Closer Look at Structure of Fully Hydrated Fluid Phase DPPC Bilayers

ABSTRACT

X-ray data are neared for the benchmark dipalmitoylphosphatidylcholine lipid bilayer in the greatest in quantity biologically relevant state in which the bilayers are largely hydrated and in the fluid (liquid-crystalline) phase. Form factors F(q^sub z^) are obtained from a combination of sum of two units sample preparations, oriented stacks of bilayers for q^sub z^ extending to 085 [Angstrom]^sup -1^ and unilamellar vesicles for smaller q^sub z^ Modeling obtains the electron density profile and values for the area through molecule, for the locations of the composing groups, and for the different impressed signs of thicknesses of the bilayer, of that kind as the hydrocarbon thickness and the steric thickness.

The greatest in quantity studied lipid bilayer is dipalmitoylphosphatidylcholine (DPPC) sometimes said to be the inflammable air atom of lipids. The spread in the literature rises at 50?°C emphasizes the difficulty of obtaining structural comes for fully hydrated, fluid phase bilayers (12) This is especially serious for the area by molecule, A, which is a central quantity that plays a pivotal part in simulations (3,4).

The major difficulty in obtaining bilayer mode of building arises from the fluctuations that degrade the Bragg peaks that provide the basic data for the crystallographic approach to membrane conformation A decade ago, we used a liquid crystallographic means to recover the intensity not to be found from the peaks, and we reported a mode of building of DPPC (5). Since then, we have discovered a novel method that focuses on the diffuse scattering from oriented stacks of bilayers (67) This rule provides better primary data to obtain the bilayer form factor F(q^sub z^) mainly because the data stretch out to larger q^sub z^ in reciprocal space, thereby providing better real space resolution to locate more features in the bilayer, on the contrary also because the data are continuous in q^sub z^ instead of discrete as in liquid crystallography. mode of buildings of several lipids have newly been reported using the of recent origin diffuse scattering method (8,9). We now use the of recent origin method, enhanced by data from unilamellar vesicles that are more robust for small q^sub z^ to report an improved constitution of the benchmark lipid DPPC

The basic F(q^sub z^) ensues are shown in Fig. 1 For unilamellar samples, background powerfully exceeds signal as q^sub z^ increases, with equal reason larger uncertainties are assigned and no data are used beyond the next to the first lobe. Complementarily, the scattering from oriented stacks becomes problematic for F(q^sub z^) smaller than 02 [Angstrom]^sup -1^ to be paid to distortion from the real strong h = 2 order reflection (7) with equal reason these data are not used. Larger uncertainties are also applied near obvious distortions from the plain behavior required by the sampling theorem. Comparison of the sum of two units types of data requires a scaling factor that is provided through modeling in Fig. 1, on the other hand the scaling factor depends chiefly on the overlap of the data for 02 [Angstrom]^sup -1^

Fig. 1 also exhibits the F(q^sub z^) results obtained from multilamellar vesicles in our earlier liquid crystallography application of mind (5) that used 21 different samples. solitary three of those samples had athletic enough fourth orders to provide F(qz) for q^sub z^ > 04 [Angstrom]^sup -1^ and those three samples had to be partially dehydrated with osmotic compressing A major point was, owed to the degradation of the fluctuations, the intensities had to be corrected by dint of a factor that became increasingly > 1 for greater lamellar repeat spacings D and as the x-ray order h increased. No correction l to the incorrect be derived that the structure of lipid bilayers changed precipitously as filled hydration was approached. However, comparison with the rises presented here indicates that the intensities were overcorrected, further motivating revisitation of the construction of DPPC in the fluid phase.

Structural be deriveds are obtained by modeling with functional forms for the electron density profiles. Fig. 2 present to views the result of fitting our older hybrid baseline (HB) design and Fig. 3 shows the accrue using a recently developed H2 hybrid prototype (4). These forms have been shown to show the electron density profiles of simulations actual well (4). Both models occupy the method of Mclntosh and Simon (10) that uses well-established gel phase configuration (11) as a reference. These figures exhibit the distribution functions for the submolecular constitutings and the Gibbs dividing surfaces for the various thicknesses of the bilayer.

Numerical values are displayed in Table 1 The area A is not a great quantity [i]or[/i] amount of changed from our previous value of 64 [Angstrom]^sup 2^ (1) This is perhaps surprising in view of the large differences in Fig. 1 on the other hand it was pointed out in Figs. 6 and 11 in Nagle et al. (5) that above or under correcting mostly affects the widths of the molecular distribution functions rather than their positions, for a like reason the gel phase reference means (10) appears to have obtained A reasonably well. However, the older analysis had to use a simpler, depressed spatial resolution model with alone one Gaussian in the headgroup region because the data did not reach forth to high q^sub z^; more structural detail is now obtained.