Oxime-phosphorus-distances calculations.

The ability of the cyclodextrin-oxime construct 6-OxP-CD to bind and degrade the nerve agents Cyclosarin (GF), Soman (GD) and S-[2-[Di(propan-2-yl)amino]ethyl] O-ethyl methylphosphonothioate (VX) has been studied using 31P-nuclear magnetic resonance (NMR) under physiological conditions. While 6-OxP-CD was found to degrade GF instantaneously under these conditions, it was found to form an inclusion complex with GD and significantly improve its degradation (t1/2 ~ 2 hrs) relative over background (t1/2 ~ 22 hrs). Consequently, effective formation of the 6-OxP-CD:GD inclusion complex results in the immediate neutralization of GD and thus preventing it from inhibiting its biological target. In contrast, NMR experiments did not find evidence for an inclusion complex between 6-OxP-CD and VX, and the agent’s degradation profile was identical to that of background degradation (t1/2 ~ 24 hrs). As a complement to this experimental work, molecular dynamics (MD) simulations coupled with Molecular Mechanics-Generalized Born Surface Area (MM-GBSA) calculations have been applied to the study of inclusion complexes between 6-OxP-CD and the three nerve agents. These studies provide data that informs the understanding of the different degradative interactions exhibited by 6-OxP-CD with each nerve agent as it is introduced in the CD cavity in two different orientations (up and down). For its complex with GF, it was found that the oxime in 6-OxP-CD lies in very close proximity (PGF⋯OOxime ~ 4–5 Å) to the phosphorus center of GF in the ‘downGF’ orientation for most of the simulation accurately describing the ability of 6-OxP-CD to degrade this nerve agent rapidly and efficiently. Further computational studies involving the center of masses (COMs) for both components (GF and 6-OxP-CD) also provided some insight on the nature of this inclusion complex. Distances between the COMs (ΔCOM) lie closer in space in the ‘downGF’ orientation than in the ‘upGF’ orientation; a correlation that seems to hold true not only for GF but also for its congener, GD. In the case of GD, calculations for the ‘downGD’ orientation showed that the oxime functional group in 6-OxP-CD although lying in close proximity (PGD⋯OOxime ~ 4–5 Å) to the phosphorus center of the nerve agent for most of the simulation, adopts another stable conformation that increase this distance to ~ 12–14 Å, thus explaining the ability of 6-OxP-CD to bind and degrade GD but with less efficiency as observed experimentally (t1/2 ~ 4 hr. vs. immediate). Lastly, studies on the VX:6-OxP-CD system demonstrated that VX does not form a stable inclusion complex with the oxime-bearing cyclodextrin and as such does not interact in a way that is conducive to an accelerated degradation scenario. Collectively, these studies serve as a basic platform from which the development of new cyclodextrin scaffolds based on 6-OxP-CD can be designed in the development of medical countermeasures against these highly toxic chemical warfare agents.

本研究在生理条件下，采用31P核磁共振波谱法（31P-nuclear magnetic resonance, NMR），探究了环糊精-肟构建体6-OxP-CD（cyclodextrin-oxime construct 6-OxP-CD）结合并降解神经毒剂（nerve agents）环沙林（Cyclosarin, GF）、梭曼（Soman, GD）以及S-[二(异丙基)氨基]乙基 O-乙基甲基硫代膦酸酯（VX）的能力。研究发现，6-OxP-CD可在该条件下瞬时降解GF，但会与GD形成包合物（inclusion complex），且相较于本底降解（半衰期t1/2约22小时），可显著提升GD的降解效率（半衰期t1/2约2小时）。因此，6-OxP-CD与GD形成的包合物可快速中和GD，阻止其抑制生物靶点。与之相反，核磁共振实验未发现6-OxP-CD与VX形成包合物的证据，该毒剂的降解曲线与本底降解完全一致（半衰期t1/2约24小时）。作为本实验研究的补充，本研究结合分子动力学（molecular dynamics, MD）模拟与分子力学-广义玻恩表面积（Molecular Mechanics-Generalized Born Surface Area, MM-GBSA）计算，探究了6-OxP-CD与三种神经毒剂形成的包合物。这些研究数据有助于理解6-OxP-CD与每种神经毒剂的不同降解相互作用——当毒剂以两种不同取向（向上与向下）进入环糊精（cyclodextrin, CD）空腔时的相互作用。针对GF与6-OxP-CD的复合物，研究发现在多数模拟时长内，当GF以“向下GF”取向进入空腔时，6-OxP-CD中的肟基与GF的磷中心距离极近（PGF⋯OOxime约4–5 Å），这准确反映了6-OxP-CD快速高效降解该神经毒剂的能力。进一步针对两种组分（GF与6-OxP-CD）的质心（center of masses, COMs）开展的计算研究，也为该包合物的本质提供了一些见解。质心间距（ΔCOM）在“向下GF”取向中相较于“向上GF”取向更短；这一相关性不仅适用于GF，对其同系物GD同样成立。就GD而言，“向下GD”取向的计算结果显示，尽管多数模拟时长内6-OxP-CD的肟官能团与GD的磷中心距离较近（PGD⋯OOxime约4–5 Å），但该体系会形成另一种稳定构象，使该间距增至约12–14 Å，这解释了实验中观察到的6-OxP-CD结合并降解GD的效率较低的现象（半衰期t1/2约4小时 vs 瞬时降解）。最后，针对VX:6-OxP-CD体系的研究表明，VX无法与该含肟基环糊精形成稳定包合物，因此不会以利于加速降解的方式发生相互作用。综上，本研究可为基于6-OxP-CD的新型环糊精骨架开发提供基础平台，助力针对这类高毒性化学战剂（chemical warfare agents）的医学对抗手段研发。