Assessment of the Role of 2,2,2-Trifluoroethanol Solvent Dynamics in Inducing Conformational Transitions in Melittin: An Approach with Solvent 19F Low-Field NMR Relaxation and Overhauser Dynamic Nuclear Polarization Studies

Abstract

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 19F NMR relaxation and 19F 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–D2O 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 via solvent dynamics by measuring the 19F TFE relaxation rates at 0.34 T for various TFE–D2O 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 19F 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, 19F 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 19F 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.