A one- and two-dimensional NMR study of the B to Z transition of (m5dC-dG)3in methanolic solution

Abstract

The deoxyribose hexanucleoside pentaphosphate (m ⁵ dC-dG) ₃ has been studied by 500 MHz ¹ H NMR in D ₂ O (0.1 M NaCl) and in D ₂ O/deuterated methanol mixtures. Two conformations, in slow equilibrium on the NMR time scale, were detected in methanolic solution. Two-dimensional nuclear Overhauser effect (NOE) experiments were used to assign the base and many of the sugar resonances as well as to determine structural features for both conformations. The results were consistent with the an equilibrium in solution between B-DNA and Z-DNA. The majority of the molecules have a B-DNA structure in low-salt D ₂ O and a Z-DNA structure at high methanol concentrations. A cross-strand NOE between methyl groups on adjacent cytosines is observed for Z-DNA but not B-DNA. The B-DNA conformation predominates at low methanol concentrations and is stabilized by increasing temperature, while the Z-DNA conformation predominates at high methanol Concentrations and low temperatures. ³¹ P NMR spectra gave results consistent with those obtained by ¹ H NMR. Comparison of the ³¹ P spectra with those obtained on poly(dG-m ⁵ dC) allow assignment of the lower field resonances to GpC in the Z conformation.