Interaction of Gadolinium (III), Manganese (II) and Copper (II) with Cyclic Nucleotides
Reactions of nucleic acids in biological systems are generally mediated by the presence of metal ions . Because of the variety of sites available for metal binding (base and phosphate) this interaction can be complex as the metal ion does not necessarily occupy a unique site. As models for nucleic acids the cyclic nucleotides being phosphodiesters have been chosen. With monoesters there is the additional complication that first row transition metal ions may form a backbound chelate linking the base and the phosphate through a bridging water molecule. This has been suggested for Mn(II) and Cu(II) complexes with ATP [2, 3]. Although Mn(II) might be expected to bind predominantly to the phosphate and Cu(II) to base nitrogen donors the enhanced stability of the Cu(II) complex of 5’ AMP relative to that of adenosine  and the observation of line broadening of the 31P resonance of 5’AMP in the presence of Cu(II)  suggests that direct phosphate binding cannot be eliminated. Two furthers factors complicate the interpretation of n.m.r. line broadening studies. In employing high concentrations of ligand relative to paramagnetic ion multinuclear species may be present in solution and in, for example a 1:2 metal ligand complex the metal may be bound to different sites on the two coordinated ligands. Also as the n.m.r. data on species undergoing rapid comformational changes reflects the time averaged conformation it is necessary to determine what conformations exist in solution and their contribution to the overall effect to interpret line broadening studies. For the cyclic nucleotides rapid interconversion between syn and anti forms and rapid flipping of the ribose may be expected.
KeywordsHydroxyl Manganese Titration Adenosine Gadolinium
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