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Origins of the "nucleation" free energy in the hybridization thermodynamics of short duplex DNA.

Thermodynamic parameters deltaH(cal), deltaS(cal), and deltaG(cal) of the melting transitions for 19 short DNA/DNA duplexes ranging in length from 6 to 35 base pairs were systematically evaluated by differential scanning calorimetry melting experiments carried out at four salt concentrations from 85 mM to 1.0 M [Na+]. As expected, thermodynamic stabilities of the DNA duplexes increased with length and increasing [Na+]. From plots of deltaG25 versus duplex length, extrapolation to N = 0 provided estimates on values of deltaG(cal)25 (N = 0) as a function of [Na+], corresponding to the free-energy of the "hypothetical duplex" having zero base pairs, but occupying precisely the same molar volume as the fully base paired duplex. The values obtained for deltaG(cal)25 (N = 0) were 3.68, 5.59, 7.86, and 8.68 kcal/mol in 1.00, 0.60, 0.30, and 0.085 M Na+, respectively. These values are in reasonable agreement with published values of the nucleation or initiation free-energy, attributed to formation of the first base pair in a short duplex compared to formation of the remaining base pairs. A statistical thermodynamic formulation of the association of two strands accounting for displaced solvent was utilized to relate [Na+]-dependent deltaG(cal)25 (N = 0) values to configuration integrals for both single and duplex strands. Relative differences between two single strands in their standard states and the duplex (in its standard state), and solvent displaced during the annealing process was taken into account. This analysis provides a new vantage point to view what has historically been referred to as the helix initiation or nucleation parameter and provides an alternate interpretation and mechanism for the nucleation complex in duplex formation.