Study of Metal-Ion Binding to Nucleic Acids by 31P Nuclear Magnetic Resonance

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Abstract
We have measured the 31P NMR in aqueous solutions of RNA (ribonucleic acid) and AMP (adenosine monophosphate) containing Mn2+ and Co2+ ions. Measurements of T1, T2, and Δω, the shift from γIH0, were made as functions of the metal‐ion concentration and the temperature. Near‐room‐temperature linewidths were determined by the slow chemical exchange. At higher temperatures the Co2+ complexes enter the exchange‐narrowed region where 31P shifts in the CoAMP system are observed. The room‐temperature value of T1 for CoAMP and CoRNA complexes, the high‐temperature shifts of the CoAMP, and the high‐temperature values of T2 in the MnAMP all independently determine the strength of the isotropic transferred hyperfine coupling A between the metal ions and the 31P nuclei. Within the experimental accuracy which ranges from ∼5% (for Δω) to ∼50% (for T1 measurements) the values of A agreed with the values derived from 31P NMR measurements in crystals of Mn3(PO4)2 and Co3(PO4)2. From the similarity of A in crystals and complexes, we conclude that Mn2+ and Co2+ bind to the phosphates of AMP and RNA.