Solid-State NMR Investigations of 67Zn and 27Al Nuclei in Zinc-Amino Acid Complexes, Zinc-Insulin Hexamers, and Aluminum-Centered Dyes
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Modern solid-state nuclear magnetic resonance (NMR) methodologies are applied to investigate two spin-5/2 nuclei, Zn-67 and Al-27, in different coordination environments in order to characterize the magnitudes and orientations of their electric field gradient (EFG) and nuclear magnetic shielding tensors. The advantages of using high (21.1 T) applied magnetic fields for detecting Zn-67 directly at ambient temperatures, using the quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequence and the stepped-frequency technique, are demonstrated by the successful investigation of the different zinc sites in several zinc-amino acid coordination compounds, and in the more complex polymorphs of the zinc-insulin hexamers. In all systems, the high-field Zn-67 NMR spectra are dominated only by the quadrupolar interaction. The first two Zn-67 NMR spectra of pentacoordinated zinc sites are reported and analyzed. The experimental results are corroborated by ab initio and density functional theory (DFT) calculations of the Zn-67 NMR parameters in order to gain better understanding of the zinc local electronic environments. Solid-state Al-27 NMR is applied to study three commercial aluminum-phthalocyanine dyes. Solid-state Al-27 NMR experiments, including multiple-quantum magic-angle-spinning (MQMAS) and quadrupolar Carr-Purcell Meiboom-Gill (QCPMG), are employed at multiple high magnetic field strengths (11.7, 14.1 and 21.1 T) to determine the composition and number of aluminum distinct sites in these dyes. The quadrupolar parameters for each Al-27 site are determined from spectral simulations, with quadrupolar coupling constants ranging from 5.40 to 10.0 MHz and asymmetry parameters ranging from 0.10 to 0.50, and compare well with the results of quantum chemical calculations of these tensors. The largest Al-27 chemical shielding anisotropy (CSA), with a span of 120 ppm, observed directly in a solid material is also reported. The combination of MQMAS and computational chemistry are used to interpret the presence of multiple aluminum sites in two of the three samples.