Abstract
Contrary to earlier reports on the interaction between polyadenylic acid (poly (A)) and poly-5-bromouridylic acid (poly (br5U)), poly (A) was demonstrated to form both 1 : 1 and 1 : 2 complexes with poly (br5U) in 0.1 MNa+, pH 7.3, at 25°C by reconstructing mixing curves. Under comparable conditions, newly synthesized poly-2-methyladenylic acid (poly (m2A)) and poly-2-ethyladenylic acid (poly (e2A) formed only 1 : 1 complexes with poly (br5U), possibly having non Watson-Crick base pairing due to steric hindrance between the C (2)-alkyl substituents of adenine and the C (2)-carbonyl oxygen of bromouracil in polynucleotides, whereas poly-2-isopropyladenylic acid (poly (iso-pr2A)) did not interact with poly (br5U). On the other hand, with polyinosinic acid (poly (I)), poly (m2A) formed a 1 : 2 complex with hydrogen bonding at both N (1) and N (7) of 2-methyladenine, similar to the structure of poly (A)·2poly (I), whereas poly (e2A) formed only a 1 : 1 complex, possibly having non Watson-Crick type base pairing due to steric hindrance of a bulkier substituent which blocks pairing at the N (1) bonding site of 2-ethyladenine to hypoxanthine, and poly (iso-pr2A) again gave no complex at all. The ultraviolet (UV) and circular dichroism (CD) spectra and thermal stabilities of these duplexes and triplexes were compared under various conditions.
