Nuclear magnetic resonance studies of the helix-coil transition of poly (dA-dT) in aqueous solution.

The well-resolved base and sugar proton resonances in the high resolution proton nuclear magnetic resonance (NMR) spectra of poly(dA-dT) can be monitored during the helix-coil transition. The observable resonances shift upfield on helix formation and the temperature-dependent chemical shifts exhibit a melting temperature t 1/2 = 69.9 +/- 0.3 degrees for 18.8 mM (with respect to phosphorus) poly(dA-dT) in 0.5 M Tris, 0.1 M cacodylate, D2O, pH 7.05. The observable protons are in fast exchange throughout the poly (dA-dT) helix-coil transition. The adenine H2 resonance that shifts upfield by about 1 ppm on helix formation exhibits uncertainty broadening in the fast exchange region...