Study of calcium-soy protein interactions by isothermal titration calorimetry and pH cycle.

The aim of this work was to understand Ca-induced soy protein (nonhydrolyzed, NH; or hydrolyzed, H) aggregation and to characterize the involved interactions using ITC and pH cycle. The endothermic signals obtained upon titration of soy proteins with Ca were fitted with a one set of sites model. NH soy proteins bound more Ca than H soy proteins ( approximately 52 and approximately 2 mg of Ca/g of proteins, respectively). The binding constant K indicated the easier Ca binding onto H soy proteins than for NH soy proteins. The exothermic part involved by electrostatic interactions was completely hidden by the strong endothermic signal from the water molecule release. Ca binding onto soy proteins should be described as a H(+)/Ca(2+) exchange. Whatever the soy proteins, the positive value of heat capacity changes indicated a reduction in the number of surface-exposed polar residues. Ca-induced soy protein aggregation was irreversible for pH cycle to 3.5.