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.

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)5Cl]2+ and Co(NH3)4Cl2]+ 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 teh 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.