Local Water Dynamics in Coacervated Polyelectrolytes Monitored through Dynamic Nuclear Polarization-Enhanced 1H NMR
Local Water Dynamics in Coacervated Polyelectrolytes Monitored through Dynamic Nuclear Polarization-Enhanced 1H NMR
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DOI：
10.1021/ma901137g
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发表时间：
2009-10-13
期刊：
影响因子：
5.5
通讯作者：
Han, Songi
中科院分区：
文献类型：
--
作者：
Kausik, Ravinath;Srivastava, Aasheesh;Han, Songi
本文首次研究了在环境条件下完全分散于水中的体系的油水界面扩散系数。这样的测量是可能的，通过实施最近推出的动态核极化（DNP）技术，选择性地放大的核磁共振（NMR）信号的水合水，是相互作用的具体定位自旋标签上的表面。在本报告中，通过测量两种类型的带相反电荷的聚电解质表面上的溶剂微粘度来证明这种新能力的优点：当自由溶解时与当复合形成称为复合凝聚层的液-液胶体相时。这些复合凝聚层是通过基于咪唑的阳离子均聚物聚（N-乙烯基咪唑）（PVIm）和阴离子多肽聚天冬氨酸酯（PAsp）之间在pH 4.5-6.0范围内的静电络合形成的，在该条件下凝聚层液滴是高度流体性的，但致密地填充有聚电解质。我们还研究了旋转扩散系数的自旋标签共价结合到Mill和PAsp的双链，显示了5倍的旋转相关时间以及凝聚后的各向异性参数的变化，这代表了一个令人惊讶的小的减少，给定的高聚合物浓度内的致密微滴。对于DNP和ESR实验，聚合物用稳定的氮氧自由基自旋标记物（类似于1重量%）共价标记以探测局部溶剂和聚合物链段动力学。我们发现，在pH 8下，未复合的PVlm和PAsp附近的表面水扩散系数不同，大约为D，类似于1.3 x 10（-9）m（2）/S。相比之下，在复合凝聚层相内，紧邻两个水分子的水扩散系数为D，类似于0.25 × 10（-9）m（2）/s，这大约比本体水自扩散系数小一个数量级，但比缔合的结合水的自扩散系数大一个数量级。这一观察结果表明，复杂凝聚层内存在可测量的水，具有相对较高的扩散和交换动力学，这意味着水在纳米尺度的孔隙空间中移动，而不是在结构上被束缚或甚至不存在。我们从我们的观察中推断PVlm和PAsp链正在经历大致成对的缔合，使得大部分电荷中和的物质组成浓缩物。还具有流体性和部分水合的凝聚核。
We present the first study of quantifying the diffusion coefficient of interfacial water oil polyelectrolyte surfaces of systems fully dispersed in bulk water under ambient conditions. Such measurements were made possible through the implementation of a recently introduced dynamic nuclear polarization (DNP) technique to selectively amplify the nuclear magnetic resonance (NMR) signal of hydration water that is interacting with specifically located spin-labels on polyelectrolyte surfaces. The merit of this novel capability is demonstrated in this report through the measurement of solvent microvisosity on the surface of two types of oppositely charged polyelectrolytes: when freely dissolved versus when complexed to form a liquid-liquid colloidal phase called complex coacervates. These complex coacervates were formed through electrostatic complexation between the imidazole-based cationic homopolymer poly(N-vinylimidazole) (PVIm) and anionic polypeptide polyaspartate (PAsp) in the pH range of 4.5-6.0, under which conditions the coacervate droplets are highly fluidic yet densely packed with polyelectrolytes. We also investigated the rotational diffusion coefficients of the spin-labels covalently bound to the polyelectrolyte chains for both Mill and PAsp, showing a 5-fold change in the rotational correlation time as well as anisotropy parameter upon coacervation, which represents a surprisingly small decrease given the high polymer concentration inside the dense microdroplets. For both DNP and ESR experiments, the polymers were covalently tagged with stable nitroxide radical spin-labels (similar to 1 wt %) to probe the local solvent and polymer segment dynamics. We found that the surface water diffusion coefficients near uncomplexed PVlm and PAsp at pH 8 differ and are around D similar to 1.3 x 10(-9) m(2)/S. In contrast, inside the complex coacervate phase, the water diffusion coefficient in the immediate vicinity of either polyelectrolyte was D similar to 0.25 x 10(-9) m(2)/s, which is about in order of magnitude smaller than the bulk water self-diffusion coefficient and yet orders of magnitude greater than that of associated, bound, hydration water. This observation suggests the existence of measurable water inside complex coacervates with relatively high diffusion and exchange dynamics, implying that water moves in nanometer-scale pore spaces as opposed to being structurally bound or even absent. We infer from our observation that the PVlm and PAsp chains are undergoing roughly pairwise association, so that largely charge-neutralized species compose the concentrated., yet fluidic, and partially hydrated coacervate cores.