Stradner Anna, Cardinaux Frédéric, Schurtenberger Peter
Department of Physics, University of Fribourg, Chemin du Musée 3, CH-1700 Fribourg, Switzerland.
J Phys Chem B. 2006 Oct 26;110(42):21222-31. doi: 10.1021/jp0639804.
We use small-angle scattering experiments to investigate the structural properties of aqueous lysozyme solutions under conditions where the existence of equilibrium clusters has recently been demonstrated (Nature 2004, 432, 492). We also discuss the possible emergence of a low angle scattering contribution, which recently attracted interest due to its appearance in solutions of various proteins. We demonstrate that in lysozyme solutions under our experimental conditions such rising low q intensities can only be observed under special circumstances and can thus not be attributed to the existence of a universal long-range attraction. We then focus on the structural properties of the equilibrium clusters as a function of protein concentration, temperature, and ionic strength. We show that the experimental structure factors obtained from the scattering measurements exhibit the typical cluster-cluster peak q(c) reflecting the mean distance between charged clusters as well as a monomer-monomer peak q(m), which represents the nearest neighbor shell of monomers within a single cluster. The underlying principle for the formation of these structures is the coexistence of two opposing forces, a short-range attraction and a long-range repulsion due to residual charges. We can quantitatively analyze our scattering data by applying a simple equilibrium cluster model and calculate an average cluster aggregation number, N(c). The thus obtained cluster aggregation number increases linearly with volume fraction. We also observe an increasing N(c) as temperature decreases and as the screening of residual charges increases. We point out the importance of the existence of equilibrium clusters and the universality of this phenomenon for self-assembling processes observed in nature. Finally, we discuss the limitations of our simple globular cluster model in view of recent findings from computer simulations.
我们利用小角散射实验来研究溶菌酶水溶液在最近已证实存在平衡聚集体的条件下的结构特性（《自然》，2004年，432卷，492页）。我们还讨论了低角散射贡献可能出现的情况，这种贡献最近因其在各种蛋白质溶液中的出现而受到关注。我们证明，在我们的实验条件下，溶菌酶溶液中这种低q强度的上升只有在特殊情况下才能观察到，因此不能归因于普遍存在的长程吸引力。然后，我们关注平衡聚集体的结构特性随蛋白质浓度、温度和离子强度的变化。我们表明，从散射测量中获得的实验结构因子呈现出典型的聚集体 - 聚集体峰q(c)，它反映了带电聚集体之间的平均距离，以及单体 - 单体峰q(m)，它代表了单个聚集体内单体的最近邻壳层。这些结构形成的基本原理是两种相反力的共存，即由于残余电荷产生的短程吸引力和长程排斥力。我们可以通过应用一个简单的平衡聚集体模型对散射数据进行定量分析，并计算平均聚集体聚集数N(c)。由此获得的聚集体聚集数随体积分数线性增加。我们还观察到，随着温度降低和残余电荷屏蔽增加，N(c)也会增加。我们指出了平衡聚集体存在的重要性以及这种现象在自然界中自组装过程中的普遍性。最后，鉴于计算机模拟的最新发现，我们讨论了我们简单球状聚集体模型的局限性。
