Thermodynamics of left‐handed helix formation

The thermodynamics of right‐ and left‐handed helix formation by poly[d(G‐C)]·poly[d(G‐C)] and by poly‐(dG‐m⁵dC)·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 ΔH values are −8.6 and −11.2 base pairs for the two conformations in poly[dG‐C)]·poly[d(G‐C)], and −9.0 and −12.7 base pairs, respectively, for poly(dG‐m⁵dC)·poly(dG‐m⁵dC).