Evaluating the effect of ¹H decoupling on ¹⁹F longitudinal relaxation and signal line shape in 5-Fluorotryptophan

Fluorine 19 (¹⁹F) in aromatic rings serves as a useful spin probe for studying the structure, dynamics, and intermolecular interactions of biomolecules and small-molecule ligands. Although ¹⁹F NMR involving aromatic ¹⁹F probes has a number of applications, quantitative relaxation analyses of these probes remain challenging due to the complexity of ¹H-¹⁹F spin interaction networks. In this study, we investigated ¹H-¹⁹F spin interactions in the 5-fluorotryptophan system, focusing on the effects of ¹H decoupling on ¹⁹F relaxation properties and line shape. We demonstrate that ¹H decoupling significantly slows down the ¹⁹F magnetization recovery process, leading to reduced sensitivity particularly in large proteins. In addition, ¹H decoupling effectively eliminates cross-correlations between ¹⁹F chemical shift anisotropy and ¹H-¹⁹F dipole-dipole interactions, resolving the asymmetric line shape of the aromatic ¹⁹F signal. Through experimental and theoretical analyses of the ¹H-coupled ¹⁹F spin system, we propose a model that explains these relaxation behaviors. Our results offer practical guidelines for optimizing ¹H decoupling schemes in aromatic ¹⁹F probes, thereby expanding the utility of ¹⁹F NMR.