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 beta-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 (H2O)-H-1/(H2O)-H-2 mixtures was performed and gave radii of gyration at the calculated match points for the calcium phosphate (88.2% (H2O)-H-2) and phosphopeptide (41.3% (H2O)-H-2) 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 CaHPO4 . 2 H2O 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.