The low-resolution structure of casein (CN) clusters in sodium caseinate (NaCas) solution and its conformational dynamics were obtained by size-exclusion chromatography (SEC), analytical ultracentrifugation (AUC), small-angle X-ray scattering (SAXS), and molecular dynamics (MD) simulations. The results of sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) and native PAGE revealed that the casein clusters consisted predominantly of α- and β-CN complexes, and a trace amount of κ-CN. The AUC analysis indicated that the casein clusters were composed of 34.6% of casein monomers, 19.2%, 20.4%, and 25.8% of complexes with molar weight (M_w) of ~50.3, ~70.6, and ~133 kDa, respectively. The volume fractions of components in casein clusters were quantified as 64.3% of α_s1-β-α_s2-CN, 22.3% of α_s1-CN, 8.5% of α_s2-CN, and 4.4% of α_s1-α_s2-CN, respectively. The ensemble optimization method (EOM) gave a fitting result where α_s1-β-α_s2-CN species coexisted in ~35.3% under compact conformation and ~64.7% in elongated conformation in solution. The three-dimensional structures of α_s1-β-α_s2-CN from EOM showed a good overlay on the casein clusters ab initio model obtained from DAMMIN and DAMMIX program. MD simulations revealed that α_s1-β-α_s2-CN underwent a conformational change from the elongated state into the compact state within the initial 200 ns of simulations. The addition of nonionic surfactants affected little the backbone-to-backbone interactions in the formation of the casein clusters. We propose that α_s1-CN, β-CN, α_s2-CN, and κ-CN associated in consecutive steps into casein clusters, and a trace of κ-CN may be located at the surface of the assemblies limiting the growth of casein aggregates.

酪蛋白酸钠 (NaCas) 溶液中酪蛋白 (CN) 簇的低分辨率结构及其构象动力学通过尺寸排阻色谱 (SEC)、分析超速离心 (AUC)、小角 X 射线散射 (SAXS)、和分子动力学 (MD) 模拟。十二烷基硫酸钠-聚丙烯酰胺凝胶电泳 (SDS-PAGE) 和非变性 PAGE 结果表明酪蛋白簇主要由 α- 和 β-CN 复合物以及微量的 κ-CN 组成。AUC 分析表明酪蛋白簇由 34.6% 的酪蛋白单体、19.2%、20.4% 和 25.8% 的复合物组成，摩尔重量 ( M _w ) 分别为~50.3、~70.6 和~133 kDa。酪蛋白簇中组分的体积分数被量化为 α _{s1 的}64.3%-β-α _s2 -CN、22.3%的α _s1 -CN、8.5%的α _s2 -CN和4.4%的α _s1 -α _s2 -CN。集成优化方法 ( EOM ) 给出了拟合结果，其中 α _s1 -β-α _s2 -CN 物种在紧凑构象下以~35.3% 的比例共存，在溶液中以~64.7% 的细长构象共存。来自EOM的 α _s1 -β-α _s2 -CN的三维结构与从DAMMIN和DAMMIX程序获得的酪蛋白簇ab initio模型有很好的叠加。MD 模拟显示 α _s1-β-α _s2 -CN 在模拟的初始 200 ns 内经历了从伸长状态到紧凑状态的构象变化。添加非离子表面活性剂对酪蛋白簇形成过程中骨架与骨架的相互作用影响很小。我们提出 α _s1 -CN、β-CN、α _s2 -CN 和 κ-CN 在连续步骤中与酪蛋白簇相关联，并且微量 κ-CN 可能位于限制酪蛋白生长的组件表面聚合。