Determination of the quaternary structural states of bovine casein by small-angle X-ray scattering: submicellar and micellar forms.
Determination of the quaternary structural states of bovine casein by small-angle X-ray scattering: submicellar and micellar forms.
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DOI：
10.1016/0003-9861(88)90288-3
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发表时间：
1988-11
影响因子：
3.9
通讯作者：
T. F. Kumosinski;H. Pessen;H. M. Farrell;H. Brumberger
中科院分区：
文献类型：
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作者：
T. F. Kumosinski;H. Pessen;H. M. Farrell;H. Brumberger
整个酪蛋白以蛋白质和盐的球形胶体复合体形式存在于牛奶中，称为酪蛋白胶束，通过电子显微镜测定，其平均半径约为650？钙离子的去除被认为会导致解离成更小的非胶体蛋白质复合体，称为亚胶束。对整个酪蛋白亚胶束的流体动力学和光散射研究表明，它们主要是带有疏水核心的球形粒子。为了研究在静电稳定的胶束结构中是否保持疏水稳定的亚胶束结构的完整性，对牛乳中的整个酪蛋白在亚胶束(不含钙)和胶束(添加10mCaCl2)条件下进行了小角X射线散射(SAXS)实验。所有的SAXS结果都表现出多重高斯特征，可以用非线性回归代替传统的Guinier曲线图进行分析。对亚胶束酪蛋白的SAXS数据分析表明，有两个高斯分量可以用具有两个不同电子密度的同心区域的粒子来解释，分别被命名为“致密的”C核和“松散的”L壳层。亚胶束的平均分子质量为285,000±14,600，较高电子密度核的质量分数k为0.212±0.028。核的回转半径Rc为37.98±0.01ra，电子密度差ΔρC为0.0148±0.0014 e−/A3，疏松区的R值为R L=88.2±0.8Å，ΔρL=0.0091±0.0003 e−/Å3。计算的距离分布函数和归一化散射曲线也与具有较高电子密度的同心球形核的整体球形粒子相一致。这些结果，特别是壳层极低的电子密度，可以用疏水内核稳定的松散排列的球形聚集体来解释，并被更松散排列的亲水区域包围，这与其他研究的结果一致。胶体胶束酪蛋白的SAXS数据只产生与散射强度窗口相关的横截面信息，用三个没有剩余函数的高斯和进行分析。回转半径较小的两个高斯粒子再次被解释为具有相同质心的具有两个电子密度的不均匀球形粒子的指示。第三个高斯型反映了这些粒子的堆积数，该体系的堆积数为3：1。用较低转动半径的两个高斯色谱测得的分子量与用胶束下酪蛋白小角X射线光谱分析测得的值一致，k值和ΔρC值也一致。然而，在这种胶束条件下，ΔρL=0.0065±0.0003。这些结果表明，胶体胶束结构中存在含有疏水稳定内核的亚胶束不均匀粒子。
Whole casein occurs in milk as a spherical colloidal complex of protein and salts called the casein micelle, with approximate average radii of 650 Å as determined by electron microscopy. Removal of Ca 2+ is thought to result in dissociation into smaller noncolloidal protein complexes called submicelles. Hydrodynamic and light scattering studies on whole casein submicelles suggest that they are predominantly spherical particles with a hydrophobic core. To investigate whether the integrity of a hydrophobically stabilized submicellar structure is preserved in the electrostatically stabilized colloidal micellar structure, small-angle X-ray scattering (SAXS) experiments were undertaken on whole casein from bovine milk under submicellar (without Ca 2+) and micellar (with added 10 m m CaCl 2) conditions. All SAXS results showed multiple Gaussian character and could be analyzed best by nonlinear regression in place of the customary Guinier plot. Analysis of the SAXS data for submicellar casein showed two Gaussian components which could be interpreted in terms of a particle with two concentric regions of different electron density, designated as a “compact” C core and a “loose” L shell, respectively. The submicelle was found to have an average molecular weight of 285,000±14,600 and a mass fraction of higher electron density core, k, of 0.212±0.028. The radius of gyration of the core, R c, was 37.98±0.01 Å with an electron density difference, Δρ C, of 0.0148±0.0014 e−/Å 3, while the loose region had values of R L= 88.2±0.8 A ̊ with Δ ρ L= 0.0091±0.0003 e−/A ̊ 3. Calculated distance distribution functions and normalized scattering curves also were consistent with an overall spherical particle with a concentric spherical inner core of higher electron density. These results, and in particular the remarkably low electron densities of the shells, can be interpreted in terms of a loosely packed spherical aggregate stabilized by a hydrophobic inner core and surrounded by an even more loosely packed hydrophilic region, in agreement with the results of other studies. The SAXS data for the colloidal micellar casein, which yield only cross-sectional information related to a window of scattered intensity, were analyzed by a sum of three Gaussians with no residual function. The two Gaussians with the lower values of the radius of gyration were interpreted again as an indication of an inhomogeneous spherical particle of two electron densities with the same centroid. The third Gaussian was shown to reflect the packing number of these particles, which was 3: 1 for this system. The molecular weight determined from the two Gaussians of lower radii of gyration was in agreement with the value obtained from SAXS of submicellar casein, as were the k and Δρ C values. However, a lower value of Δρ L= 0.0065±0.0003 was observed under these micellar conditions. These results are an indication of the existence of submicellar inhomogeneous particles containing a hydrophobically stabilized inner core within the colloidal micellar structure.