title: 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.
abstract: The 1H 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(III)(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/R1p) 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, R1p for the purine H(8) signal was larger than for the H(2) signal with the [Cr(III)(H2O)6]3+ probe, while R1p for the H(2) signal was larger with all the other Cr(III) probes. Molecular mechanics computations on the [Cr(III)(H2O)4(HPP)(alpha,beta)], [Cr(III)(NH3)4(HPP)(alpha,beta)], [Co(III)(NH3)3(H2PPP)(alpha,beta,gamma)] and [Co(III)(NH3)4(HPP)(alpha,beta)] 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)(alpha,beta,gamma)] 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)(alpha,beta,gamma)] complex shows the existence of a hydrogen bond interaction between a water ligand and the adenine N(7)(O...N = 2.82 A). The structure is also stabilized by intramolecular hydrogen bonds involving the -O(2')H group and the adenine N(3) (O...N = 2.80 A) as well as phosphate oxygen atoms and a water molecule (O...O = 2.47 A). The metal center has an almost regular octahedral coordination geometry. The structures of the two outer-sphere species reveal that the phosphate group interacts strongly with the hexa-aquochromium probe. In both complexes, the nucleotides have a similar "anti" conformation around the N(9)-C(1') glycosidic bond. However, a very important difference characterizes the two structures. For the (HIMP)- complex, strong hydrogen bond interactions exist between one and two water ligands and the inosine N(7) and O(6) atoms, respectively (O...O = 2.63 A; O...N = 2.72, 2.70 A). For the H2AMP complex, the [Cr(III)(H2O)6]3+ cation does not interact with N(7) since it is far from the purine system. Hydrogen bonds occur between water ligands and phosphate oxygens. The Cr-H(8) and Cr-H(2) distances revealed by the energy-minimized geometries for the two outer sphere species were used to calculate the R1p values for the H(8) and H(2) signals for comparison with the observed R1p values: 0.92(c), 1.04(ob) (H(8)) and 0.06(c), 0.35(ob) (H(2)) for H2AMP; and 3.76(c), 4.53(ob) (H(8)) and 0.16(c), 0.77(ob) s-1 (H(2)) for HIMP-.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Dipartimento di Chimica della Università di Siena, Italy.
authors.forename: R
authors.initials: R
authors.lastname: Cini
authors.forename: G
authors.initials: G
authors.lastname: Giorgi
authors.forename: F
authors.initials: F
authors.lastname: Laschi
authors.forename: C
authors.initials: C
authors.lastname: Rossi
authors.forename: L G
authors.initials: LG
authors.lastname: Marzilli
mesh_terms: D002857:Chromium; D003035:Cobalt; D008274:Magnesium; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D011685:Purine Nucleotides
chemical_list: D011685:Purine Nucleotides; D002857:Chromium; D003035:Cobalt; D008274:Magnesium