Lipid membrane structure by Small-angle X-ray scattering (SAXS): Scattering density profiles of chemical groups.

Small Angle X-ray Scattering (SAXS) is one of the most suitable experimental techniques to investigate, at nanometer resolution, the structure of self-associating/self-organizing systems formed by amphiphilic molecules. The information that SAXS can provide concerns the shape of the aggregated structures and how they interact.
The electronic density contrast in the membrane structure gives rise to the RX scattering signal. In fact, the SAXS signal is related to the Fourier transform of the electronic density of the system, so that any variation of the electronic density is mapped onto the experimental SAXS intensity and recorded as a function of the scattering angle.
In recent years, our research group has developed the Modified Scattering Density Profile (MSDP) method. Instead of using Gaussian functions, we analyze the data given the probability distribution of chemical groups along the bilayer axis, modeled by a combination of Error functions considered to be more suitable for groups, forming a nearly flat electronic density. The MSDP method was integrated with the GENFIT software, developed by Prof. Spinozzi, which adopts the so-called global fitting strategy: not only a single SAXS curve can be analyzed, but also a complete set of SAXS curves, measured on the same amphiphilic systems varying physical-chemical conditions, analyzed as model parameters and describing the sample condition.
The global fitting approach allows the reduction of the number of free parameters, leading to a more robust and reliable interpretation of the SAXS data, allowing us to extract structural information of the molecules involved. So far we have studied lipids that compose the plasma membrane (such as POPC, POPC-OOH, SM, DPPC...) in order to understand how they behave in the presence of other lipids in the same membrane and how this changes for different compositions.
This research has been supported by FAPESP project 2022/07024-1.