Proton NMR study of model substrate binding in hemoproteins. Intercalation of mercuric triiodide in sperm whale met-aquo myoglobin.

1H-NMR spectra have been recorded for sperm whale met-aquo myoglobin intercalated with xenon, cyclopropane, mercuric triiodide and auric triiodide. All four agents are known to intercalate on the proximal side of the heme over pyrrole A. The complexes of xenon and cyclopropane exhibit insignificant shifts for all four heme methyls, suggesting that these species fit into an existing hole without causing significant perturbations on the heme cavity. Mercuric and auric triiodide, on the other hand, induce substantial changes in the hyperfine-shifts for the heme methyls. Based on the previously assigned methyl peaks in met-aquo myoglobin, we find that methyl-1, closest to the intercalating agent, is affected most, with the influence decreasing with distance from the binding site. These results indicate that determination of the relative perturbations on the assigned heme methyl shifts due to substrate binding can be utilized to ascertain the substrate-heme stereochemistry in high-spin ferric hemoproteins such as peroxidases.