Viewing the bilayer hydrocarbon core using neutron diffraction

Abstract

Membrane proteins fold, assemble and function within their native fluid lipid environment. Structural studies of fluid lipid bilayers are thus critically important for understanding processes in membranes. Here, we propose a simple approach to visualize the hydrocarbon core using neutron diffraction and deuterated lipids that are commercially available. This method should have broad utility in structural studies of the bilayer response to protein insertion and folding in membranes.

Figure 1

Absolute structure factors of POPC bilayers with 0, 5%, 10% and 20% POPC^D, as a function of D₂O mole percent. The experimental uncertainties were calculated as described previously (9). The solid lines are the linear fit to the data. For a given D₂O mole percent, a point on the line is the best estimate of the structure factor.

Figure 2

Absolute structure factors of POPC bilayers with 4 mole% FGFR3 TM domain in the absence and presence of 20% POPC^D as a function of D₂O mole percent.

Figure 3

Absolute scattering length density profiles for POPC bilayers with various POPC^D concentrations. (A) 0 and 20 % POPC^D (B) 5 and 20 % POPC^D (C) 10 and 20 % POPC^D. The obvious difference between each pair of profiles is due to the isomorphous replacement of 31 hydrogens with 31 deuterons in the palmitoyl chain of POPC^D, giving the palmitoyl chain distribution across the bilayer.

Figure 4

Difference profiles, obtained via direct subtraction between the pairs of profiles shown in Figure 3. These difference profiles report the palmitoyl chain distribution of POPC. The hydrocarbon core width is defined as the width at 1/e of the maximal amplitude in the difference profiles (inset).

Figure 5

(A) Absolute scattering length density profiles for POPC bilayers with 4 mole% FGFR3 TM domain in the presence (dashed line) and absence (solid line) of 20 mole% POPC^D. (B) Difference profile showing the palmitoyl chain distribution in the presence of 4 mole% FGFR3 TM domain.