A core‐shell model of calcium phosphate nanoclusters stabilized by β‐casein phosphopeptides, derived from sedimentation equilibrium and small‐angle X‐ray and neutron‐scattering measurements

Calcium phosphate nanoclusters were prepared under standardised conditions using 10 mg ml −1 of the 25‐amino‐acid N‐terminal tryptic phosphopeptide of bovine β‐casein as a stabilising agent. The M r determined by sedimentation equilibrium was 197 600 ± 13 700 and the apparent radius of gyration determined by X‐ray scattering was 2.80 ± 0.05 nm. A small‐angle neutron scattering contrast variation study in 1 H 2 O/ 2 H 2 O mixtures was performed and gave radii of gyration at the calculated match points for the calcium phosphate (88.2 % 2 H 2 O) and phosphopeptide (41.3 % 2 H 2 O) of 3.39 ± 0.08 nm and 1.85 ± 0.05 nm, respectively. Measurements at larger scattering wave vector showed a subsidiary maximum at about Q  = 1.6 nm −1 . The results are consistent with a model of the nanoclusters comprising a spherical core of 355 ± 20 CaHPO 4 2 H 2 O units, density 2.31 g ml −1 and radius 2.30 ± 0.05 nm surrounded by 49 ± 4 peptide chains with a partial specific volume of 0.7 cm 3  g −1 , forming a tightly packed shell with an outer radius of 4.04 ± 0.15 nm. This model suggests that the phosphopeptide is able to arrest the process of growth of the precipitating phase of calcium phosphate at its earliest stages. A similar role for whole casein could be vital to the normal functioning of the mammary gland during milk secretion.