INTERACTIONS OF ADENOSINE AND ADENINE HEPTANUCLEOSIDE HEXAPHOSPHATE WITH POLYURIDYLIC ACID

Studies on the interaction of the adenine heptanucleoside hexaphosphate (A(pA)6) with polyuridylic acid show that there exists only one type of A(pA)6-polyuridylic acid complexunder the present experimental conditions (0.4 m salt, pH 7.5, 0-35°) with stoichiometry of 1A to 2U, regardless of the ratio of the oligo A to the polyuridylic acid in the solution. This conclusion was based on quantitative evaluation of the mixing curve and of the hypochromicity in the transition observed for solutions of varying oligo A/polyuridylic acid ratios, as well as on the fact that the transition can be observed simultaneously by absorbance at 260 and 280mμ. The concentration dependence of the Tm of the oligo A-2-polyuridylic acid complex has been observed. In this system, the formation of the A(pA)6 2polyuridylic acid complex with the polyuridylic acid in excess of 10-20-fold is not completely cooperative but could be partially cooperative. The over-all binding of adenosine to the remaining polyuridylic acid appears to be enhanced slightly by the presence of A(pA)6 in forming the A(pA)6-polyuridylic acid complex. Furthermore, a new site for the binding of adenosine has been created in the A(pA)6-polyuridylic acid complex with polyuridylic acid in large excess. The binding of adenosine to this new site is also cooperative in nature and is favored by about 0.8 kcal than the over-all binding of adenosine to the remaining sites of the polyuridylic acid. The standard free-energy change of the binding of the adenosine to the new site is about 3.5 kcal. We propose that these new sites in the complex are located between two strands of polyuridylic acid and at the end of the A(pA)6, and are benefited by the conformational restraint imposed by A(pA)6 to the two polyuridylic acid strands in the complex and by the stacking energy of adenosine monophosphate residue at the end of the oligomer. © 1969, American Chemical Society. All rights reserved.