COMPLEXES OF POLY-RIBOTHYMIDYLIC ACID WITH POLY-ADENYLIC ACIDS AND SOME PROPERTIES OF POLY-DEOXYRIBOADENYLIC ACID

A study has been made of the complexing ability of poly‐ribothymidylic acid [poly (T)] with poly‐riboadenylic acid [poly (A)] and with poly‐deoxyriboadenylic acid [poly d(A)] in neutral medium in the presence of 0.01 M to 0.15 M Na+. Mixing curve techniques and analyses of temperature profiles demonstrated that poly (T) reacted with poly (A) or poly d(A), irrespective of the ratio of the two components, to give a three‐stranded helix, poly (T2‐A) or poly (T2‐dA). Melting of the complexes occurred without formation of an intermediate twin‐stranded form. The complex of poly (T) with poly (A) was more stable than that with poly d(A). The rate of complexing of poly (T) with poly d(A) was much slower than with poly (A). Preliminary experiments indicated that poly‐5‐ethyluridylic acid also forms only a triple‐stranded helix with poly d(A). While the temperature profile of neutral poly d(A) is similar to that for poly (A), indicating that the former also exists as a single strand stabilized by stacking forces, there are several minor differences which remain to be clarified. Titration of poly d(A) from neutral to acid medium, and its ability to react rapidly with poly (U) at pH 4.9 to give a complex identical with that formed in neutral medium, both indicate that poly d(A) does not form a twin‐helix like the acid form of poly (A). However, at pH 4 the structure of poly d(A) is appreciably different from that in neutral medium. The overall results are discussed in relation to the structure of synthetic polynucleotides, with particular reference to the role of the 2′‐hydroxyl and a 5‐methyl pyrimidine substituent. Copyright © 1968, Wiley Blackwell. All rights reserved