High-Field Solid-State 67Zn NMR Spectroscopy of Several Zinc−Amino Acid Complexes
Kamal H. Mrou and William P. Power*
We report the results of our solid-state 67Zn NMR study of the various zinc sites in four zinc−amino acid coordination complexes: bis(glycinato)zinc(II) monohydrate; bis(l-alaninato)zinc(II); bis(l-histidinato)zinc(II) dihydrate; and sodium bis(l-cysteinato)zincate(II) hexahydrate; as well as a related complex, bis(imidazole)zinc(II) chloride. We demonstrate the advantages of using high (21.1 T) applied magnetic fields for detecting 67Zn directly at ambient temperatures using the quadrupolar Carr−Purcell Meiboom−Gill (QCPMG) pulse sequence. The stepped-frequency technique was employed in cases where the central-transition (CT) 67Zn NMR spectra were too broad to be uniformly excited. The parameters of the anisotropic zinc tensors were extracted by iterative simulations of the experimental spectra. In all cases, the quadrupolar interaction is found to dominate the central-transition 67Zn NMR spectra; no convincing effects from chemical shift anisotropy (CSA) on the NMR spectra of the five complexes could be reliably detected at this field strength. Analyses of the experimental NMR spectra reveal that the 67Zn quadrupolar coupling constants (CQ) range from 7.05 to 26.4 MHz, the isotropic chemical shifts (δiso) range from 140 to 265 ppm, and the quadrupolar asymmetry parameters (ηQ) range from 0.20 to 0.95. The first report of the NMR spectral features of pentacoordinated zinc sites is included for two complexes. Quantum chemical calculations of the electric field gradient (EFG) and magnetic shielding tensors reproduced the experimental results to a reasonable extent. Moreover, the computationally determined orientations of both tensors permit correlations between NMR tensor properties and zinc local environments to be understood.