Thermodynamics of left-handed helix formation.

The thermodynamics of right- and left-handed helix formation by poly[d(G-C)] X poly[d(G-C)] and by poly-(dG-m5dC) X poly(dG-m5dC) were measured spectrophotometrically and calorimetrically. From the spectrophotometric measurements the thermal stabilities of the alternative helical conformations were evaluated as a function of counterion concentration. From the calorimetric measurements the enthalpies of either right-handed or left-handed helix formation were determined. The corresponding experimental delta H values are -8.6 and -11.2 kcal/mol base pairs for the two conformations in poly[dG-C)] X poly[d(G-C)], and -9.0 and -12.7 kcal/mol base pairs, respectively, for poly(dG-m5dC) X poly(dG-m5dC).