Analysis of SANS profiles using CREASE to characterize the nanoscopic domains in lipid bilayer vesicles

Lipid bilayers in cellular membranes contain laterally heterogeneous domains rich in saturated lipids known as ‘lipid rafts’. Characterizing the sizes and shapes of lipid rafts to understand their functionality is challenging due to their transient nature and nanometer-scale dimensions. The absence of invasive probes in small-angle neutron scattering (SANS) and its ability to provide structural information at the 10-100 nm length scale have led to its extensive use in the study of lipid vesicles. Additionally, the use of selective deuteration and contrast matching in SANS enables distinguishing between the laterally heterogeneous domains within the lipid vesicle. Interpretation of SANS profiles to obtain physical insights about the lipid vesicles requires a consideration of size dispersity in the vesicles and irregular shapes adopted by the laterally heterogeneous domains within the lipid bilayer. We use ‘machine-learning enhanced computational reverse engineering of scattering experiments’ (i.e., ML-CREASE) to interpret SANS profiles of polydisperse lipid vesicles with irregularly shaped phase boundaries. The SANS data are obtained from measurements on model lipid membranes composed of tail-deuterated DPPC (dDPPC) and DLPC lipids at different temperatures. The temperature variation leads to varying extents of phase separation in the lipid bilayer into gel and liquid domains, rich in dDPPC and DLPC, respectively. The CREASE interpretation confirms that the liquid domains exhibit irregular shapes, and that their composition at each temperature is consistent with the dDPPC-DLPC gel and liquid phase behavior [Reference: Krzyzanowski, Natalie, et al. Membranes 13.3 (2023): 323]. CREASE also predicts decreasing size of liquid domains in the lipid bilayer with decreasing temperature, which is in agreement with the available literature on model lipid membranes [Reference: Heberle, Frederick A., et al. Journal of the American Chemical Society 135.18 (2013): 6853-6859].