Laboratoire Léon Brillouin, UMR CEA-CNRS, CE Saclay, Gif-sur-Yvette, France.
Adv Colloid Interface Sci. 2011 Sep 14;167(1-2):71-84. doi: 10.1016/j.cis.2011.05.007. Epub 2011 May 26.
We review, based on structural information, the mechanisms involved when putting in contact two nano-objects of opposite electrical charge, in the case of one negatively charged polyion, and a compact charged one. The central case is mixtures of PSS, a strong flexible polyanion (the salt of a strong acid, and with high linear charge density), and Lysozyme, a globular protein with a global positive charge. A wide accurate and consistent set of information in different situations is available on the structure at local scales (5-1000Å), due to the possibility of matching, the reproducibility of the system, its well-defined electrostatics features, and the well-defined structures obtained. We have related these structures to the observations at macroscopic scale of the phase behavior, and to the expected mechanisms of coacervation. On the one hand, PSS/Lysozyme mixtures show accurately many of what is expected in PEL/protein complexation, and phase separation, as reviewed by de Kruif: under certain conditions some well-defined complexes are formed before any phase separation, they are close to neutral; even in excess of one species, complexes are only modestly charged (surface charges in PEL excess). Neutral cores are attracting each other, to form larger objects responsible for large turbidity. They should lead the system to phase separation; this is observed in the more dilute samples, while in more concentrated ones the lack of separation in turbid samples is explained by locking effects between fractal aggregates. On the other hand, although some of the features just listed are the same required for coacervation, this phase transition is not really obtained. The phase separation has all the macroscopic aspects of a fluid (undifferentiated liquid/gas phase) - solid transition, not of a fluid-fluid (liquid-liquid) one, which would correspond to real coacervation). The origin of this can be found in the interaction potential between primary complexes formed (globules), which agrees qualitatively with a potential shape of the type repulsive long range attractive very short range. Finally we have considered two other systems with accurate structural information, to see whether other situations can be found. For Pectin, the same situation as PSS can be found, as well as other states, without solid precipitation, but possibly with incomplete coacervation, corresponding to differences in the globular structure. It is understandable that these systems show smoother interaction potential between the complexes (globules) likely to produce liquid-liquid transition. Finally, we briefly recall new results on Hyaluronan/Lysozyme, which present clear signs of coacervation in two liquid phases, and at the same time the existence of non-globular complexes, of specific geometry (thin rods) before any phase separation. These mixtures fulfill many of the requirements for complex coacervation, while other theories should also be checked like the one of Shklovskii et al.
我们基于结构信息，综述了将带相反电荷的两个纳米物体接触时所涉及的机制，其中一个带负电荷的多离子和一个紧凑带电的多离子。中心案例是 PSS（一种强柔性聚阴离子，强酸的盐，具有高线性电荷密度）和溶菌酶的混合物，溶菌酶是一种带有整体正电荷的球形蛋白质。由于匹配的可能性、系统的可重复性、明确的静电特性以及获得的明确结构，在不同情况下，局部尺度（5-1000Å）的结构信息具有广泛、准确和一致的特点。我们将这些结构与宏观尺度上的相行为观察结果以及预期的凝聚机制联系起来。一方面，PSS/溶菌酶混合物准确地显示了 de Kruif 综述的 PEL/蛋白质复合和相分离中所预期的许多特征：在某些条件下，在任何相分离之前都会形成一些定义明确的复合物，它们接近中性；即使在一种物质过量的情况下，复合物的电荷也只是适度的（PEL 过量时的表面电荷）。中性核相互吸引，形成负责大浊度的更大物体。它们应该导致系统发生相分离；在更稀的样品中观察到这种情况，而在更浓的样品中，混浊样品中没有分离的现象可以用分形聚集体之间的锁定效应来解释。另一方面，尽管列出的一些特征是凝聚所必需的，但这种相变实际上并没有发生。相分离具有流体（未分化的液体/气体相）-固体转变的所有宏观特征，而不是真正的凝聚所对应的流体-流体（液体-液体）转变。这种情况的原因可以在形成的初级复合物（球）之间的相互作用势能中找到，该势能定性上与排斥长程吸引短程的势能形状一致。最后，我们考虑了另外两个具有准确结构信息的系统，以观察是否可以找到其他情况。对于果胶，可以找到与 PSS 相同的情况，以及其他状态，没有固体沉淀，但可能不完全凝聚，这对应于球型结构的差异。可以理解的是，这些系统中复合物（球）之间的相互作用势能更平滑，可能产生液体-液体转变。最后，我们简要回顾了透明质酸/溶菌酶的新结果，它们在两个液相中表现出明显的凝聚迹象，同时存在非球型复合物，具有特定的几何形状（细棒），在任何相分离之前。这些混合物满足复杂凝聚的许多要求，同时也应该检查其他理论，如 Shklovskii 等人的理论。
