An ab initio quantum chemical investigation of 43Ca NMR interaction parameters for the Ca2+ sites in organic complexes and in metalloproteins.

We have carried out an extensive ab initio quantum chemical (QC)43Ca NMR study on a series of Ca-O organic compounds and three different Ca-bound proteins and found that the HF/6-31G* level of function can reliably predict 43Ca NMR interaction parameters (delta(iso) and chi(q)), especially for organic solids. This QC study finds correlations between Ca-O bond environment (mean distance and coordination number) and delta(iso)(43Ca). Although relatively small values of chi(q)(43Ca) are found for Ca-O organic compounds with a coordination number between 6 and 10, the QC shows that chi(q)(43Ca) is sensitive to the Ca-O coordination geometry of the Ca2+ sites in metalloproteins--a potentially important observation. An application of such ab initio QC 43Ca NMR studies is in characterizing the Ca-O bonding environment around target Ca2+ sites. As an example, we propose a new potential analytical approach using the absolute (43)Ca chemical shielding constant to investigate the hydration shell of Ca2+ in a dilute CaCl2 aqueous solution. Furthermore, by adopting a NMR methodology similar to that reported in Wong et al. Chem. Phys. Lett. 2006, 427, 201, natural abundance 43Ca MAS NMR spectra of Ca(L-glutamate)(2) x 4H2O were recorded, and delta(iso)(43Ca) and the quadrupolar parameter (Pq) were estimated to be 6.6 ppm and 0.8 MHz, respectively.