Characterization and Elucidation of Coordination Requirements of Adenine Nucleotides Complexes with Fe(II) Ions

Abstract

In spite of the significant role of iron ions-nucleotide complexes in living cells, these complexes have been studied only to a limited extent. Therefore, we fully characterized the ATP:Fe(II) complex including stoichiometry, geometry, stability constants, and dependence of Fe(II)-coordination on pH. A 1:1 stoichiometry was established for the ATP:Fe(II) complex based on volumetric titrations, UV and SEM/EDX measurements. The coordination sites of ferrous ions in the complex with ATP, established by ¹H-, ³¹P-, and ¹⁵N-NMR, involve the adenine N7 as well as Pα, Pβ, and Pγ. Coordination sites remain the same within the pH range of 3.1–8.3. By applying fluorescence monitored Fe(II)-titration, we established a log K value of 5.13 for the Fe(ATP)^2? complex, and 2.31 for the Fe(HATP)^? complex. Ferrous complexes of ADP^3? and AMP^2? were less stable (log K 4.43 and 1.68, respectively). The proposed major structure for the Fe(ATP)^2? complex is the ‘open’ structure. In the minor ‘closed’ structure N7 nitrogen is probably coordinated with Fe(II) through a bridging water molecule. The electronic and stereochemical requirements for Fe(II)-coordination with ATP^4? were probed using a series of modified-phosphate or modified-adenine ATP analogues. We concluded that: Fe(II) coordinates solely with the phosphate-oxygen atom, and not with sulfur, amine, or borane in the cases of phosphate-modified analogues of ATP; a high electron density on N7 and an anti conformation of the adenine-nucleotide are required for enhanced stability of ATP analogues:Fe(II) complexes as compared to ATP complexes (up to more than 100-fold); there are no stereochemical preferences for Fe(II)-coordination with either Rp or Sp isomers of ATP-α-S or ATP-α-BH₃ analogues.