High resolution nuclear magnetic resonance studies of [formula omitted], β and κ-caseins

Abstract

1. 1. Nuclear magnetic resonance (NMR) studies have been made at 60, 100 and 220 Mcycles/sec of α S1 , β and κ-caseins. These proteins are examples of highly aggregated systems with sufficient internal flexibility to enable high resolution NMR signals to be obtained. 2. 2. The NMR spectrum of β-casein is consistent with previous optical rotatory dispersion and viscosity data, which suggests that a low degree of structural organisation is present in the molecule. Aggregation of β-casein leads to a structure in which the individual amino acid residues must still possess considerable molecular freedom. 3. 3. The NMR spectra of κ-casein differs markedly from β-casein in that signals from the aromatic and terminal methyl protons are fully developed only after heating or treatment with urea. The spectra are consistent with a structure for the κ-casein aggregate in which the aromatic and most of the non-polar residues are restricted in motion, but the hydrophilic groups have considerable molecular motion. 4. 4. Increase of temperature or addition of urea to α S1 -casein leads to a prominent enhancement of the methyl signal from the non-polar residues.