Interaction between the Heme and a G-Quartet in a Heme-DNA Complex

The structure of a complex between heme(Fe3+) and a parallel G-quadruplex DNA formed from a single repeat sequence of the human telomere, d(TTAGGG), has been characterized by H-1 NMR The study demonstrated that the heme(Fe3+) is sandwiched between the 3'-terminal G-quartets of the G-quadruplex DNA. Hence, the net +1 charge of the heme(Fe3+) in the complex is surrounded by the eight carbonyl oxygen atoms of the G-quartets. Interaction between the heme Fe3+ and G-quartets in the complex was clearly manifested in the solvent H-1/H-2 isotope effect on the NMR parameters of paramagnetically shifted heme methyl proton signals, and interaction of the heme Fe3+ with the eight carbonyl oxygen atoms of the two G-quartets was shown to provide a strong and axially symmetric ligand field surrounding the heme Fe3+, yielding a heme(Fe3+) low-spin species with a highly symmetric heme electronic structure. This finding provides new insights as to the design of the molecular architecture and functional properties of various heme-DNA complexes.