A solid-state 17O NMR, X-ray, and quantum chemical study of N-α-Fmoc-protected amino acids

We report the results of a solid-state 17O NMR and X-ray investigation of 17O-enriched N-α-fluoren-9-ylmethoxycarbonyl-l-alanine (Fmoc-l-ALA) and N-α-fluoren-9-ylmethoxycarbonyl-O-t-butyl-l-serine (Fmoc-l-SER). The present X-ray results for Fmoc-l-SER show that the compound crystallized in the monoclinic space group P21 with unit-cell dimensions a = 5.843, b = 11.937, c = 15.042 Å, and β = 96.19°. Analysis of 17O magic-angle spinning (MAS) spectra and stationary NMR spectra recorded at multiple magnetic fields of the present Fmoc-protected amino acids yields the magnitudes of hydroxyl and carbonyl 17O electric-field-gradient (EFG) and chemical shielding (CS) tensors with the relative orientations between the two NMR tensors. The 17O quadrupole coupling constants (CQ) are found to be 7.05–7.60 MHz and 7.90–8.35 MHz, and the spans of the CS tensors are 218–236 ppm and 450–521 ppm, for hydroxyl and carbonyl oxygen atoms, respectively. We also carry out quantum chemical calculations using density functional theory in order to investigate the effects of hydrogen-bond angles on 17O NMR parameters. It is demonstrated that, in addition to the hydrogen bond distances, hydrogen bond angles are an important factor in determining the magnitudes of these tensor components.