QUANTITATIVE-EVALUATION OF ELECTROSTATIC AND HYDROGEN-BONDING CONTRIBUTIONS TO METAL COFACTOR BINDING TO NUCLEIC-ACIDS

The binding free energy (Delta G(b)) of a hydrated alkali or alkaline earth ion to double-strand nucleic acids is dominated by electrostatic (Delta G(es) and hydrogen-bonding (Delta G(hb) contributions. We have estimated the relative magnitudes of these two terms by use of metal complexes of defined charge and hydrogen bonding capability. A strategy is described where Delta G(b) far M(n+)(aq) is compared with values obtained from substitutionally-inert cobaltic-ammine complexes of similar charge ([Co(NH3)(6).(z)X(z)](n+), X = NH3, NO2- ). Values for the latter are cominated by the electrostatic term, and so Delta G(hb) can be determined by direct comparison with the hydrated Mg2+(aq) or Na+(aq) ion of equivalent charge. Apparent binding affinities (K-a, M(-1) for a series of aquated metal ions (Mg2+(aq), Na+(aq)) and cobalt coordination complexes (Co(NH3)(6)(3+), [CO(NH3)(5)NO2](2+), [CO(NH3)(4)(NO2)(2)](+)) to B- and A-configuration nucleic acids have been determined in 20 mM Tris (pH 7.0) by NMR line-shape analysis and the neighbor exclusion model of McGhee-von Hippel. B-conformer nucleic acids: [Co(NH3)(6)](3+), 14 800 M(-1); [Co(NH3)(5)(NO2](+), 1500 M(-1); [Co(NH3)(4)(NO2)(2)](+), 20 M(-1); Mg2+(aq), 12 800 M(-1); Na+(aq), 150 M(-1). A-conformer nucleic acids: [Co(NH3)(6)](3+), 4200 M(-1); [Co(NH3)(5)- NO2](2+), 250 M(-1); [Co(NH3)(4)(NO2)(2)](+), undetermined; Mg2(+)(aq), 2500 M(-1); Na+(aq), 8 M(-1). Individual contributions from electrostatic attraction and hydrogen bonding have been evaluated and found to be additive for each specific configuration. The results are consistent with the expectations of polyelectrolyte theory. Stronger binding to B-conformers results from electrostatic terms, while the contribution from hydrogen bonding is apparently conformation independent. Variable temperature experiments demonstrate that the main factor dictating the exchange region for bound and free metal species derives from extensive hydrogen bonding to the polynucleotide.