Salt-dependent complex formation in lysozyme-alginate mixture

International audience

英语. The associative interaction between oppositely charged macromolecules proceeds through eitherliquid-liquid phase separation (LLPS) forming complex coacervates or liquid-solid phase separation(LSPS) forming aggregates. In this work, we investigated the assembly of the basic protein lysozyme(LYS) with the negatively charged polysaccharide alginate (ALG) at pH 7 under varying conditionsof mixing ratios, total concentrations, and ionic strengths.1The droplet-based millifluidic device coupled with optical microscopy gave an extensive qualitativeanalysis of the phase behaviour of the system by probing different experimental conditions.2 Greylevel analysis associated with the droplet millifluidic experiment allowed both the quantification andthe qualitative definition of phase separation. We constructed a three-dimensional phase diagram,incorporating salt, LYS, and ALG concentrations as coordinates, offering a detailed depiction ofmonophasic regions, liquid-solid and liquid-liquid phase separation domains, and areas ofcoexistence of both solid and liquid phases. The thermodynamic characterization of the formation ofdifferent LYS/ALG assemblies was carried out using isothermal titration calorimetry (ITC) wheredistinct ITC profiles were associated with coacervation and aggregation. The interaction affinity (Ka)for aggregation was three orders of magnitude higher than for coacervation, without significantchange in binding stoichiometry. Structural differentiation of various assemblies in the nanometerrange was achieved through small-angle X-ray scattering (SAXS) experiments. To gain deeperinsights into the mechanisms underlying both LSPS and LLPS processes, further investigation ofadditional polyelectrolyte couples is needed, facilitating a more comprehensive understanding ofthese phase separation phenomena.