Franck John M, Han Songi
Department of Chemistry, Syracuse University, Syracuse, NY, United States.
Department of Chemistry and Biochemistry, University of California Santa Barbara, Santa Barbara, CA, United States; Department of Chemical Engineering, University of California Santa Barbara, Santa Barbara, CA, United States.
Methods Enzymol. 2019;615:131-175. doi: 10.1016/bs.mie.2018.09.024. Epub 2018 Dec 11.
We outline the physical properties of hydration water that are captured by Overhauser Dynamic Nuclear Polarization (ODNP) relaxometry and explore the insights that ODNP yields about the water and the surface that this water is coupled to. As ODNP relies on the pairwise cross-relaxation between the electron spin of a spin probe and a proton nuclear spin of water, it captures the dynamics of single-particle diffusion of an ensemble of water molecules moving near the spin probe. ODNP principally utilizes the same physics as other nuclear magnetic resonance (NMR) relaxometry (i.e., relaxation measurement) techniques. However, in ODNP, electron paramagnetic resonance (EPR) excites the electron spins probes and their high net polarization acts as a signal amplifier. Furthermore, it renders ODNP parameters highly sensitive to water moving at rates commensurate with the EPR frequency of the spin probe (typically 10GHz). Also, ODNP selectively enhances the NMR signal contributions of water moving within close proximity to the spin label. As a result, ODNP can capture ps-ns movements of hydration waters with high sensitivity and locality, even in samples with protein concentrations as dilute as 10 µM. To date, the utility of the ODNP technique has been demonstrated for two major applications: the characterization of the spatial variation in the properties of the hydration layer of proteins or other surfaces displaying topological diversity, and the identification of structural properties emerging from highly disordered proteins and protein domains. The former has been shown to correlate well with the properties of hydration water predicted by MD simulations and has been shown capable of evaluating the hydrophilicity or hydrophobicity of a surface. The latter has been demonstrated for studies of an interhelical loop of proteorhodopsin, the partial structure of α-synuclein embedded at the lipid membrane surface, incipient structures adopted by tau proteins en route to fibrils, and the structure and hydration profile of a transmembrane peptide. This chapter focuses on offering a mechanistic understanding of the ODNP measurement and the molecular dynamics encoded in the ODNP parameters. In particular, it clarifies how the electron-nuclear dipolar coupling encodes information about the molecular dynamics in the nuclear spin self-relaxation and, more importantly, the electron-nuclear spin cross-relaxation rates. The clarification of the molecular dynamics underlying ODNP should assist in establishing a connection to theory and computer simulation that will offer far richer interpretations of ODNP results in future studies.
我们概述了通过奥弗豪泽动态核极化（ODNP）弛豫测量法所捕获的水化水的物理性质，并探讨了ODNP对水以及与该水耦合的表面所产生的见解。由于ODNP依赖于自旋探针的电子自旋与水的质子核自旋之间的成对交叉弛豫，它捕获了在自旋探针附近移动的水分子集合的单粒子扩散动力学。ODNP主要利用与其他核磁共振（NMR）弛豫测量法（即弛豫测量）技术相同的物理原理。然而，在ODNP中，电子顺磁共振（EPR）激发电子自旋探针，其高净极化充当信号放大器。此外，它使ODNP参数对以与自旋探针的EPR频率（通常为10GHz）相当的速率移动的水高度敏感。而且，ODNP选择性地增强了在自旋标记附近移动的水的NMR信号贡献。因此，即使在蛋白质浓度低至10µM的样品中，ODNP也能以高灵敏度和局部性捕获水化水的皮秒至纳秒级运动。迄今为止，ODNP技术的实用性已在两个主要应用中得到证明：表征蛋白质或其他具有拓扑多样性的表面的水化层性质的空间变化，以及识别高度无序的蛋白质和蛋白质结构域中出现的结构性质。前者已被证明与分子动力学模拟预测的水化水性质密切相关，并已被证明能够评估表面的亲水性或疏水性。后者已在视紫红质的螺旋间环、嵌入脂质膜表面的α-突触核蛋白的部分结构、tau蛋白形成纤维过程中采用的初始结构以及跨膜肽的结构和水化轮廓的研究中得到证明。本章重点在于提供对ODNP测量以及ODNP参数中编码的分子动力学的机理理解。特别是，它阐明了电子-核偶极耦合如何在核自旋自弛豫中，更重要的是在电子-核自旋交叉弛豫率中编码有关分子动力学的信息。对ODNP背后分子动力学的阐明应有助于建立与理论和计算机模拟的联系，这将在未来的研究中为ODNP结果提供更丰富的解释。
