The Structure ofa Lipid Bilayer

High-resolution structures cannot be obtained for bilayers in the liquid crystalline phase because the thermal motion and disorder in the bilayer means that the positions of the atoms are relatively ill-defined. Nevertheless, a picture of the bilayer can be developed, showing the average spatial distribution of atoms or groups of atoms, projected along the direction normal to the bilayer surface (White and Wiener 1995). The structure of a bilayer of dioleoylphosphatidylcho-line (di(C18:l)PE) in the liquid crystalline phase at relatively low hydration (5.4 water molecules per lipid molecule), determined by a combination of X-ray and neutron diffraction methods, is shown in Fig. 6.1 (Wiener and White 1992). The structure is represented by a number of fragments, and Fig. 6.1 shows the fraction of each fragment to be found (the probability of finding the fragment) at any given position along the direction of the bilayer normal. The width of the peak representing each fragment provides an estimate of the range of thermal motion for the fragment, in the direction of the bilayer normal. The narrowest of the regions is that corresponding to the glycerol backbone region, indicating that this is the most rigid part of the structure. Extents of motion generally increase with increasing distance from the backbone, both out to the choline of the headgroup and down the fatty acyl chains to the terminal methyl groups, but the wider distribution for the C=C double bonds than for either the carbonyl or the terminal

Springer Series in Biophysics C.R.Mateo etal. (Ed.) Protein-Lipid Interactions © Springer-Verlag Berlin Heidelberg 2006

Interface Hydrocarbon Interface

choline phosphate glycerol

Distance

Interface Hydrocarbon Interface

choline phosphate glycerol

Distance

Fig. 6.1. The structure of a bilayer of di(C18:1)PC at 23°C at low hydration. The figure shows projections onto the bilayer normal of the time-averaged transbilayer distributions of the principal structural groups. Fragments shown are the methyls (CH3), methylenes (CH2), double bonds (C=C), carbonyls, the glycerol backbone, the phosphate and choline groups ofthe headgroup,and water. Modified from White et al. (2001)

methyl groups implies increased thermal motion in this region of the fatty acyl chain (Wiener and White 1992).

The glycerol backbone region lies at the extreme boundaries of both the methylene and water distributions and thus marks the water-hydrocarbon interface. The combined thicknesses of the interface regions on the two sides of the bilayer, defined as the distance from the choline to the glycerol group, are comparable to that of the hydrocarbon core of the bilayer (Fig. 6.1), so that it is clear that the bilayer cannot be represented as just a "slab" ofhydrocarbon. As described later, the hydrophobic thickness of a fully hydrated bilayer of di(C18:l)PC is signifi-cantlyless than that for the partiallyhydrated sample represented in Fig. 6.1.

Was this article helpful?

0 0
Healthy Chemistry For Optimal Health

Healthy Chemistry For Optimal Health

Thousands Have Used Chemicals To Improve Their Medical Condition. This Book Is one Of The Most Valuable Resources In The World When It Comes To Chemicals. Not All Chemicals Are Harmful For Your Body – Find Out Those That Helps To Maintain Your Health.

Get My Free Ebook


Post a comment