Electron Paramagnetic Resonance Measurements of Four Nitroxide Probes in Supercooled Water Explained by Molecular Dynamics Simulations
Electron Paramagnetic Resonance Measurements of Four Nitroxide Probes in Supercooled Water Explained by Molecular Dynamics Simulations
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DOI：
10.1021/acs.jpcb.0c00684
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发表时间：
2020-05-14
影响因子：
3.3
通讯作者：
Luchko，Tyler
中科院分区：
文献类型：
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作者：
McMillin，Patrick J.;Alegrete，Matthew;Luchko，Tyler
电子顺磁共振（EPR）测量的旋转扩散的小氮氧探针已被证明是一个强大的技术实验研究过冷液体，如水的性质。然而，由于仅测量探针分子的旋转扩散，并且EPR测量是间接的，因此不清楚水的行为和探针分子之间的关系。为了解决这个问题，我们进行了分子动力学模拟的四氮氧探针TIP4P-Ew和OPC水模型直接与EPR实验比较，并确定水的行为和水和探针之间的潜在微观耦合。在253和283 K之间的23个温度下，对所有四个探针进行了200 ns的模拟，每个水模型的模拟时间总计为36.8 μ s。两个水模型的模拟系统低估了水和探针的旋转扩散系数，虽然OPC模拟一般比TIP4P-Ew模拟更好地与实验一致。尽管如此，当数据的温度依赖性符合幂律时，TIP4P-Ew的拟合参数通常比OPC更符合实验。对于探针分子，实验测得其奇异温度为T0 = 226.5 ± 0.4K，对于OPC水为T0 = 208 ± 2K，对于TIP4P-Ew水为T0 = 215 ± 2K。而对于水分子，实验测得其奇异温度为T0 = 220.3 ± 0.2K，OPC水为T0 = 208 ± 2K，TIP4P-Ew水为T0 = 220 ± 1K。系统的旋转扩散系数的低估是最明显的在较低的温度下，并清楚地观察到的变化的Arrhenius活化能。在最大密度温度T ρ max = 277 K以上，实验探针的EA活化能为16.7 kJ/mol，OPC的EA活化能为15.2 kJ/mol，TIP4P-Ew的EA活化能为14.6 kJ/mol。在最大密度温度以下，实验的活化能为EA = 32.5kJ/mol，而OPC和TIP4P-Ew的活化能分别为EA = 23kJ/mol和EA = 22kJ/mol。在所有情况下，我们看到探针分子和水的行为之间有很好的一致性。为了理解原因，我们计算了探针分子和水之间氢键的平均数量。由此，我们能够解释所有探针的旋转扩散时间。这些结果表明，目前的分子模型足以捕捉EPR观察到的物理现象，并有助于阐明为什么探针提供准确的见解过冷水的行为。
Electron paramagnetic resonance (EPR) measurements of the rotational diffusion of small nitroxide probes have been demonstrated to be a powerful technique for experimentally investigating the properties of supercooled liquids, such as water. However, since only the rotational diffusion of the probe molecules is measured and EPR measurements are indirect, it is not clear what the relationship between the behavior of water and the probe molecule is. To address this, we have performed molecular dynamics simulations of four nitroxide probes in TIP4P-Ew and OPC water models to directly compare with EPR experiments and to determine the behavior of the water and the underlying microscopic coupling between the water and the probes. In all, 200 ns simulations were run for 23 temperatures between 253 and 283 K for all four probes with each water model for an aggregate of 36.8 μs of simulation time. Simulations for both water models systematically underestimated the rotational diffusion coefficients for both water and probes, though OPC simulations were generally in better agreement with the experiments than TIP4P-Ew simulations. Despite this, when the temperature dependence of the data was fit to a power law, fit parameters for TIP4P-Ew were generally in better agreement with the experiments than OPC. For probe molecules, the singular temperature was found to beT0= 226.5 ± 0.4 K from experiments,T0= 208 ± 2 K for OPC water, andT0= 215 ± 2 K for TIP4P-Ew water. While for water molecules, the singular temperature was found to beT0= 220.3 ± 0.2 K from experiments,T0= 208 ± 2 K for OPC water, andT0= 220 ± 1 K for TIP4P-Ew water. Systematic underestimation of the rotational diffusion coefficients was most pronounced at lower temperatures and was clearly observed in changes to the Arrhenius activation energy. Above the maximum density temperature ofTρmax= 277 K, an activation energy ofEA≈ 16.7 kJ/mol was observed for the probes from experiments, while OPC hadEA≈ 15.2 kJ/mol and TIP4P-Ew hadEA≈ 14.6 kJ/mol. Below the maximum density temperature, the activation energy jumped toEA≈ 32.5 kJ/mol for experiments but onlyEA≈ 23 kJ/mol for OPC andEA≈ 22 kJ/mol for TIP4P-Ew. In all cases, we saw good agreement between the behavior of the probe molecules and water. To understand why, we calculated the average number of hydrogen bonds between the probe molecules and water. From this, we were able to explain the rotational diffusion times for all of the probes. These results show that current molecular models are sufficient to capture physical phenomena observed with EPR and to help elucidate why the probes provide accurate insights into the behavior of supercooled water.