Raman Spectroscopic Study of Casein Structure

The secondary structure of caseins was investigated with resolution-enhanced laser Raman spectroscopy. Raman spectra in the 1580 to 1720 cm -1 region were obtained from the following lyophilized proteins: 1) asl-casein , 2) /3-casein, 3) a natural mixture of bovine whole casein, and 4) micelles of the natural mixture in the presence of Ca 2+ ions. In addition, /3-casein was also investigated in D20 solution. The spectra obtained were Fourier deconvolved and curve fitted with Gaussian components. The results suggest that both as1 and /3-casein have around 10% helical structure, around 20% /3-structure, and from 20 to 35% turns. The turns are clearly distinguishable from the moiety usually called undefined, random, or structureless. Freeze-dried micelles in the presence of Ca 2+ ions and submicelles in the presence of K + ions appear to contain an increased amount of turns and of /3-structure as compared with the as1-and 3-caseins. The increase in turns is at the expense of the amount of undefined structure. All conformational designations here are based on spectroscopic assignments derived from crystallized proteins with well characterized structures. These designations thus have a more qualitative, descriptive meaning for caseins than for other milk proteins, such as a-lactalbumin or 3-1actoglobulin.