Heme {Fe(II)- or Fe(III)-protoporphyrin IX complex [heme(Fe(2+)) or heme(Fe(3+)), respectively]} binds selectively to the 3'-terminal G-quartet of a parallel G-quadruplex DNA formed from a single repeat sequence of the human telomere, d(TTAGGG), through a π-π stacking interaction between the porphyrin moiety of the heme and the G-quartet. The binding affinities of some chemically modified hemes(Fe(3+)) for DNA and the structures of complexes between the modified hemes(Fe(2+)) and DNA, with carbon monoxide (CO) coordinated to the heme Fe atom on the side of the heme opposite the G6 G-quartet, have been characterized to elucidate the interaction between the heme and G-quartet in the complexes through analysis of the effects of the heme modification on the structural properties of the complex. The study revealed that the binding affinities and structures of the complexes were barely affected by the heme modification performed in the study. Such plasticity in the binding of heme to the G-quartet is useful for the versatile design of the complex through heme chemical modification and DNA sequence alteration. Furthermore, exchangeable proton signals exhibiting two-proton intensity were observed at approximately -3.5 ppm in the (1)H nuclear magnetic resonance (NMR) spectra of the CO adducts of the complexes. Through analysis of the NMR results, together with theoretical consideration, we concluded that the heme(Fe(2+)) axial ligand trans to CO in the complex is a water molecule (H2O). Identification of the Fe-bound H2O accommodated between the heme and G-quartet planes in the complex provides new insights into the structure-function relationship of the complex.

The molecular structure and heme electronic structure of a complex (heme–(d(TTAGGGT))4 complex) between heme and a parallel G-quadruplex DNA formed from heptanucleotide d(TTAGGGT), i.e., (d(TTAGGGT))4, have been characterized using 1H NMR. The study demonstrated that the heme is accommodated between G6 and T7 of the G-quadruplex DNA to form a 1:1 complex between the heme and DNA. The spin state of the heme Fe3+ of the heme(Fe3+)–(d(TTAGGGT))4 complex exhibited a characteristic pH-dependent change from a high spin (HS) state, i.e., S = 5/2, at low pH to a thermal spin equilibrium between the HS and low spin state (LS), i.e., S = 1/2, at high pH, with a midpoint at the pH value of 8.9 ± 0.2. The pH-dependent spin state change of the heme(Fe3+)–(d(TTAGGGT))4 complex was similar to that of metmyoglobin (T. M. McGrath and G. N. La Mar, Biochim. Biophys. Acta, 1978, 534, 99–111), demonstrating that these two systems are highly alike in terms of the heme environment. This finding provides novel insights as to the design of the heme–DNA complexes that exhibit catalytic activities similar to those of hemoproteins.

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