In-depth study of the heme-binding ability to five heavy metals: Hg, Cd, Pb, Cr, and As

The molecular structural characteristics of the complexes formed by the substitution of the central atom of heme (iron, Fe) by mercury (Hg), cadmium (Cd), lead (Pb), chromium (Cr), or arsenic (As) were studied using density functional theory (DFT). The simulated infrared and ultraviolet-visible spectra, Wiberg bond levels, frontier molecular orbital energy levels, and the binding capacity of the heme ligand to the central atom were assessed for all complexes. The results showed that when Fe was replaced by Hg, Cd, Pb, Cr, or As, the resulting complex was stable, with unique spectroscopic characteristics. Interestingly, the binding capacity of the heme ligand to Pb was about 10 times greater than that of the heme ligand to Fe and more than 10 times greater than that of the heme ligand to any of the other four heavy metals. The complex in which Fe was replaced by Cd had the greatest chemical stability. These results will provide an important reference for the future pollution control programs and will support efforts to enrich the five studied heavy metals using heme ligands, especially the specific capture of Pb and the enrichment of Cd.