THERMODYNAMICS OF TRIPLE-HELIX AND DOUBLE-HELIX FORMATIONS BY OCTAMERS OF DEOXYRIBOADENYLIC AND DEOXYRIBOTHYMIDYLIC ACIDS

The triple-helix and double-helix formations by octamers of deoxyriboadenylic and deoxyribothymidylic acids, (dA)8 and (dT)8, have been studied energetically and dynamically by UV and CD measurements, a melting analysis, curve-fitting and nearest-neighbor calculations. The UV mixing curves and CD spectra showed that the triple-helix of (dA)8 . 2(dT)8 mainly existed in 0.05 mol dm-3 MgCl2 buffer at a low temperature range, while the double helix of (dA)8 . (dT)8 existed in 1 and 0.1 mol dm-3 NaCl buffers. The thermodynamic parameters for the triplex and duplex formations were obtained with analysis for the UV melting curves. The free-energy changes at 25-degrees-C obtained from the melting temperature vs. oligomer concentration plots in the MgCl2 buffer were -20.7 kJ mol-1 for the triplex formation and -27.3 kJ mol-1 for the duplex formation, respectively, which were consistent with the values obtained from the curve-fitting calculations. The free-energy changes for the duplex formation at 25-degrees-C were -28.5 kJ mol-1 in 1 mol dm-3 NaCl buffer, and -11.6 kJ mol-1 in 0.1 mol dm-3 NaCl buffer, respectively. These values were discussed in comparison with the predicted values by the nearest-neighbor calculation.