Dept. of Chemistry, Ecole Normale Supérieure, rue Lhomond, Paris, France.
Faraday Discuss. 2010;146:263-81; discussion 283-98, 395-401. doi: 10.1039/b925673c.
The dynamics of water next to hydrophobic groups is critical for several fundamental biochemical processes such as protein folding and amyloid fiber aggregation. Some biomolecular systems, like melittin or other membrane-associated proteins, exhibit extended hydrophobic surfaces. Due to the strain these surfaces impose on the hydrogen (H)-bond network, the water molecules shift from the clathrate-like arrangement observed around small solutes to an anticlathrate-like geometry with some dangling OH bonds pointing toward the surface. Here we examine the water reorientation dynamics next to a model hydrophobic surface through molecular dynamics simulations and analytic modeling. We show that the water OH bonds lying next to the hydrophobic surface fall into two subensembles with distinct dynamical reorientation properties. The first is the OH bonds tangent to the surface; these exhibit a behavior similar to the water OHs around small hydrophobic solutes, i.e. with a moderate reorientational slowdown explained by an excluded volume effect due to the surface. The second is the dangling OHs pointing toward the surface: these are not engaged in any H-bond, reorient much faster than in the bulk, and exhibit an unusual anisotropy decay which becomes negative for delays of a few picoseconds. The H-bond dynamics, i.e. the exchanges between the different configurations, and the resulting anisotropy decays are analyzed within the analytic extended jump model. We also show that a recent spectroscopy technique, two-dimensional time resolved vibrational spectroscopy (2D-IR), can be used to selectively follow the dynamics of dangling OHs, since these are spectrally distinct from H-bonded ones. By computing the first 2D-IR spectra of water next to a hydrophobic surface, we establish a connection between the spectral dynamics and the dynamical properties that we obtain directly from the simulations.
紧邻疏水基团的水分子动力学对于许多基本的生化过程至关重要，如蛋白质折叠和淀粉样纤维聚集。一些生物分子系统，如蜂毒肽或其他与膜相关的蛋白质，表现出扩展的疏水表面。由于这些表面对氢键网络施加的应变，水分子从围绕小分子的笼形排列转变为反笼形几何形状，一些悬垂的 OH 键指向表面。在这里，我们通过分子动力学模拟和分析建模研究了模型疏水表面附近的水分子重取向动力学。我们表明，紧邻疏水表面的水分子 OH 键分为两个亚群，具有不同的动力学重取向性质。第一个是与表面相切的 OH 键；这些键表现出与小分子周围水分子 OH 相似的行为，即由于表面的排斥体积效应，其重取向速度适度减缓。第二个是指向表面的悬垂 OH 键：这些键不参与任何氢键，重取向速度比在体相快得多，并且表现出异常的各向异性衰减，在几个皮秒的延迟后变为负值。在分析扩展跳跃模型中分析了氢键动力学，即不同构型之间的交换，以及由此产生的各向异性衰减。我们还表明，最近的一种光谱技术，二维时间分辨振动光谱（2D-IR）可用于选择性地跟踪悬垂 OH 的动力学，因为它们与氢键的 OH 明显不同。通过计算疏水表面附近水的第一个 2D-IR 光谱，我们建立了光谱动力学与我们直接从模拟中获得的动力学性质之间的联系。
