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

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 (19)F NMR strategy, the crystal structure and (19)F 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 sigma(11)=0.9, sigma(22)=-63.3, and sigma(33)=-82.9 ppm relative to TFA in D(2)O. 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.