Interaction of Purine Nucleotides with Cobalt-Hexammine, Cobalt-Pentammine and Cobalt- Tetrammine Cations. Evidence for the Rigidity of Adenosine and Flexibility of Guanosine and Deoxyguanosine Sugar Conformations

Abstract

The interaction of adenosine-5′-monophosphate (5′-AMP), guanosine-5′-monophosphate (5′-GMP) and 2′-deoxyguanosine-5′-monophosphate (5′-dGMP) with the [Co(NH₃)₆]³⁺, [CO(NH₃)₅C1]²⁺ and [CO(NH₃)₄C1₂]⁺ cations has been investigated in aqueous solution with metal/nucleotide ratios (r) of 1/2, 1 and 2 at neutral pH. The solid complexes have been isolated and characterized by FT-IR and ¹H-NMR spectroscopy. The complexes are polymeric in nature both in the crystalline solid and aqueous solution. The binding of the cobalt-hexammine cation is indirectly (via NH₃) through the N-7 and the PO₃ ^2- groups of the AMP and via O-6, N-7 and the PO₃ ^2- of the GMP and dGMP anions (outer-sphere). The cobalt-pentammine and cobalt-tetrammine bindings are through the phosphate groups (inner-sphere) and the N-7 site (outer-sphere) of these nucleotide anions. The ribose moiety shows C2′-endo/anti conformation, in the free AMP and GMP anions as well as in the cobalt-ammine - AMP complexes, whereas a mixture of the C2′-endo/anti and C3′-endo/anti sugar puckers were observed for the Co(NH₃)₆-GMP, Co(NH₃)₅-GMP and a C3′-endo/anti conformer for the Co(NH₃)₄-GMP complexes. The deoxyribose showed an O4′-endo/anti conformation for the free dGMP anion and a C3′-endo/anti for the Co(NH₃)₆-dGMP, Co(NH₃)₅-dGMP and Co(NH₃)₄-dGMP complexes.