Interactions between proteins and surfactants are both of fundamental interest and relevant for applications in food, cosmetics and detergency. The anionic surfactant sodium dodecyl sulfate (SDS) denatures essentially all proteins. Denaturation typically involves a number of distinct steps where growing numbers of SDS molecules bind to the protein, as seen in multidisciplinary approaches combining several complementary techniques. We adopt this approach to study the SDS-induced unfolding of Ca²⁺-depleted α-lactalbumin (aLA), a protein particularly sensitive toward denaturation by surfactants. By combining stopped-flow mixing of protein and surfactant solutions with stopped-flow synchrotron small-angle X-ray scattering (SAXS), circular dichroism (CD) and Trp fluorescence, together with information from previous calorimetric studies, we construct a detailed picture of the unfolding process at the level of both protein and surfactant. A protein-surfactant complex is formed within the dead time of mixing (2.5 ms). Initially a cluster of SDS molecules binds asymmetrically, i.e., to one side of the protein, after which aLA redistributes around the SDS cluster. This occurs in two kinetic steps where the complex grows in number of both SDS and protein molecules, concomitant with protein unfolding. During these steps, the core-shell complex undergoes changes in shell thickness as well as core shape and radius. The entire process is very sensitive to SDS concentration and completes within 10 s at an SDS:aLA ratio of 9, decreasing to 0.2 s at 60 SDS:aLA. The number of aLA molecules per SDS complex drops from 1.9 to 1.0 over this range of ratios. While both CD and Trp kinetics reveal a fast and a slow conformational transition, only the slow transition is observed by SAXS, indicating that the protein-SDS complex (which is monitored by SAXS) adjusts to the presence of the unfolded protein. We attribute the rapid unfolding of aLA to its predominantly α-helical structure, which persists in SDS (albeit as isolated helices), enabling aLA to unfold without undergoing major secondary structural changes unlike β-sheet rich proteins. Nevertheless, the overall unfolding steps are broadly similar to those of the more β-rich protein β-lactoglobulin, suggesting that this unfolding model is representative of the general process of SDS-unfolding of proteins.

蛋白质和表面活性剂之间的相互作用既是基础性的，又与食品，化妆品和去污剂中的应用有关。阴离子表面活性剂十二烷基硫酸钠（SDS）基本上使所有蛋白质变性。变性通常涉及许多不同的步骤，其中越来越多的SDS分子与蛋白质结合，如在结合多种互补技术的多学科方法中所见。我们采用这种方法来研究SDS诱导的Ca ^2+的展开贫化的α-乳白蛋白（aLA），一种对表面活性剂变性特别敏感的蛋白质。通过将蛋白质和表面活性剂溶液的停流混合与停流同步加速器小角X射线散射（SAXS），圆二色性（CD）和Trp荧光相结合，再结合以前的量热研究得出的信息，我们构建了一张在蛋白质和表面活性剂水平上的展开过程。在混合的停滞时间内（2.5毫秒）会形成蛋白质表面活性剂复合物。最初，一簇SDS分子不对称地结合，即与蛋白质的一侧结合，之后aLA在SDS簇周围重新分布。这发生在两个动力学步骤中，其中复合物的SDS和蛋白质分子数量均增加，同时蛋白质展开。在这些步骤中，核-壳复合体的壳厚度，核的形状和半径都会发生变化。整个过程对SDS浓度非常敏感，并且在SDS：aLA比率为9的10 s内完成，在60 SDS：aLA的情况下降至0.2 s。在该比例范围内，每个SDS复合物的aLA分子数量从1.9降至1.0。虽然CD和Trp动力学都揭示了快速和缓慢的构象转变，但SAXS只能观察到缓慢的转变，这表明蛋白质-SDS复合物（由SAXS监控）可以适应未折叠蛋白质的存在。我们将aLA的快速展开归因于其主要是α螺旋结构，该结构在SDS中仍然存在（尽管是孤立的螺旋），从而使aLA展开而不会经历与富含β-sheet的蛋白质不同的主要二级结构变化。不过，