Chemical shielding and electric field gradient tensors of disodium 5′-adenosine triphosphate trihydrate determined by solid-state NMR spectroscopy and ab initio calculations

The tri-phosphate and sodium ion environments in well crystallized disodium 5′adenosine triphosphate trihydrate (ATP•3H₂O) are investigated using high-resolution solid-state NMR spectroscopy and ab initio calculations. The six inequivalent phosphorus sites resolved in ³¹P MAS spectra have been assigned on the basis of experimentally obtained results and theoretical ab initio quantum chemical calculations of ³¹P chemical shielding tensors with increased numerical accuracy. Aided by ³¹P-³¹P dipolar connectivity established in DQ-SQ correlation experiment and DFT calculations carried out at the B3LYP/(aug-cc-pDVZ(3s,2p,1d), 6–31G(d,p)) level, phosphorus resonance assignments in MAS spectra have been made and the ³¹P chemical shielding anisotropies and tensor orientations in the molecule-fixed frame have been determined. ³¹P shielding tensors are found to be oriented with the most shielded direction nearly perpendicular to the O-P-O plane involving the two adjoining oxygens. The complete resolution of the four sodium sites has been achieved from 3Q-MAS experiments performed at 7.05 T and this has enabled ²³Na quadrupole parameters to be determined experimentally and compared with theoretical calculations. Besides aiding a partial resonance assignment of the ²³Na 3Q-MAS spectrum, our HF/6–311++G(2d,2p) calculations of ²³Na EFG tensors show that for two sodium sites which are tightly coordinated the orientation of the EFG tensor is distinct. These exhibit unique directions along which the field gradient is the smallest and largest.