A solid-state 17O NMR study of L-phenylalanine and L-valine hydrochlorides

We have presented an experimental investigation of the oxygen-17 chemical shielding (CS) and electric-field-gradient (EFG) tensors for alpha-COOH groups in polycrystalline amino acid hydrochlorides. The 170 CS and EFG tensors including the relative orientations between the two NMR tensors are determined in [O-17]-L-phenylaianine hydrochloride and [ O-17]-L-valine hydrochloride by the analysis of the 170 magic-angle-spinning (MAS) and stationary NMR spectra obtained at 9.4,11.7,16.4, and 21.8T. The quadrupole coupling constants (C-Q) and the span of the CS tensors are found to be 8.41-8.55 MHz and 7.35-7.41 MHz, and 548-570 ppm and 225-231 ppm, for carbonyl and hydroxyl oxygen atoms, respectively. Extensive quantum chemical calculations using density functional theory (DFT) have been also carried out for a hydrogen-bonding model. It is demonstrated that the behavior of the dependence of hydrogen-bond distances on O-17 NMR tensors for the halogen ions is different from those for the water molecule. Copyright (c) 2008 John Wiley & Sons, Ltd.