A 1H/19F minicoil NMR probe for solid-state NMR:: Application to 5-fluoroindoles

We show that it is feasible to use a minicoil for solid-state F-19 H-1 NMR experiments that has short pulse widths, good RF homogeneity, and excellent signal-to-noise for small samples while using low power amplifiers typical to liquid-state NMR. The closely spaced resonant frequencies of H-1 and F-19 and the ubiquitous use of fluorine in modern plastics and electronic components present two major challenges in the design of a high-sensitivity, high-field H-1/F-19 probe. Through the selection of specific components, circuit design, and pulse sequence, we were able to build a probe that has low 19F background and excellent separation of H-1 and F-19 signals. We determine the principle components of the chemical shift anisotropy tensor of 5-fluoroindole-3-acetic acid (5FIAA) and 5-fluorotryptophan. We also solve the crystal structure of 5FIAA, determine the orientation dependence of the chemical shift of a single crystal of 5FIAA, and predict the F-19 chemical shift based on the orientation of the fluorine in the crystal. The results show that this H-1/F-19 probe is suitable for solid-state NMR experiments with low amounts of biological molecules that have been labeled with F-19. (c) 2005 Elsevier Inc. All rights reserved.