Interactions of DNA with divalent metal ions. II. Proton relaxation enhancement studies

The extent and modes of binding of the divalent metal ions Mn 2+ and Co 2+ to DNA and the effects of salt on the binding have been studied by measurements of the effects of these paramagnetic metal ions on the longitudinal and transverse relaxation rates of the protons of the solvent water molecules, a technique that is sensitive to overall binding. The number of water molecules coordinated to the DNA–bound Mn 2+ and Co 2+ is found to be between five and six, and the electron spin relaxation times and the electron‐nuclear hyperfine constants associated with Mn 2+ and Co 2+ are little or not affected by the binding. These observations indicate little disturbance of the hydration sphere of Mn 2+ and Co 2+ upon binding to DNA. An average 2–3‐fold reduction in the exchange rate of the water of hydration of the bound metal ions and an order‐of‐magnitude increase in their rotational correlation time are attributed to hydrogen‐bond formation with the DNA. The binding constants of Mn 2+ to DNA, at metal concentrations approaching zero, are found to be inversely proportional to the second power of the salt concentration, in agreement with the predictions of Manning's polyelectrolyte theory. A remarkable quantitative agreement with the polyelectrolyte theory is also obtained for the anticooperativity in the binding of Mn 2+ to DNA, although the experimental results can be well accounted for by another simple electrostatic model. The various modes of binding of divalent metal ions to DNA are discussed.