A core-shell model of calcium phosphate nanoclusters stabilized by beta-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 Mr 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 1H2O/2H2O mixtures was performed and gave radii of gyration at the calculated match points for the calcium phosphate (88.2% 2H2O) and phosphopeptide (41.3% 2H2O) 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 x 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 cm3 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.
Holt
C
C
Hannah Research Institute, Ayr, UK.
Timmins
P A
PA
Errington
N
N
Leaver
J
J
D002130:Calcium Phosphates / Q000737:chemistry*; D002364:Caseins / Q000737:chemistry*; D008823:Micelles; D008968:Molecular Conformation; D008970:Molecular Weight; D009502:Neutrons; D010446:Peptide Fragments / Q000494:pharmacology; D010748:Phosphopeptides / Q000302:isolation & purification / Q000494:pharmacology*; D012542:Scattering, Radiation; D014461:Ultracentrifugation; D014965:X-Rays
D002130:Calcium Phosphates; D002364:Caseins; D008823:Micelles; D010446:Peptide Fragments; D010748:Phosphopeptides; C485828:alpha-tricalcium phosphate; C485830:tetracalcium phosphate; C485838:calcium phosphate, monobasic, anhydrous; C020243:calcium phosphate; C485829:calcium phosphate, dibasic, anhydrous