Triple Helix Formation of Poly dA · Poly dT · Poly rU

Abstract

AbstractThe enthalpy of the triple helix formation of polydeoxythymidylic acid (poly dT), polydeoxyadenylic acid (poly dA), and polyribouridylic acid (poly rU) has been measured by the help of an isothermal mixing calorimeter. The reaction can be split under proper condition into two consecutive steps, the double helix formation of poly dA · poly dT and the addition of the poly U strand to form a trip · helix poly dA · poly dT · poly rU. The interaction energy of poly dA and dT has been determined to 34.3 kJ per mole base pairs and the interaction energy of a poly U single strand with the poly (dA‐dT) double helix has been determined to 9.8 kJ per mole U. The formation energy of the triple helix thus amounts to 44.3 kJ per mole base triplet. The stoichiometry of the complex formation of poly dA · poly dT and of poly dA · poly dT · poly rU was derived from the continuous mixing curves of the constituents. The intrinsic sedimentation coefficient of the components has also been used as an independent method to check the base ratio. Thermal denaturation of the helices has been followed by recording the UV‐absorbance as a function of temperature. Finally the template activity of the triple helix has been tested.