A calorimetric study of monomer-polymer complexes formed by polyribouridylic acid and some adenine derivatives

Abstract

This paper describes a calorimetric study of the reaction between various adenine derivatives with the common substrate polyribouridylic acid to form monomer-polymer complexes of the stoichiometry A:2 poly U † † Abbreviations used: A refers to the adenine derivative employed. It may stand for adenine, adenosine, deoxyadenosine, 2-aminoadenosine, or 2,6-diaminopurine. It may also stand for all of these in general. The meaning will be obvious from the text. The three-strand monomer-polymer complex is denoted as A:2 poly U, the three-strand polymer-polymer complex between one monomole of polyriboadenylic acid (poly A) and two monomoles of polyuridylic acid (poly U) is written as poly A:2 poly U. . A heat of reaction of −12.8 kcal./mole of A:2 poly U complex was found for the interaction between poly U and either adenine, adenosine or deoxyadenosine in 0.6 m-NaCl at 20 °C. This result indicates that the presence or absence of the sugar group or the 2′OH group contributes little to the ΔH of these monomer-polymer complexes. Complexes of poly U with 2-aminoadenosine and 2,6-diaminopurine, which can form three hydrogen bonds with the first strand of poly U, were found to be 3 kcal./mole more exothermic; that is, ΔH is −15.8 kcal./mole of A:2 poly U complex. These results were independently confirmed by direct calorimetric measurement of the energy absorbed in the melting of these complexes. It was found that the 2-amino derivatives are 3-kcal./mole more stable with respect to ΔH than the adenosine derivatives at their respective melting temperatures, T m. The standard entropy changes at T m calculated for dissociating these complexes are found to be large, positive and different for each system studied, with ΔS ° varying between 42 and 49 cal./deg. mole. It is suggested that the additional favorable enthalpy change accompanying the addition of the second polymer strand to form the 1:2 complex is decisive for overcoming the large unfavorable entropy change accompanying the immobilization of the monomer species upon incorporation into the 1:1 complex. This would account for the observation that monomer-polymer complexes are usually of 1:2 stoichiometry.