Determination of the 19F NMR chemical shielding tensor and crystal structure of 5-fluoro- dl-tryptophan

Abstract

5-Fluoro- dl-tryptophan (5F-Trp) is a very sensitive probe used to investigate orientation and dynamics of biomacromolecules at the in situ level. In order to establish a 19F NMR strategy, the crystal structure and 19F chemical shielding tensor of 5F-Trp are reported. A novel approach was developed to use F–F homonuclear dipole–dipole coupling information to analyze single-crystal NMR data without determining crystal orientations. The measured values for the principal components of the shielding tensor are σ 11 = 0.9, σ 22 = −63.3, and σ 33 = −82.9 ppm relative to TFA in D 2O. The principal axes of the shielding tensors coincide with the indole ring symmetry, which makes it a straightforward and powerful tool to monitor protein alignment in oriented environments. Hartree–Fock (HF) and density functional theory (DFT) calculations of the chemical shielding tensors are also reported.