Chaubey Bhawna, Dey Arnab, Banerjee Abhishek, Chandrakumar N, Pal Samanwita
Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur, Rajasthan 342037, India.
MRI-MRS Centre, Indian Institute of Technology Madras, Madras, Tamil Nadu 600036, India.
J Phys Chem B. 2020 Jul 16;124(28):5993-6003. doi: 10.1021/acs.jpcb.0c03544. Epub 2020 Jul 7.
2,2,2-Trifluoroethanol (TFE) is one of the fluoroalcohols that have been known to induce and stabilize an open helical structure in many proteins and peptides. The current study has benchmarked low-field F NMR relaxation and F Overhauser dynamic nuclear polarization (ODNP) by providing a brief account of TFE solvent dynamics in a model melittin (MLT, an antimicrobial peptide) solution with a TFE-DO cosolvent mixture at pH 7.4. Further, this approach has been employed to reveal the solvation of MLT by TFE in a nonbuffered solution with pH 2.8 for the first time. The structural transition of MLT has been elucidated solvent dynamics by measuring the F TFE relaxation rates at 0.34 T for various TFE-DO compositions in the absence (bulk TFE) and in the presence of MLT at both the pH values. A complementary initial record of circular dichroism experiments on these aqueous MLT solutions with TFE as the cosolvent at two different pH conditions demonstrated the structural transition from a random coil to a helical or from a folded helical to an open helical structure. The molecular correlation time derived from the corresponding relaxation rates shows that TFE resides on the MLT surface in both pH conditions. However, the trends in the variation of molecular correlation time ratio as a function of TFE concentration represent that the mechanism and the extent to which TFE affects the MLT structural integrity are different at different pH values. The extraction of the DNP coupling parameter from steady-state F ODNP experiments performed in the presence of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl at 0.34 T revealed changes in the solvation dynamics of TFE concomitant with the MLT structural transition. In summary, F relaxation and ODNP measurements made at a low field have allowed direct monitoring of TFE dynamics during the MLT structural transition in terms of preferential solvation. The choice of experiments performed at a moderately low field (0.34 T) enabled us to exploit on the one hand almost 1200-fold mitigation of the strong contribution of F chemical shift anisotropy at 11.76 T, whereas on the other hand, the ODNP experiment offered a window for probing molecular dynamics on timescales of the order of 10-1000 ps.
2,2,2-三氟乙醇（TFE）是一种已知能在许多蛋白质和肽中诱导并稳定开放螺旋结构的氟代醇。当前的研究通过简要描述在pH 7.4的含TFE-DO共溶剂混合物的模型蜂毒素（MLT，一种抗菌肽）溶液中的TFE溶剂动力学，对低场氟核磁共振弛豫和氟奥弗豪泽动态核极化（ODNP）进行了基准测试。此外，该方法首次被用于揭示在pH 2.8的非缓冲溶液中TFE对MLT的溶剂化作用。通过测量在两种pH值下，无MLT（纯TFE）和有MLT存在时，不同TFE-DO组成在0.34 T下的氟- TFE弛豫率，阐明了MLT的结构转变与溶剂动力学的关系。在两种不同pH条件下，以TFE作为共溶剂的这些水性MLT溶液的圆二色性实验的补充初始记录表明，其结构从无规卷曲转变为螺旋结构，或从折叠螺旋转变为开放螺旋结构。从相应弛豫率得出的分子相关时间表明，在两种pH条件下TFE都位于MLT表面。然而，分子相关时间比随TFE浓度变化的趋势表明，在不同pH值下，TFE影响MLT结构完整性的机制和程度是不同的。在0.34 T下，在4-羟基-2,2,6,6-四甲基哌啶-1-氧基存在的情况下进行稳态氟-ODNP实验，提取DNP耦合参数，揭示了伴随MLT结构转变的TFE溶剂化动力学变化。总之，在低场下进行的氟弛豫和ODNP测量能够从优先溶剂化的角度直接监测MLT结构转变过程中的TFE动力学。在中等低场（0.34 T）下进行实验的选择，一方面使我们能够利用几乎1200倍的程度减轻在11.76 T下氟化学位移各向异性的强烈贡献，而另一方面，ODNP实验为探测10 - 1000 ps量级时间尺度上的分子动力学提供了一个窗口。
