Charge reversal by salt-induced aggregation in aqueous lactoferrin solutions

We have observed salt-induced aggregation in lactoferrin solutions using dynamic light scattering (DLS). Aggregates start to form once the ionic strength exceeds 10 mM, and are of opposite charge to their monomer building blocks. The presence of aggregates was monitored by electrophoretic measurements, in which the change of isoelectric point in lactoferrin solutions was observed and found to depend on the concentration of background electrolyte. Complimentary atomic force microscopy (AFM) imaging of adsorbed lactoferrin films demonstrated that for negatively charged surfaces (mica, glass) the topography of the adsorbed film remains invariant to changes in ionic strength, whilst for positively charged surfaces (chitosan coated mica) we observed a salt-induced transition in deposited architecture, with ∼100 nm aggregates being deposited together with monomers for ionic strengths in excess of 10 mM. The size of aggregates observed with AFM is consistent with those observed using DLS. These results suggest that negatively charged lactoferrin aggregates adsorb only onto positively charged surfaces, whereas isolated lactoferrin molecules are sufficiently amphiphilic and adsorb at surfaces of either charge, although without producing a charge inversion effect.