Characterization of the Interaction between Heme and a Parallel G-Quadruplex DNA Formed from d(TTAGGGT)

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 (HNMR)-H-1. 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.