Heme binds selectively to the 3'-terminal G-quartet (G6 G-quartet) of an all parallel-stranded tetrameric G-quadruplex DNA, [d(TTAGGG)], to form a heme-DNA complex. Complexes between [d(TTAGGG)] and a series of chemically modified hemes possessing a heme Fe atom with a variety of electron densities were characterized in terms of their peroxidase activities to evaluate the effect of a change in the electron density of the heme Fe atom (ρ) on their activities. The peroxidase activity of a complex decreased with a decreasing ρ, supporting the idea that the activity of the complex is elicited through a reaction mechanism similar to that of a peroxidase. In the ferrous heme-DNA complex, carbon monoxide (CO) can bind to the heme Fe atom on the side of the heme opposite the G6 G-quartet, and a water molecule (HO) is coordinated to the Fe atom as another axial ligand, trans to the CO. The stretching frequencies of Fe-bound CO (ν) and the Fe-C bond (ν) of CO adducts of the heme-DNA complexes were determined to investigate the structural and electronic natures of the axial ligands coordinated to the heme Fe atom. Comparison of the ν and ν values of the heme-DNA complexes with those of myoglobin (Mb) revealed that the donor strength of the axial ligation trans to the CO in a complex is considerably weaker than that of the proximal histidine in Mb, as expected from the coordination of HO trans to the CO in the complex.

Heme in its ferrous and ferric states [heme(Fe 2+ ) and heme(Fe 3+ ), respectively] binds selectively to the 3′-terminal G-quartet of all parallel-stranded monomeric G-quadruplex DNAs formed from inosine(I)-containing sequences, i.e., d(TAGGGTG-GGTTGGGTGIG) DNA(18mer) and d(TAGGGTGGGTTGG-GTGIGA) DNA(18mer/A), through a π−π stacking interaction between the porphyrin moiety of the heme and the G-quartet, to form 1:1 complexes [heme−DNA(18mer) and heme−DNA-(18mer/A) complexes, respectively]. These complexes exhibited enhanced peroxidase activities, compared with that of heme(Fe 3+ ) alone, and the activity of the heme(Fe 3+ )−DNA(18mer/A) complex was greater than that of the heme(Fe 3+ )−DNA(18mer) one, indicating that the 3′-terminal A of the DNA sequence acts as an acid−base catalyst that promotes the catalytic reaction. In the complexes, a water molecule (H 2 O) at the interface between the heme and G-quartet is coordinated to the heme Fe atom as an axial ligand and possibly acts as an electron-donating ligand that promotes heterolytic peroxide bond cleavage of hydrogen peroxide bound to the heme Fe atom, trans to the H 2 O, for the generation of an active species. The intermolecular nuclear Overhauser effects observed among heme, DNA, and Fe-bound H 2 O indicated that the H 2 O rotates about the H 2 O−Fe coordination bond with respect to both the heme and DNA in the complex. Thus, the H 2 O in the complex is unique in terms of not only its electronic properties but also its dynamic ones. These findings provide novel insights into the design of heme− deoxyribozymes and −ribozymes.

Heme in the ferric state (heme(Fe3+)) binds to G-quadruplex DNAs to form stable complexes that exhibit enhanced peroxidase activities. The complexes are considered DNAzymes possessing heme as a prosthetic group (heme-DNAzymes), and have been extensively investigated as promising catalysts for a variety of applications. On ESR and stopped-flow measurements, an iron(IV)oxo porphyrin π-cation radical known as Compound I was detected in reaction mixtures of heme-DNAzymes and hydrogen peroxide. This finding not only resolved the long-standing issue of the mechanism underlying the enhancement of the peroxidase activity of heme(Fe3+) in the scaffold of a G-quadruplex DNA, but also provided new insights as to the design of novel heme-DNAzymes.

Heme binds to a parallel-stranded G-quadruplex DNA to form a peroxidase-mimicking heme-DNAzyme. An interpolyelectrolyte complex between the heme-DNAzyme and a cationic copolymer possessing protonated amino groups was characterized and the...