Department of Chemistry and Biochemistry and Materials Research Laboratory, University of California, Santa Barbara, California 93106, USA.
J Am Chem Soc. 2009 Dec 30;131(51):18254-6. doi: 10.1021/ja9060849.
Measurements of the interfacial diffusion coefficient of the surface hydration layer of lipid vesicles in dilute solutions are presented. This was made possible by the greatly enhanced sensitivity and unique contrast provided by the site-specific and selective Overhauser dynamic nuclear polarization of solvent molecules that approach nitroxide radical-based spin labels within <5-10 A. All experiments were carried out using minute microliter sample volumes of lipid vesicle solutions, using low spin label concentrations (<2 mol %) and under physiological conditions. This presents unprecedented sensitivity for analyzing interfacial solvent diffusion of macromolecules and their assemblies in solutions and highlights the feasibility of investigating precious samples. Interfacial diffusion on DOTAP (1,2-DiOleoyl-3-TrimethylAmmonium-Propane) and DPPC (1,2-DiPalmitoyl-sn-glycero-3-PhosphoCholine) surfaces are further analyzed as a function of temperature to determine the activation energy of their hydration layer dynamics. The temperature-dependent analysis across the phase transition of DPPC concludes that the hydration water with 100-200 ps dynamics displays Arrhenius behavior and does not undergo a phase transition unlike the lipid chains. We also discuss the advantages of determining the activation energy of diffusion as a general approach to comparing interfacial diffusivity on surfaces that have vastly different charge topologies and, thus, may display different distances of closest approach between the spin label placed at the surface and the protons of hydration water. The further development and application of this technique is expected to facilitate the study of membrane dynamics and their phase behavior, including the formation of lipid rafts, with lipid-specific resolution.
本文介绍了在稀溶液中测量脂质体表面水合层的界面扩散系数的方法。这种方法的灵敏度非常高，并且具有独特的对比度，这是通过接近位于 <5-10 A 内的硝氧自由基自旋标记的溶剂分子的位置特异性和选择性的 Overhauser 动态核极化来实现的。所有实验均使用脂质体溶液的微小微升样品体积、低自旋标记浓度（<2 mol%）和生理条件下进行。这为分析大分子及其在溶液中的组装的界面溶剂扩散提供了前所未有的灵敏度，并突出了研究珍贵样品的可行性。进一步分析 DOTAP（1,2-二油酰基-3-三甲基铵丙烷）和 DPPC（1,2-二棕榈酰基-sn-甘油-3-磷酸胆碱）表面的界面扩散作为温度的函数，以确定其水合层动力学的活化能。在 DPPC 的相变温度范围内的温度依赖性分析得出结论，具有 100-200 ps 动力学的水合水显示 Arrhenius 行为，并且不像脂质链那样经历相变。我们还讨论了确定扩散活化能作为比较具有极大不同电荷拓扑结构的表面的界面扩散率的一般方法的优点，因此，可能显示出放置在表面上的自旋标记与水合质子之间的最近距离不同。预计这项技术的进一步发展和应用将有助于研究膜动力学及其相行为，包括具有脂质特异性分辨率的脂质筏的形成。
