The conformation of α-lactalbumin as a function of pH, heat treatment and adsorption at hydrophobic surfaces studied by FTIR

The secondary structure of alpha-lactalbumin (alpha-la) from bovine milk has been studied using Fourier Transform Infra Red (FTIR) spectroscopy. It was found that the secondary structure of alpha-la, after having being heated above its denaturation temperature, was permanently altered even if the temperature was brought back to room temperature. The reorganized structure still contained a large proportion of the helical structure but it differed from the native structure. The reorganized structure was similar for samples heated at 70 degrees C and 80 degrees C at either pH 6 or 7 and at 90 degrees C at pH 7 for 2 min, but after heating at 90 degrees C at pH 6, the reorganized structure was less well-defined, possibly because of alterations to the disulfide bonds. When alpha-la was adsorbed to an oil-water interface, the 3(10)-helix denatured first, giving an increase in unfolded structure. Additional beta-sheet was formed at the expense of the helical structure. The two different absorption bands for the alpha-helix at 1650 and 1657 cm(-1) became even more distinctive after the adsorption on the interface. The structure of adsorbed alpha-la depended on the concentration of the protein rather than the pH and the age of the emulsion. At higher protein concentration, more beta-sheet structure was formed and the adsorption for 3(10)-helix was shifted from 1639 to 1641 cm(-1), below the frequency of unordered structure. At 1% alpha-la, the 3(10)-helical structure became unordered structure (1643 cm(-1)) and there was less beta-sheet structure. (C) 1998 Elsevier Science Ltd. All rights reserved.