INTERACTION OF PURINE NUCLEOTIDES WITH INERT PARAMAGNETIC CR(III) PROBES EVALUATED BY NMR RELAXATION EFFECTS - MOLECULAR MECHANICS CALCULATIONS ON CR(III) AND CO(III) POLYPHOSPHATE COMPLEXES

The1H NMR relaxation effects produced by paramagnetic Cr(III) complexes on nucleoside 5’−mono− and −triphosphates in D2O solution at pH’=3 were measured. The paramagnetic probes were [Cr(III)(H2O)6]3+, [Cr(III)(H2O)3(HATP)], [Cr(III)(H2O)3(HCTP)] and [Cr(IlI) (H2O)3 (UTP)−, while the matrix nucleotides (0.1 M) were H2AMP, HIMP−, and H2ATP2−. For the aromatic base protons, the ratios of the transverse to longitudinal paramagnetic relaxation rates (R2p/Rlp) for the [Cr(III)(H2O)6]3+/H2ATP2−, [Cr(III)H2O)3(HATP)]/H2ATP2−, [Cr(III)(H2O)3(HCTP)]/H2ATP2 and [Cr(III)(H2O)3(UTP)]−/H2ATP2 systems were below 2.33 so the dipolar term predominates. For a given nucleotide, Rlp for the purine H(8) signal was larger than for the H(2) signal with the [Cr(III)(H2O)6]3+ probe, while Rlp for the H(2) signal was larger with all the other Cr(III) probes. Molecular mechanics computations on the [Cr(III)(H2O)4(HPP)(α,β)], [Cr(III)(NH3)4(HPP)(α,β)], [Co(III)(NH3)3(H2PPP)(α,β,γ)] and [Co(III)(NH3)4(HPP)(α,β)] complexes gave calculated energy−minimized geometries in good agreement with those reported in crystal structures. The molecular mechanics force constants found were then used to calculate the geometry of the inner sphere [Cr(III)(H2O)6]3+ and [Cr(III)(H2O)3(HATP)(α,β,γ)] complexes as well as the structures of the outer sphere [Cr(III) (H2O)6]3+−(H2AMP) and [Cr(III)(H2O)6]−(HIMP)− species. The gas−phase structure of the [Cr(III)(H2O)3(HATP)(α,β,γ)] complex shows the existence of a hydrogen bond interaction between a water ligand and the adenine N(7) (O…N = 2.82 Å). The structure is also stabilized by intramolecular hydrogen bonds involving the −O(2’)H group and the adenine N(3) (O…N = 2.80Å) as well as phosphate oxygen atoms and a water molecule (O…O = 2.47 Å). The metal center has an almost regular octahedral coordination geometry. © 1990 Adenine Press.