Nature of a H2O Molecule Confined in the Hydrophobic Interface between the Heme and G-Quartet Planes in a Heme-DNA Complex

Heme binds selectively to the 3'-terminal G-quartet of all parallel G-quadruplex DNAs to form stable heme-DNA complexes. Interestingly, the heme-DNA complexes exhibit various spectroscopic and functional properties similar to those of hemoproteins. Since the nature of the axial ligands is crucial in determining the physicochemical properties of heme, identification and characterization of the axial ligands in a heme-DNA complex are essential to elucidate the structure-function relationship in the complex. NMR studies of a complex possessing a low-spin ferric heme with a water molecule (H₂O) and cyanide ion (CN^-) as the axial ligands allowed detailed characterization of the physicochemical nature of the axial H₂O ligand. We found that the in-plane asymmetry of the heme electronic structure of the complex is not largely affected by the axial H₂O coordination, indicating that the H₂O confined in the hydrophobic interface between the heme and G-quartet planes of the complex rotates about the coordination bond with respect to the heme. The effect of the hydrogen(H)/deuterium(D) isotope replacement of the axial H₂O on the heme electronic structure was manifested in the isotope shifts of paramagnetically shifted heme methyl proton signals of the complex in such a manner that three resolved peaks associated with axial H₂O, HDO, and D₂O were observed for each of the heme methyl proton signals. These findings provide not only the basis for an understanding of the nature of the unique axial H₂O but also an insight into the molecular mechanism responsible for the control of the heme reactivity in the heme-DNA complex.

Spectroscopic investigation of the reaction of metallo-protoporphyrins with hydrogen sulfide

Hydrogen sulfide (H₂S) is the most recently discovered gasotransmitter molecule joining nitric oxide and carbon monoxide. In addition to being biologically important gases, these gasotransmitters also provide distinct modes of reactivity with biomimetic metal complexes. The majority of previous investigations on the reactivity of H₂S with bioinorganic models have focused on Fe-based porphyrin systems, whereas investigations with other metals remains underinvestigated. To address this gap, we report here an examination of the reactions of H₂S, HS^-, and S₈ with Mg^II, Cu^II, Co^II, Zn^II, Cr^II, Sn^IV, and Mn^II/III protoporphyrins.

NMR study of the molecular and electronic structure of the heme cavity in Dolabella met-cyano myoglobin

The molecular and electronic structure of the active site of the cyanide-ligated ferric complex of the myoglobin from the mollusc Dolabella auricularia has been investigated using NMR. Analysis of nuclear Overhauser effects has revealed that the correlation times for the internal motion of the heme propionate alpha-CH2 and beta-CH2 groups at ambient temperature are about 5 and 4 ns, respectively. These correlation times indicate that the terminal carboxylate groups of both the heme propionates are not bound to the protein via salt bridges. Although the absence of the propionate-protein interaction does not influence the equilibrium population of the two heme orientational isomers involving rotation about the alpha,gamma-meso axis, it allows the heme to rotate about the iron-His bond in the active site of the myoglobin. Such rotational motion of the heme resulted in an anomalous temperature-dependence of the heme methyl-proton hyperfine shift. Thus the present myoglobin studies provide the first example demonstrating the rotation of the heme about the iron-His bond in native myoglobin.