FT-IR分析乳清分离蛋白-葡聚糖接枝物的结构变化

Study of Structural Changes in Whey Protein Isolate-dextran Conjugate Using Fourier Transform Infrared Spectroscopy

A294、褐变程度和游离氨基含量的测定结果证实乳清分离蛋白与葡聚糖在干热处理条件下确实发生了以美拉德反应为机理的接枝反应。使用傅里叶变换红外光谱技术研究了接枝反应对乳清分离蛋白分子结构的影响。与原蛋白相比，乳清分离蛋白－葡聚糖接枝物在3700～3200cm^-1范围内出现一宽峰及在1260～1000cm^-1范围内吸收增强；接枝物在酰胺I带（1600～1700cm^-1）和酰胺Ⅱ带（1500～1600cm^-1）内的峰形发生变化，峰强增加，对应的吸收峰住发生红移。结合去卷积和二阶导数处理，对酰胺I带进行曲线拟合，定量分析了接枝反应前后蛋白质分子二级结构的变化。结果显示，由于大分子葡聚糖的接入，WPI的二级结构遭到破坏，α-转角和无规则卷曲含量增加，小螺旋和β-折叠含量减少，蛋白质分子结构有序性减弱。

Measurement of A294, the degree of browning, and the content of free amino groups showed that a grafting reaction based on the Maillard-type reaction had occurred between the whey protein isolate （WPI） and dextmn under dry-heating treatment. The effect of this grafting reaction on the structure of WPI was investigated by using Fourier transform infrared spectroscopy （FT-IR）. Compared to the original WPI, the conjugate showed a broad peak in the range of 3700～3200 cm^-1 and an increase in absorption between 1260 and 1000 cm^-1. The peak shapes of amide I and amide Ⅱ bands showed remarkable changes, the peak intensity increased, and the red shift phenomenon occurred in the corresponding absorption peaks. Using deconvolution and second derivative methods, the curve of amide I band was fitted for quantitative analysis of the secondary structural changes in WPI before and after the grafting reaction. The results showed that the secondary structure of WPI had been destroyed owing to the introduction of macromolecule dextmn. The number of β-turns and random coils increased, whereas the number of α-helix and β-sheet decreased after the graft reaction, indicating that the structure of WPI had become relatively unordered.