Small-Angle Neutron Scattering Study of a Phosphatidylcholine-Phosphatidylethanolamine Mixture

Abstract

The properties of single-component phospholipid lipid bilayers have been extensively characterized. Natural cell membranes are not so simple, consisting of a diverse mixture of lipids and proteins. While having detailed structural information on complex membranes would be useful for understanding their structure and function, experimentally characterizing such membranes at a level of detail applied to model phospholipid bilayers is challenging. Here, small-angle neutron scattering with selective deuteration was used to characterize a binary lipid mixture composed of 1,2-dimyristoyl-3-sn-glycero-phosphatidylcholine and 1,2-dimyristoyl-3-sn-glycero-phosphatidylethanolamine. The data analysis provided the area per lipid in each leaflet as well as the asymmetry of the composition of the inner and outer leaflets of the bilayer. The results provide new insight into the structure of the lipid bilayer when this lipid mixture is used to prepare vesicles.

Figure 1

Schematic of the four-layer self-consistent slab model used for the SANS data analysis.

Figure 2

(A) SANS data collected for the 7:3 DMPC/DMPE vesicles as a function of temperature. The curves are 30 °C (black square), 40 °C (red circle), 45 °C (blue triangle), 50 °C (green down-pointing triangle), and 55 °C (magenta diamond). The SANS data set collected at 55 °C was presented previously in Figure 3A of reference 35 and is reused with permission Copyright 2022, by the author under the CC-BY 4.0 license (https://creativecommons.org/licenses/by/4.0/). The solid black lines in panel (A) are example model intensity profiles found during SANS data analysis. The data were offset from the 30 °C data for clarity. The vertical dashed lines are provided as guides for the eye. (B) Zoomed-in view of the SANS data collected at 55 °C (deg) and example best fit model curves for the three models described in Section 2.4. Three model curves are presented: the asymmetric model (solid black line), the symmetric composition model that can have different A_L values in the leaflets (solid red line), and the fully symmetric model (blue dotted line). The small, sharp feature in the two symmetric model curves near 0.20 Å^–1 visible on the broad peak due to the bilayer structure is an artifact of the underlying mathematical functions of Python used in the model calculation that has been convoluted with the EQ-SANS q-resolution (see Section 2.4). Similar features at different locations in q can be seen in panel (A).

Figure 3

(A) A_L determined by fitting the SANS data and (B) Δ parameter as a function of temperature from the SANS data analysis. In panel (A), A_L,in and A_L,out from the SANS data fitting are in red and blue, respectively. The average A_L,ave is black.

Figure 4

Simplified representation of the vesicle structure at 55 °C. DMPC is colored white and gray, while DMPE is colored shades of blue. The left image shows the vesicle at a scale with a 90° wedge removed to reveal the content of the inner and outer leaflets. The right image shows a zoomed-in view of the top of the vesicle that better presents the differences in A_L and leaflet thicknesses of the inner and outer leaflets. The image was rendered using Persistence of Vision Raytracer software from an input file produced by software written for the generation of this kind of vesicle schematic that has been used previously.^, The software was modified to show the differences in the structures of the inner and outer leaflets.