Abstract

Many biological systems are inherently polydisperse. In other words, they present multiple co-existing species with relative populations that are governed by thermodynamic and kinetic laws. Conformational fluctuations, transient biomolecular interactions, or irreversible aggregation processes are examples of biological polydisperse systems. The structural characterization of these phenomena is arduous as traditional approaches are not adapted to study complex equilibriums. Small-Angle X-ray Scattering (SAXS), which probe the overall size and shape of particles in solution, is in principle a very well adapted technique to structurally describe polydisperse systems. Experimental SAXS profiles correspond to averaged values from all co-existing species of the sample. Two strategies to disentangle complex polydisperse biological systems developed in our group will be presented. The first approach uses previous structural information and integrative strategies to build atomistic models of the (supposedly) existing species. These models are subsequently used to describe experimental SAXS data in terms of ensembles of multiple species in equilibrium. The second strategy aims at disentangling complex systems for which no structural information is available. Concretely, the formation of insulin and α-synuclein E46K familial mutant amyloids have been monitored by SAXS in a time-dependent manner. These large SAXS datasets are decomposed using specifically modified chemometric routines to yield the species-pure SAXS profiles and their relative populations linked to the fibrillation kinetics. In both cases an oligomeric species has been detected and characterized. The perspectives and limitations of the approaches used to disentangle complex mixtures will be discussed.