Effects of spermidine and hexaamminecobalt(III) on thymine imino proton exchange.

We examine the influence of spermidine and hexaamminecobalt(III) binding on DNA base pair dynamics. Proton NMR line-width measurements are used to monitor the exchange of the thymine imino proton in poly[dA].poly[dT] and in the alternating copolymer [d(AT)]15.[d(AT)]15. We employ the treatment by Benight et al. (1988) of diffusion effects in imino proton exchange to estimate catalytic rate constants and to extract apparent equilibrium constants for the interconversion between closed (nonexchanging) and solvent-accessible (exchangeable) states of the base pairs. The salt dependence of the spermidine-catalyzed imino proton exchange in poly[dA].poly[dT] is qualitatively described by this theory. van't Hoff analysis of the temperature dependence of the apparent equilibrium constants suggests that the binding of spermidine and hexaamminecobalt(III) lowers the energy difference between the nonexchanging and exchanging states of the AT base pair in both duplexes.