Entropy and heat capacity of DNA melting from temperature dependence of single molecule stretching

When a single molecule of double-stranded DNA is stretched beyond its B-for m contour length, the measured force shows a highly cooperative overstretch ing transition. We have measured the force at which this transition occurs as a function of temperature. To do this, single molecules of DNA were capt ured between two polystyrene beads in an optical tweezers apparatus. As the temperature of the solution surrounding a captured molecule was increased from 11 degreesC to 52 degreesC in 500 mM NaCl, the overstretching transiti on force decreased from 69 pN to 50 pN. This reduction is attributed to a d ecrease in the stability of the DNA double helix with increasing temperatur e. These results quantitatively agree with a model that asserts that DNA me lting occurs during the overstretching transition. With this model, the dat a may be analyzed to obtain the change in the melting entropy DeltaS of DNA with temperature. The observed nonlinear temperature dependence of DeltaS is a result of the positive change in heat capacity of DNA upon melting, wh ich we determine from our stretching measurements to be DeltaC(p) = 60 +/- 10 cal/mol K bp, in agreement with calorimetric measurements.