different lipids, with or without addition of cholesterol, and somewhat different levels of hydration, there is general agreement that water may penetrate into bilayers down to the level of hydrocarbon chain carbonyl groups, while water content in the hydrophobic bilayer center is below the detection limits.…”

Hydrophobic interactions of peptides with membrane interfaces

“…projected onto an axis normal to the bilayer plane [41]. Complete and fully resolved bilayer structures comprised of the collection of water and structural-group distributions can be determined through the joint re¢nement of X-ray and neutron di¡raction data obtained by using lipids that have been labeled at speci¢c sites with heavy metals [43,50,51] or deuterium [44,52,53].…”

Pore Hydration States of KcsA Potassium Channels in Membranes

“…The water profile, calculated on a per lipid, absolute scale (22), was then modeled and analyzed in terms of a minimal set of Gaussian distributions. We could describe the overall 2 H 2 O profile across the repeat unit (Ϫd/2 to d/2) of samples with KcsA by a minimum set of six Gaussian distributions (three pairs, symmetrically positioned relative to the bilayer center).…”