Helix-Coil Transition of the Self-Complementary dG-dG-dA-dA-dT-dT-dC-dC Duplex

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Abstract
The helix‐coil transition of the octanucleotide self‐complementary duplex dG‐dG‐dA‐dA‐dT‐dT‐dC‐dC has been monitored at the Watson‐Crick protons, the base and sugar nonexchangeable protons and the backbone phosphates by high‐resolution nuclear magnetic resonance (NMR) spectroscopy. The melting transition of the octanucleotide monitored by ultraviolet absorbance spectroscopy is characterized by the thermodynamic parameters ΔH°=–216.7 kJ/mol and ΔS° (25°C) =–0.632 kJ mol−1 K−1 in 0.1 M NaCl, 10 mM phosphate solution. Correlation of the transition midpoint values monitored by the ultraviolet absorbance studies at strand concentrations below 0.2 mM and by NMR studies at 5.3 mM suggest that both methods are monitoring the octanucleotide duplex‐to‐strand transition. The NMR spectra of the Watson‐Crick ring NH protons of the octanucleotide duplex have been followed as a function of temperature. The resonance from the terminal dG · dC base pairs broadens out at room temperature while the resonances from the other base pairs broaden simultaneously with the onset of the melting transition. The nonexchangeable base and sugar H‐1′ protons are resolved in the duplex and strand states and shift as average peaks through the melting transition. The experimental shifts on duplex formation have been compared with calculated values based on ring‐current and atomic diamagnetic anisotropy contributions for a B‐DNA base‐pair‐overlap geometry in solution. Several nonexchangeable proton resonances broaden in the fast‐exchange region during the duplex‐to‐strand transition and the excess widths yield a duplex dissociation rate constant for the octanucleotide of 1.9 × 103 s−1 at 32 °C (fraction of duplex = 0.86) in 0.1 M NaCl, 10 mM phosphate buffer. The 31P resonances of the seven internucleotide phosphates are distributed over 0.6 ppm in the duplex state, shift downfield during the duplex‐to‐strand transition and undergo additional downfield shifts during the stacked‐to‐unstacked strand transition with increasing temperature.