Ionized and wobble base-pairing for bromouracil-guanine in equilibrium under physiological conditions. A nuclear magnetic resonance study on an oligonucleotide containing a bromouracil-guanine base-pair as a function of pH

A one and two-dimensional nuclear magnetic resonance study of a non-selfcomplementary oligonucleotide containing a central 5-bromouracil-guanine pair is reported. For these two bases three types of hydrogen bonding schemes could exist; wobble, rare tautomer and ionized. The two-dimensional spectra of non-exchangeable protons together with onedimensional spectra recorded in water show that at pH 7.0 the predominant species is a right-handed B-form DNA in which the brU · G pair has wobble geometry. On raising the pH we observe a transition monitored by proton chemical shift changes for the brU · G and adjacent base-pairs. The mid-point of the transition was observed at pH 8.6. Spectra recorded at pH 9.8 show that the helix remains intact with B form conformation. It is shown that this high pH form has an ionized brU · G base-pair now in Watson-Crick geometry. Thus under physiological conditions an equilibrium exists between wobble and ionized structures.