Kinetics of aggregation of αs1 -casein/ca2+ mixtures: charge and temperature effects

Time-dependent light-scattering studies have been made on mixtures of α_s1 -casein and Ca²⁺ at fixed temperature over a range of Ca²⁺] and α_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 monomeroctamer 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 α_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 α_s1 -casein/Ca²⁺ complex caused by changed binding of Ca²⁺ at different temperatures.