The conformation and aggregation of bovine β‐casein A. I. Molecular aspects of thermal aggregation

Abstract

SynopsisThe conformation of (3-casein A in the monomeric and thermally aggregated states has been investigated by a range of techniques. P-Casein exists as a monomer in solution a t 4°C and a t concentrations up to a t least 3 g/dl. The molecule is flexible and exhibits a lot of segmental motion, but its secondary structure is not wholly random coil; about one-third of the polypeptide chain is ordered and the likely locations of these regions are discussed. The radius of gyration, representing the time-average distribution of the flexible chain, is 46 A. Increasing temperature leads to aggregation of the P-casein molecules. The degree of association is very sensitive to experimental conditions, and under our conditions a 14-mer exists a t 20°C. The aggregate is spherical with a radius of about 100 A. The interior of the aggregate is relatively disordered, and the P-casein molecules remain in a largely flexible, hydrated conformation. The volume restriction of the protein molecules which occurs on association leads tvsome immobilization of the hydrophobic C-terminal region, which is packed toward the center of the aggregate.

Study of structure and scaling behavior of chemically unfolded β‐casein by means of small‐angle neutron scattering

“…A previous report has given a value of 69 ± 2 Å at 4 mol L −1 of GdmCl and at 4.4 °C 28. Also, small‐angle X‐ray scattering data yield a predicted radius of gyration of 54 ± 3 Å 34. This value is slightly lower than ours, which was obtained for deuterated solution, but is still within the error bars of our measurements.…”

Study of chemically unfolded β-casein by means of small-angle neutron scattering

“…Previous report has given value 6972 Å at 4 M of GdmCl and at 4.4 1C [18]. Also, small angle X-ray scattering data yield a predicted radius of gyration of 5473 Å [29]. This value is slightly lower than ours, which obtained for deuterated solution, but is still within the error bars of our measurements.…”

Electrochemical impedance spectroscopy study of the adsorption behaviour of α-lactalbumin and β-casein at stainless steel

“…b-CN (M r 23 980) has a random coil configuration with a net charge of À13 at pH 6.7 on the N-termed segment of the molecule while the remainder of the molecule contains no net charge and is very hydrophobic [22]. However, spectroscopic measurements have revealed a moderate helical content and a small content of other types of regular secondary structures [39] with a more compact structure than that expected for a random coil and which tends to aggregate at the higher temperatures [40]. The largest molecule, BSA (M r 66 267, pI 5.2 [41]), consists of a continuous polypeptide chain of 582 amino acids and has an ellipsoidal tertiary structure [41].…”