Reactivity of [Ru<SUP>III</SUP>(edta)(H<SUB>2</SUB>O)]<SUP>-</SUP> with nucleic bases, nucleosides and DNA (calf-thymus) in aqueous solution (etda = ethylenediamine-N,N,N',N'-tetraacetate)

Reactions of [Ru<SUP>III</SUP>(edta)(H<SUB>2</SUB>O)]<SUP>-</SUP>(edta = ethylenediamine-N,N,N',N'-tetraacetate) with L'(L'= adenine, adenosine, cytosine, cytidine or thymine) and calf-thymus DNA were studied by spectrophotometric, electrochemical and kinetic methods. Spectral features of the substituted product [Ru<SUP>III</SUP>(edta)L']<SUP>-</SUP> complexes were characterised by a strong ligand-to-metal charge transfer (l.m.c.t.) band in the UV region (293-309 nm). The E<SUB>1/2</SUB> values for the Ru<SUP>III</SUP>-Ru<SUP>II</SUP> couple for [RuIII(edta)L']<SUP>-</SUP> are in the range -0.28 to -0.15 V (vs. saturated calomel electrode). Equilibration kinetics of complexation of [Ru<SUP>III</SUP>(edta)(H<SUB>2</SUB>O)]- with L' have been investigated using stopped-flow methods at 25 &#176;C and pH 5.2. Equilibrium constants determined kinetically are in good agreement with those obtained spectrophotometrically. The reactivity of L' towards aqua-substitution of [Ru<SUP>III</SUP>(edta)(H<SUB>2</SUB>O)]<SUP>-</SUP> complex follows the sequence adenine &gt; adenosine &gt; cytosine &gt; cytidine. On the basis of spectrophotometric, electrochemical and kinetic data and comparing the reactivity of [Ru<SUP>III</SUP>(edta)(H<SUB>2</SUB>O)]<SUP>-</SUP> with DNA bases and DNA (calf-thymus) itself it is proposed that the interaction of [RuIII(edta)(H<SUB>2</SUB>O)]<SUP>-</SUP> with DNA takes place through the adenine base unit in a kinetically preferred pathway