Detection of N-3 and N-7-coordinated [Ru II(edta)(5′-GMP)] 4− complexes and the N-1 protonation equilibrium of the Ru III derivative

Abstract

The reaction of 5′-GMP (5′-GMP=guanosine-5′-monophosphate) with [Ru II(edta)(H 2O)] 2− and [Ru III(edta)(H 2O)] − (edta 4−=ethylenediamine- N, N, N′, N′-tetraacetate) was followed by 1 H 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 upfield 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 ≅ 6, T = 22°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 (p K a = 7.2). The origin of the LMCT band is attributed to deprotonation of coordinated 5′-GMP at N-1. At pH = 8.0, μ = 0.2, T = 27°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.