Nuclear magnetic resonance study of hydrogen-bonded ring protons in Watson-Crick base pairs

We have studied the low-field proton magnetic resonance spectrum of the double-helical complex of d(A-A-C-A-A) with d(T-T-G-T-T). Well-resolved resonances for the thymidine N(3) and the guanosine N(1) protons are seen at 1 degrees C, which broaden by 4 degrees C and disappear above 9 degrees C. The actual T(m) of the complex was found to be 28 degrees C by measurement of UV-absorbance change. There are several factors that could limit the observed exchange broadening of the resonance lines; the model that best fits the data is one in which the rate of double-helix dissociation limits the rate of exchange of the H-bonding protons with water.