Structures and interactions among lysozyme proteins below the isoelectric point in presence of divalent ions

Highlights: • Protein-protein interactions strongly depend upon the dissolved ions. • With increasing salt concentration, repulsive interaction decreases gradually. • Attractive interaction increases gradually with increasing salt concentration. • Fractal aggregates also form by a very small amount (≈0.5%). • Fractal structure factor dominates for Na⁺ and Mg²⁺ ions at higher concentration. Small angle neutron scattering study reveals that below the isoelectric point of the globular protein lysozyme and in the presence of different divalent (Mg²⁺, Ca²⁺ and Ba²⁺) and monovalent (Na⁺) ions, the long-range repulsive and the short-range attractive interactions decreases and increases respectively with the increase of salt concentration. In addition with these short- and long-range interactions, fractal aggregates also form even in pure lysozyme solution by a very small amount (≈0.5%). Protein-protein interactions are mostly dominated by the fractal structure factor only for Na⁺ and Mg²⁺ ions at higher salt concentrations as more aggregates are formed.