Association of Enzymatically and Nonenzymatically Functionalized Caseins Analyzed by Size-Exclusion Chromatography and Light-Scattering Techniques.

The influence of covalent protein modifications resulting from the Maillard reaction (glycation) of casein and lactose on the noncovalent association behavior of the protein was studied. Nonenzymatic cross-linking with methylglyoxal (MGO) and glutaraldehyde (GTA) as well as enzymatic cross-linking with microbial transglutaminase (mTG) was investigated in comparison. Molar mass, particle size, and conformational characteristics of nonmicellar casein associates as well as the extent of intraparticle protein cross-linking were examined utilizing size-exclusion chromatography (SEC) combined with UV detection and static and dynamic light scattering. Cross-linking resulted in the stabilization of a certain fraction of casein associates, with particle sizes of approximately 30 nm in radius of gyration (), and promoted an incorporation of further casein molecules into those particles, yielding molar masses () of 1.0-1.2 × 10 g/mol. When caseins were additionally conjugated with lactose during the early Maillard reaction, a further growth of the associates up to approximately 50 nm in with a of 2.1 × 10 g/mol was observed. Furthermore, glycation reactions induced a transition from slightly elongated, random-coil structures toward more anisotropic conformations. Associates consisting of caseins cross-linked with GTA appeared to preserve the original particle conformation.