Chodankar S, Aswal V K, Kohlbrecher J, Vavrin R, Wagh A G
Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai-400 085, India.
Phys Rev E Stat Nonlin Soft Matter Phys. 2008 Sep;78(3 Pt 1):031913. doi: 10.1103/PhysRevE.78.031913. Epub 2008 Sep 17.
Coacervation is a dense liquid-liquid phase separation and herein we report coacervation of protein bovine serum albumin (BSA) in the presence of polyelectrolyte sodium polystyrene sulfonate (NaPSS) under varying solution conditions. Small-angle neutron scattering (SANS) measurements have been performed on above protein-polyelectrolyte complexes to study the structural evolution of the process that leads to coacervation and the phase separated coacervate as a function of solution pH , protein-polyelectrolyte ratio and ionic strength. SANS study prior to phase separation on the BSA-NaPSS complex shows a fractal structure representing a necklace model of protein macromolecules randomly distributed along the polystyrene sulfonate chain. The fractal dimension of the complex decreases as pH is shifted away from the isoelectric point ( approximately 4.7) of BSA protein, which indicates the decrease in the compactness of the complex structure due to increase in the charge repulsion between the protein macromolecules bound to the polyelectrolyte. Concentration-dependence studies of the polyelectrolyte in the complex suggest coexistence of two populations of polyelectrolytes, first one fully saturated with proteins and another one free from proteins. Coacervation phase has been obtained through the turbidity measurement by varying pH of the aqueous solution containing protein and polyelectrolyte from neutral to acidic regime to get them to where the two components are oppositely charged. The spontaneous formation of coacervates is observed for pH values less than 4. SANS study on coacervates shows two length scales related to complex aggregations (mesh size and overall extent of the complex) hierarchically branched to form a larger network. The mesh size represents the distance between cross-linked points in the primary complex, which decreases with increase in ionic strength and remains the same on varying the protein-polyelectrolyte ratio. On the other hand, the overall extent of the complex shows a similar structure irrespective of varying ionic strength and protein-polyelectrolyte ratio. A large fraction ( approximately 50%) of protein-polyelectrolyte complexes is also found to be free in the supernatant after the coacervation.
凝聚是一种致密的液-液相分离，在此我们报道了在不同溶液条件下，蛋白质牛血清白蛋白（BSA）在聚电解质聚苯乙烯磺酸钠（NaPSS）存在下的凝聚情况。已对上述蛋白质-聚电解质复合物进行小角中子散射（SANS）测量，以研究导致凝聚的过程以及相分离凝聚层的结构演变，该演变是溶液pH值、蛋白质-聚电解质比例和离子强度的函数。在相分离之前对BSA-NaPSS复合物进行的SANS研究显示出一种分形结构，该结构代表了沿着聚苯乙烯磺酸盐链随机分布的蛋白质大分子的项链模型。随着pH值从BSA蛋白质的等电点（约4.7）偏移，复合物的分形维数降低，这表明由于与聚电解质结合的蛋白质大分子之间电荷排斥增加，复合物结构的紧密性降低。对复合物中聚电解质的浓度依赖性研究表明存在两种聚电解质群体，第一种完全被蛋白质饱和，另一种不含蛋白质。通过改变含有蛋白质和聚电解质的水溶液的pH值，从中性到酸性范围，使两种成分带相反电荷，通过浊度测量获得了凝聚相。对于pH值小于4的情况，观察到凝聚层的自发形成。对凝聚层的SANS研究显示出与复合物聚集相关的两个长度尺度（网孔尺寸和复合物的整体范围）分层分支形成一个更大的网络。网孔尺寸代表初级复合物中交联点之间的距离，它随着离子强度的增加而减小，并且在改变蛋白质-聚电解质比例时保持不变。另一方面，无论离子强度和蛋白质-聚电解质比例如何变化，复合物的整体范围都显示出相似的结构。在凝聚后，还发现很大一部分（约50%）的蛋白质-聚电解质复合物在上清液中是游离的。
