THERMODYNAMIC INVESTIGATION OF HELIX-COIL TRANSITION OF A POLYRIBONUCLEOTIDE SYSTEM

Some relative thermodynamic state functions of various types of structure which occur in the nucleic acid model system consisting of polyriboadenylic acid (poly A) and polyribouridylic acid (poly U) have been determined using the method of adiabatic calorimetry. Polymer concentrations have been calculated from the content of organic phosphorus of the components of the samples. The content of organic phosphorus has been determined by means of a chemical procedure following the method of Asmus and Baumert (1968). Since poly A is forming an intramolecular secondary structure in the temperature range covered by the calorimetric measurements, the measured transition enthalpies have been extrapolated to about 95°, where poly A exists almost entirely in the randomly coiled state. This extrapolation yields the values ΔH° (A + U) = 9.3 ± 0.5 kcal/mol of (A + U) for the helix-random coil transition of the helical duplex poly(A + U), and ΔH°(A + 2U) = 13.5 ± 0.5 kcal/mol of (A + 2U) for the dissociation of the three-stranded complex poly(A + 2U). The calorimetric procedure (measuring continuously the heat capacity of the polymer solution as a function of temperature) involves the experimental determination of the fraction of structural change. Therefore the thermal helix-coil transition of the double-stranded poly(A + U) has been analyzed in terms of current theoretical approaches. Following the calculation procedure of Applequist (1967), the stacking parameter introduced by Crothers and Zimm (1964) is found to have a value of about 400, which is considered to be representative for this double-stranded polyribonucleotide system. Additionally, numerical values of some (average) properties of poly(A + U) at the conversion temperature are given. Appropriate interpolation along measured transition enthalpies yields an approximate value of ΔH°(A) = 4.5 ± 2 kcal/mol of (A) for the transition of neutral poly A.