Some association properties of bovine SH-kappa-casein.

(1) It is shown that kappa-casein association is characterized by a critical micelle concentration which decreases as the ionic strength is increased. (2) The kappa-casein polymer molecular weight was calculated from the weight-average apparent molecular weight by taking into consideration the monomer concentration and the excluded volume. The degree of polymerization is 30 and does not depend on ionic strength between 0.1 and 1 M. (3) The non-electrical contribution to the standard free energy of association is -38 kJ/mol monomer. The electrical part is small: 1-2 kJ/mol monomer depending on the ionic strength and kappa-casein genetic variant. (4) The limitation of size and the size itself of the kappa-casein polymer can be explained by the theory of self-assembly of virus particles by Caspar and Klug (D.L.D. Caspar and A. Klug, Cold Spring Harb. Symp. Quant. Biol. 27 (1962)1). (5) Extending this theory to casein micelle assembly, it is predicted that micelles are distributed preferentially over a restricted number of sizes.