[Effect of pH on circular dichroism and Raman spectroscopy of secondary structure of beta-casein from Chinese human milk]

Abstract

To obtain a structural basis for the beta-casein in Chinese human milk, structural transitions of the beta-casein in response to variation of pH were investigated using Raman and circular dichroism (CD) spectroscopy. Both methods indicated that the secondary structures of beta-casein in the solution were induced by the pH. Secondary structural analysis of beta-casein by CD spectroscopy yielded 0.5%-2% alpha-helical, 16%-18% beta-sheet, 30%-34% beta-turn and 49%-51% random coil contents. Another result was that as pH increases, these structures change. Several distinct transitions were observed by circular dichroism in alpha-helix at pH 8 and pH 10. Raman spectrum also showed random coil as the major secondary structure in native beta-casein, for the characteristic band of the beta-casein amide I was at 1662 cm(-1): Calculations from I850/I830 suggested that the tyrosine residues of beta-casein tended to "exposure". CD and Raman spectra both showed that at neutral and alkaline pH the beta-casein existed predominantly in random coil conformation, and the proportion of alpha-helix was higher at pH 8 than under other pH conditions. Over the range of pH studied, the sheet and turn areas remained relatively constant, and in the condition of pH 8, the content of alpha-helical was higher than in the other pH conditions.