Detection of N-3 and N-7-coordinated [RuII(edta)(5′-GMP)]4- complexes and the N-1 protonation equilibrium of the RuIII derivative

The reaction of 5'-GMP (5'-GMP=guanosine-5'-monnphosphate) with [Ru-II(edta)(H2O)](2-) and [Ru-III(edta)(H2O)](-) (edta(4-) = ethylenediamine-N,NN,N-tetraacerate) was followed by H-1 NMR spectroscopy for Ru-II and by spectrophotometric and kinetic methods for Ru-III. The coordination of 5'-GMP for the Ru-II complex occurs at N-7, implicated by a downfield 0.21 ppm shift of the H-8 proton of 5'-GMP to 8.42 ppm for ca. 30% of the coordinated 5'-GMP and a 0.02 ppm upheld shift for another isomer (70%), coordinated via the N-3 purine donor. 0.01 ppm downfield shifts are also detected for the H-1' proton of ribose ring upon metallation by [Ru-II(edta)](2-). Molecular mechanics energy-minimized structures show unhindered N-7 coordination and two N-3 rotomers with energy minima that minimize steric contacts between the ribose phosphate component of 5'-GMP and the [Ru-II(edta)](2-) functionality. Figures illustrating the N-7 and N-3 modes of coordination are shown. Cyclic voltammetry and differential pulse polarography established an E-1/2 value for the [Ru-II/III(edta)(5'-GMP)](n-) couple as 0.01 V versus NHE at glassy carbon (pH congruent to 6, T = 22 degrees C). The waves for both N-3 and N-7 are highly overlapped. When the complex is generated via substitution of the Ru-III form, only the N-7 isomer is present in abundance. The Ru-III complex exhibits an LMCT band at 582 nm which appears in a pH-dependent manner (pK(a) = 7.2), The origin of the LMCT band is attributed to deprotonation of coordinated 5'-GMP at N-1. At pH = 8.0, mu = 0.2, T = 27 degrees C the addition of 5'-GMP is detectable by stopped-flow studies which established a second-order reaction between [Ru-III(edta)(OH)](2-) and 5'-GMP with k = 30 +/- 4 M-1 s(-1). (C) 1999 Elsevier Science S.A. All rights reserved.