1H NMR and circular dichroism studies of the B and Z conformations of the self-complementary deoxyhexanucleotide d(m5C-G-C-G-m5-C-G): mechanism of the Z-B-coil transitions.

The double-helical conformations of d(m5-C-G-C-G-m5-C-G) in aqueous solution were studied by circular dichroism and 1H NMR spectroscopy. In 0.1 M NaCl, only the B form is detected whereas the Z form is strongly predominant in 3 M NaCl. In the presence of 2 M NaCl, two resonance signals corresponding to the B and Z duplexes were observed for each proton below 50 degrees C, indicating a slow exchange between B and Z. However, the B-Z exchange becomes intermediate or fast in the 55-80 degrees C temperature interval. By contrast the exchange between B helix and single-stranded (or coil) forms is much faster for the same temperature conditions. The Z form is only detectable when the coil form is practically absent. With decreasing temperature the B form decreases in favor of the Z form. From proton line-width measurements under various experimental conditions, it was also shown that Z exchanges only with B, while the latter also exchanges with the single-stranded form (S): Z in equilibrium B in equilibrium S. The enthalpy value is about 8 +/- 1 kcal/mol for the B-Z transition and about 40 +/- 2 kcal/mol for the B-S dissociation (2 M NaCl solution). The activation energy is about 47 +/- 2 kcal/mol for the Z----B and 39 +/- 2 kcal/mol for the B----Z reaction. Very good agreement between the experimental results and computed data (based on the above kinetic reaction model) was found for the B, Z, and coil proportions. The B-Z transition of methylated d(C-G)n oligomers is only possible when the Watson-Crick hydrogen bonds between the CG base pairs are firmly maintained; otherwise, the transformation from B to Z would not occur, and B-S dissociation would take place instead.