Rohloff Catherine M, Shimek Justin W, Dungan Stephanie R
Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA.
J Colloid Interface Sci. 2003 May 15;261(2):514-23. doi: 10.1016/S0021-9797(03)00079-1.
We have found that the presence of <1 wt% of the globular protein alpha-lactalbumin has a significant impact on the equilibrium phase behavior of dilute sodium bis(ethylhexyl) sulfosuccinate (AOT)/brine/isooctane systems. Nuclear magnetic resonance (NMR), Karl Fischer titration, and ultraviolet spectroscopy were used to determine the surfactant, oil, water, and protein content of the organic and aqueous phases as a function of the total surfactant and protein present. As a small amount of alpha-lactalbumin is added to the mixture, there is a substantial increase (up to 80%) in the maximum water solubility in the water-in-oil microemulsion phase. Dynamic light scattering measurements indicate that this increase is due to a decrease in the magnitude of the (negative) spontaneous curvature of the surfactant monolayer, as droplets swell in size. As the molar ratio of alpha-lactalbumin to AOT surpasses approximately 1:300, the partitioning of water, protein, and surfactant shifts to the excess aqueous phase, where soluble assemblies with positive curvature are detected by dynamic light scattering. Significant amounts of isooctane are solubilized in these aggregates, consistent with the formation of oil-in-water microemulsion droplets. Circular dichroism studies showed that the tertiary structure of the protein in the microemulsion is disrupted while the secondary structure is increased. In light of these findings, the protein most likely expands to a molten-globule type conformation in the AOT interfacial environment, but does not substantially unfold to become an extended chain.
我们发现，当球状蛋白α-乳白蛋白的含量低于1 wt%时，其会对稀双（2-乙基己基）磺基琥珀酸钠（AOT）/盐水/异辛烷体系的平衡相行为产生重大影响。利用核磁共振（NMR）、卡尔费休滴定法和紫外光谱法来确定有机相和水相中表面活性剂、油、水和蛋白质的含量，这些含量是总表面活性剂和蛋白质含量的函数。当向混合物中添加少量α-乳白蛋白时，油包水微乳液相中最大水溶解度会大幅增加（高达80%）。动态光散射测量表明，这种增加是由于随着液滴尺寸增大，表面活性剂单分子层（负）自发曲率的大小减小所致。当α-乳白蛋白与AOT的摩尔比超过约1:300时，水、蛋白质和表面活性剂的分配会转移到过量水相中，通过动态光散射在该相中检测到具有正曲率的可溶性聚集体。大量异辛烷溶解在这些聚集体中，这与水包油微乳液滴的形成一致。圆二色性研究表明，微乳液中蛋白质的三级结构被破坏，而二级结构增加。鉴于这些发现，蛋白质很可能在AOT界面环境中扩展为熔融球状构象，但不会大幅展开成为伸展链。
