Kinetics of aggregation of alpha s1-casein/Ca2+ mixtures: charge and temperature effects

Time-dependent light-scattering studies have been made on mixtures of alpha s1-casein and Ca2+, at fixed temperature over a range of [Ca2+] and [alpha s1-casein], and also as functions of temperature. Measurements were also made of the extent of precipitate formation in the casein/Ca2+ mixtures, using centrifugation. The results are analysed in terms of a monomer-octamer equilibrium between calcium caseinate particles followed by a Smoluchowski aggregation in which only the octamers can participate. The equilibrium constant is dependent upon the charge on the protein/Ca2+ particles, and hence can be related to the extent of binding of Ca2+ to the alpha s1-casein. The Smoluchowski constant is likewise shown to be charge-dependent. The variation of the reaction rate with temperature can be ascribed solely to the changing charge of the alpha s1-casein/Ca2+ complex caused by changed binding of Ca2+ at different temperatures.