Solid State Physics Division , Bhabha Atomic Research Centre , Mumbai 400 085 , India.
Division of Materials and Environmental Chemistry , Stockholm University , Frescativagen 8 , Stockholm 10691 , Sweden.
Biomacromolecules. 2019 May 13;20(5):2123-2134. doi: 10.1021/acs.biomac.9b00374. Epub 2019 Apr 1.
The evolution of interactions in the bovine serum albumin (BSA) protein solution on addition of mono and multivalent (di, tri and tetra) counterions has been studied using small-angle neutron scattering (SANS), dynamic light scattering (DLS) and ζ-potential measurements. It is found that in the presence of mono and divalent counterions, protein behavior can be well explained by DLVO theory, combining the contributions of screened Coulomb repulsion with the van der Waals attraction. The addition of mono or divalent salts in protein solution reduces the repulsive barrier and hence the overall interaction becomes attractive, but the system remains in one-phase for the entire concentration range of the salts, added in the system. However, contrary to DLVO theory, the protein solution undergoes a reentrant phase transition from one-phase to a two-phase system and then back to the one-phase system in the presence of tri and tetravalent counterions. The results show that tri and tetravalent (unlike mono and divalent) counterions induce short-range attraction between the protein molecules, leading to the transformation from one-phase to two-phase system. The two-phase is characterized by the fractal structure of protein aggregates. The excess condensation of these higher-valent counterions in the double layer around the BSA causes the reversal of charge of the protein molecules resulting into reentrant of the one-phase, at higher salt concentrations. The complete phase behavior with mono and multivalent ions has been explained in terms of the interplay of electrostatic repulsion and ion-induced short-range attraction between the protein molecules.
在添加单价和多价（二价、三价和四价）抗衡离子的牛血清白蛋白（BSA）蛋白溶液中，相互作用的演变已通过小角中子散射（SANS）、动态光散射（DLS）和 ζ-电势测量进行了研究。结果发现，在单价和二价抗衡离子存在的情况下，通过将屏蔽库仑排斥与范德华吸引力结合起来，可以很好地用 DLVO 理论来解释蛋白质的行为。单价或二价盐在蛋白质溶液中的添加降低了排斥势垒，因此整个相互作用变得有吸引力，但在添加的盐的整个浓度范围内，系统仍保持单相。然而，与 DLVO 理论相反，在三价和四价抗衡离子存在下，蛋白质溶液经历了从单相到两相系统的再进入相转变，然后又回到单相系统。结果表明，三价和四价（与单价和二价不同）抗衡离子在蛋白质分子之间诱导短程吸引力，导致从单相到两相系统的转变。两相的特征是蛋白质聚集体的分形结构。这些更高价抗衡离子在 BSA 周围的双电层中的超额凝聚导致蛋白质分子的电荷反转，从而在更高盐浓度下重新进入单相。通过蛋白质分子之间的静电排斥和离子诱导的短程吸引力的相互作用，解释了单价和多价离子的完整相行为。
