McCarney Evan R, Armstrong Brandon D, Kausik Ravinath, Han Songi
Department of Chemistry and Biochemistry, University of California-Santa Barbara, CA 93106, USA.
Langmuir. 2008 Sep 16;24(18):10062-72. doi: 10.1021/la800334k. Epub 2008 Aug 14.
We present a unique analysis tool for the selective detection of local water inside soft molecular assemblies (hydrophobic cores, vesicular bilayers, and micellar structures) suspended in bulk water. Through the use of dynamic nuclear polarization (DNP), the (1)H NMR signal of water is amplified, as it interacts with stable radicals that possess approximately 658 times higher spin polarization. We utilized stable nitroxide radicals covalently attached along the hydrophobic tail of stearic acid molecules that incorporate themselves into surfactant-based micelle or vesicle structures. Here, we present a study of local water content and fluid viscosity inside oleate micelles and vesicles and Triton X-100 micelles to serve as model systems for soft molecular assemblies. This approach is unique because the amplification of the NMR signal is performed in bulk solution and under ambient conditions with site-specific spin labels that only detect the water that is directly interacting with the localized spin labels. Continuous wave (cw) electron spin resonance (ESR) analysis provides rotational dynamics of the spin-labeled molecular chain segments and local polarity parameters that can be related to hydration properties, whereas we show that DNP-enhanced (1)H NMR analysis of fluid samples directly provides translational water dynamics and permeability of the local environment probed by the spin label. Our technique therefore has the potential to become a powerful analysis tool, complementary to cw ESR, to study hydration characteristics of surfactant assemblies, lipid bilayers, or protein aggregates, where water dynamics is a key parameter of their structure and function. In this study, we find that there is significant penetration of water inside the oleate micelles with a higher average local water viscosity (approximately 1.8 cP) than in bulk water, and Triton X-100 micelles and oleate vesicle bilayers mostly exclude water while allowing for considerable surfactant chain motion and measurable water permeation through the soft structure.
我们展示了一种独特的分析工具,用于选择性检测悬浮在大量水中的软分子聚集体(疏水核心、囊泡双层和胶束结构)内的局部水。通过使用动态核极化(DNP),水的(1)H NMR信号被放大,因为它与具有大约658倍更高自旋极化的稳定自由基相互作用。我们利用共价连接在硬脂酸分子疏水尾部的稳定氮氧化物自由基,这些自由基会自行融入基于表面活性剂的胶束或囊泡结构中。在这里,我们对油酸酯胶束和囊泡以及Triton X-100胶束内的局部水含量和流体粘度进行了研究,以作为软分子聚集体的模型系统。这种方法很独特,因为NMR信号的放大是在本体溶液中且在环境条件下通过位点特异性自旋标记进行的,这些自旋标记仅检测与局部自旋标记直接相互作用的水。连续波(cw)电子自旋共振(ESR)分析提供了自旋标记分子链段的旋转动力学和可与水合性质相关的局部极性参数,而我们表明对流体样品进行DNP增强的(1)H NMR分析直接提供了自旋标记所探测局部环境的平移水动力学和渗透性。因此,我们的技术有潜力成为一种强大的分析工具,与cw ESR互补,用于研究表面活性剂聚集体、脂质双层或蛋白质聚集体的水合特性,其中水动力学是其结构和功能的关键参数。在本研究中,我们发现水大量渗透到油酸酯胶束内部,其平均局部水粘度(约1.8 cP)高于大量水,而Triton X-100胶束和油酸酯囊泡双层大多排斥水,同时允许相当大的表面活性剂链运动以及可测量的水通过软结构渗透。