Difference of the 2',5'-oligoadenylate and the 3',5'-oligoadenylate in the formation of base pairings

The 2',5'-oligoadenylates (tetramer, pentamer, and hexamer) were synthesized and their structural properties were compared with the corresponding 3',5'-oligoadenylates. Stacking effect observed with the 2',5'-oligoadenylates was weaker than that observed with the corresponding 3',5'-oligo-adenylates. With two polyuridylate (3',5') strands, a 2',5'-hexaadenylate formed a triple strand only in the presence of Mg++ion at room temperature, while a 3',5'-hexaadenylate did even without Mg++ion.