Z helix-coil transition of d(C-Br8G-C-G-C-Br8G) studied by CD, 1H-NMR and IR spectroscopies.

The conformation of d(C-Br8G-C-G-C-Br8G) in aqueous solution was studied by CD and 1H-NMR spectroscopy and in condensed phase by IR spectroscopy. Whether in 0.1 M or 3 M NaCl solution or in film the only double helical structure adopted by brominated d(C-G)3 oligomer is the Z form. The IR spectrum of the film presents all the characteristic absorptions of the Z conformation and in particular is indicative of a syn conformation for the central guanosine as well as for the brominated one. Imino proton resonances of d(C-Br8G-C-G-C-Br8G) demonstrating the duplex formation were observed up to 60 degrees C. It is interesting to note that the significant highfield shifts of the dC H5" exocyclic sugar protons characteristic of the non exchangeable proton spectra of d(C-G)3 containing 5-methyl dC residues in the Z form were also detected in the proton spectrum of brominated oligomer. Whereas formation of the Z helix of methylated d(C-G)3 oligomers dependent on the salt concentration was found to occur via the preliminary formation of a B helix even in 4 M NaCl solution, the Z helix of d(C-Br8G-C-G-C-Br8G) is obtained directly from the coil form. However, IR data suggest that in the Z form of d(C-Br8G-C-G-C-Br8G), the overlapping of the base planes should be slightly different in comparison with the stacking observed in d(C-G)3 crystals. The kinetic data (activation energy and lifetime) of the Z helix-coil transition of brominated d(C-G)3 are compared to those of the B helix-coil transition observed for methylated d(C-G)3 in 0.1 M NaCl solution while the thermodynamic data of these two reactions (enthalpy and midpoint temperature) are slightly different.