Binding site of Fe3+ at purine of ATP as studied by NMR

The binding site of Fe3+ in the purine base of adenosine 5'-triphosphate (ATP) was studied by nuclear magnetic resonance (NMR). The NMR relaxation rates (R1) of 1H and 31P in ATP solutions free of and containing ferric ions were measured in the pH range of 3-10. It was found that Fe3+ selectively enhanced the relaxation rate of protons. In the presence of Fe3+, the R1 of H2 was much bigger than that of H8 at a lower pH (3-4.5), while at a higher pH (5.5-7.5) the R1 of H8 was more enhanced than H2. At a pH of around 5, both H2 and H8, as well as all three phosphorous, showed a sudden jump in R1. When pH>8, Fe3+ failed to show appreciable enhancement of R1 to all protons and phosphorous. The quantitative data of relaxation rate enhancements suggest that the binding site of Fe3+ in ATP is strongly dependent on pH. At lower pH values, Fe3+ binds N1 but at higher pH it binds to N7. When pH is around 5, the whole purine base donates the aromatic pi-electrons to the ferric ion, forming a ferrocene-like complex, while when pH>8, ATP could not form complexes with Fe3+.