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

Wong, Alan and Laurencin, Danielle and Wu, Gang and Dupree, Ray and Smith, Mark E. (2008) An ab initio quantum chemical investigation of Ca-43 NMR interaction parameters for the Ca2+ sites in organic complexes and in metalloproteins. Journal of Physical Chemistry A, 112 (40). pp. 9807-9813. ISSN 1520-5215

Abstract

We have carried out an extensive ab initio quantum chemical (QC) Ca-43 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 Ca-43 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)(Ca-43). Although relatively small values of chi(q)(Ca-43) are found for Ca-O organic compounds with a coordination number between 6 and 10, the QC shows that chi(q)(Ca-43) 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 Ca-43 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 Ca-43 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 Ca-43 MAS NMR spectra of Ca(L-glutamate)(2)center dot 4H(2)O were recorded, and delta(iso)(Ca-43) and the quadrupolar parameter (P-q) were estimated to be 6.6 ppm and 0.8 MHz, respectively.