Ren Hao-wei, Zhang Wan-shu, Li Xiang-yi, Liu Ning
Guang Pu Xue Yu Guang Pu Fen Xi. 2015 Feb;35(2):384-9.
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.
为了获得中国母乳中β-酪蛋白的结构基础，利用拉曼光谱和圆二色光谱（CD）研究了β-酪蛋白在pH值变化时的结构转变。两种方法均表明，溶液中β-酪蛋白的二级结构受pH值诱导。通过CD光谱对β-酪蛋白进行二级结构分析，得到α-螺旋含量为0.5%-2%、β-折叠含量为16%-18%、β-转角含量为30%-34%、无规卷曲含量为49%-51%。另一个结果是，随着pH值升高，这些结构会发生变化。通过圆二色光谱在pH 8和pH 10时观察到α-螺旋中有几个明显的转变。拉曼光谱也显示，天然β-酪蛋白的主要二级结构为无规卷曲，因为β-酪蛋白酰胺I的特征峰位于1662 cm-1处：根据I850/I830计算表明，β-酪蛋白的酪氨酸残基倾向于“暴露”。CD光谱和拉曼光谱均显示，在中性和碱性pH条件下，β-酪蛋白主要以无规卷曲构象存在，且pH 8时α-螺旋的比例高于其他pH条件。在所研究的pH范围内，β-折叠和β-转角区域保持相对恒定，且在pH 8条件下，α-螺旋的含量高于其他pH条件。
