Elucidation of interactions of lysozyme with whey proteins by Raman spectroscopy

The molecular structure, conformational changes and interactive bonds in bovine whey proteins α-lactalbumin and β-lactoglobulin and hen egg lysozyme (15% w/v in D2O pD 6.8) in isolation and in binary combination were examined by Raman spectroscopy. Aggregation was observed in mixtures of lysozyme with either α-lactalbumin or β-lactoglobulin (mixed 1:1 weight ratio) which indicated bonds in addition to the reported electrostatic interactions between the oppositely charged proteins (Howell et al., 1995). Both the lysozyme-α-lactalbumin and lysozyme-β-lactoglobulin complexes showed the involvement of hydrophobic interactions by intensification of spectral bands assigned to CH & CH2 bending vibrations and decrease in the intensity of bands assigned to Trp residues in a nonpolar environment. Changes in the disulphide stretching vibrations of cystine residues as well as lower contents of both helix and sheet structures were also observed. However, the complex with α-lactalbumin had characteristic changes in its Tyr residues not shown in the complex with β-lactoglobulin. At 20°C a mixture of α-lactalbumin and β-lactoglobulin did not indicate interaction. On heating to 90°C for 30 min gelation of the three proteins and their mixtures was accompanied by changes in the disulphide bonds, in aromatic and aliphatic CH groups involved in hydrophobic interactions, and in secondary structure especially formation of β-sheet-like structures. However, interactions of lysozyme with either α-lactalbumin or β-lactoglobulin whey proteins resulted in experimental spectra differing from the theoretically calculated average spectra of the component individually heated proteins. In contrast, the overall conformational and structural changes resulting from heating of the α-lactalbumin or β-lactoglobulin mixture did not differ greatly from those observed in the individually heated proteins.