Formation and stability of α-lactalbumin-lysozyme spherical particles: Involvement of electrostatic forces

The formation and the subsequent stability of spherical microparticles resulting from the self-assembly between two oppositely charged proteins, lysozyme (LYS) and apo alpha-lactalbumin (apo alpha-LA), at pH 7.5 and 45 degrees C were studied under different physico-chemical conditions-ionic strength, type of salts, type of buffer. Increasing the ionic strength reduced the ability of the two proteins to interact together and to form microspheres. The formation of such microparticles was completely abolished at an ionic strength of 100 mM. Increasing salt concentration also allowed destabilisation and dissociation of formed microspheres in salt-dependent manner as assessed by turbidity experiments and microscopic observations. Microparticles were destabilised with either NaCl, MgCl2 or CaCl2, but the latter showed the greatest destabilising effect. The higher efficiency of calcium ions in the destabilisation experiment could be attributed to the presence of specific calcium binding site on alpha-LA. Interestingly, complete disappearance of formed particles was not reached even after adding salt concentrations as high as 250 mM of NaCl or 7.5 mM of divalent cations. Our results suggest that electrostatic interactions are clearly involved in the first events of the two-protein assembly and spherical particles building. Moreover, non-electrostatic forces are also involved in maintaining the integrity and stability of formed microparticles. (C) 2008 Elsevier Ltd. All rights reserved.