Co-acervates of lactoferrin and caseins

Abstract

On mixing positively charged lactoferrin (LF) with negatively charged caseins (*CN) it is observed that complexes are formed. The * stands for α, β, κ or Na. The size of the complex co-acervates appears to grow indefinitely and asymptotically near the point of charge equivalency. Away from the charge equivalent ratio it seems that build-up of (surface) charges limits complex size. We proposed a simple scaling law so as to predict the size of the complex. By assuming that surface charge density is constant or can reach only a maximum value, it follows that scattering intensity is proportional to |(1 − x/x_crit)|⁻³ where x is the mole (mass) fraction of the cationic protein and x_crit the value of the mole (mass) fraction at the charge equivalent ratio. Both scattering intensity and particle size obey this simple assumption. We investigated three different caseins, all of which formed co-acervate complexes with LF, but at different molar ratios. Critical composition varied inversely with pH, showing that charge neutrality is the determining factor. Sodium caseinate formed complexes as well but the growth was limited, presumably due to the intrinsic surfactant properties of whole casein. Adding NaCl diminishes the interaction and above 0.4 mol L⁻¹ of NaCl no β-CN–LF complexes are formed. The charge neutral composition shifts to the LF side on adding NaCl, probably because the casein can wrap around the LF more effectively.