Structures and Catalytic Activities of Complexes between Heme and All Parallel-stranded Monomeric G-quadruplex DNAs

Heme in its ferrous and ferric states [heme(Fe2+) and heme(Fe3+), 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(TAGGGTGGGTTGGGTGIG) DNA(18mer) and d(TAGGGTGGGTTGGGTGIGA) 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(Fe3+) alone, and the activity of the heme(Fe3+)–DNA(18mer/A) complex was greater than that of the heme(Fe3+)–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 (H2O) 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 prom...