dC-dG Alternating Oligonucleotides: Thermodynamic and Kinetic Aspects of the B-Z Transformation

Abstract
The alternating cytosine-guanine oligodeoxyribonucleotides (dCdG)n, (dGdC)n, (dCdG)n, (dCdG)ndC (n=3,4), (dGdC)7 and dG(dCdG)3 have been studied by UV and CD spectroscopy at different temperatures and NaCl concentrations. The analysis of the melting data, assuming an all-or-none model, reveals that in the B-conformation the 5′G/C3′ stacking interactions are enthalpically favoured with respect to the 5′C/G3′ one. The CD investigation of the B-Z equilibrium shows that the Z-conformation is enthalpically stabilized, while the B-conformation is entropically favoured, in the range of NaCl concentration considered (1 to 5M). The kinetic data for the B-Z transformation, obtained with a salt-jump technique for the hexamer (dCdG)3, support a mechanism by which (he Watson-Crick hydrogen bonds are broken before the bases flip over separately and eventually stack, reforming the H-bonds, in the new helix.