Submitted to: Inorganic Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 8, 1997
Publication Date: N/A
Interpretive Summary: Biologically active trivalent chromium (Cr) improves insulin function. Cr supplementation in humans and animals reduces symptoms of Type II diabetes and hypoglycemia. Cr also positively affects plasma lipids resulting in increased high-density lipoprotein (HDL)and decreased total cholesterol and triglycerides. Cr increases lean body mass in swine and humans. Commercial Cr nutritional supplements are chromium picolinate (CrPic) and chromium polynicotinate (CrNic). Despite widespread use neither has been fully characterized. We investigated the structures of CrPic and three preparations of chromium polynicotinate with nuclear magnetic resonance, infrared spectroscopy, and molecular modeling. In contrast all CrNic preparations are noncrystalline and results provide no evidence that nicotinic acid (NicA) is oxygen-coordinated to Cr. The colors of these Cr complexes are indicative of oxygen coordination, however this bonding may be due wholly or partly to oxygen coordination with water and/or hydroxide not with the oxygen in nicotinate. The information on differences in chemical structure of these Cr complexs is essential to enable optimization of biological processes involving Cr. Evidence is presented that CrPic and CrNic interchange ligands. This may provide the absorption mechanism through which important water soluble ligands that complex with Cr can actively transport across biological membranes into (and out of) cells.
The structures of chromium picolinate (CrPic) and three preparations of chromium nicotinate (CrNic1, CrNic2, CrNic3) were investigated with 1H and 13C NMR, FTIR and molecular modeling. CrPic is a crystalline bidentately-coordinated complex, in agreement with previous work. The strong Cr-PicA bonding broadens the NMR signal or shifts it so far downfield that it is not detectable. In contrast, all CrNic preparations are noncrystalline and results provide no evidence that nicotinic acid (NicA) is O-coordinated to Cr in CrNic complexes. The green to blue complex colors may be due wholly or partly to O-coordination with H2O and/or OH not NicA. 1H NMR spectra of CrNic1 have two sets of peaks. One set has a significant chemical shift with respect to NicA indicating that some NicA may be more strongly associated with Cr. CrNic1 13C data show small uniform chemical shift with respect to NicA indicating that strong localized Cr-COOH bonding is unlikely. The magnitude of chemical shift, 13C, and 1H exchange data suggest that limited Cr-N bonding may occur in CrNic1. CrNic2 and CrNic3 do not have two sets of peaks, and show little difference from NicA spectra. The FTIR spectra of all CrNic complexes, but not CrPic PicA or NicA show bound OH and/or H20. CrNic complexes are likely to be olates with Cr and NicA held loosely in an OH polymer. Although stable CrPic does exchange with CrNic1 in DMSO solution. This exchange may provide a mechanism for the absorption and active transport of Cr (and water soluble Cr bound ligands) within biological systems.