Thermodynamic stability of DNA tandem mismatches

The thermodynamics of nine hairpin DNAs were evaluated using UV-monitored melting curves and differential scanning calorimetry (DSC). Each DNA has the same five-base loop and a stem with 8-10 base pairs. Five of the DNAs have a tandem mismatch in the stem, while four have all base pairs. The tandem mismatches examined (ga/ga, aa/gc, ca/gc, ta/ac, and tc/tc) spanned the range of stability observed for this motif in a previous study of 28 tandem mismatches. UV-monitored melting curves were obtained in 1.0 M Na+, 0.1 M Na+, and 0.1 M Na+ with 5 MM Mg2+. DSC studies were conducted in 0.1 M Na+. Transition T values were unchanged over a 50-fold range of strand concentration. Modelin-dependent enthalpy changes (DeltaHdegrees) evaluated by DSC were in good agreement (+/-8%) with enthalpy values determined by van't Hoff analyses of the melting curves in 0.1 M Na+. The average heat capacity change (DeltaC(p)) associated with the hairpin to single strands transitions was estimated from plots of DeltaHdegrees and DeltaSdegrees with T(m)and ln T-m respectively, and from profiles of DSC curves. The average DeltaC(p) values (113 +/- 9 and 42 +/- 27 cal(.)K(-1.)mol(-1) of bp), were in the range of values reported in previous studies. Consideration of DeltaC(p) produced large changes in DeltaHdegrees and DeltaSdegrees extrapolated from the transition region to 37 degreesC and smaller but significant changes to free energies. The loop free energy of the five tandem mismatches at 37 degreesC varied over a range of similar to4 kcal(.)mol(-1) for each solvent.