NMR CHARACTERIZATION OF A TRIPLE-STRANDED COMPLEX FORMED BY HOMO-PURINE AND HOMO-PYRIMIDINE DNA STRANDS AT 1 1 MOLAR RATIO AND ACIDIC PH/

Homo-purine (d-TGAGGAAAGAAGGT) and homopyrimidine (d-CICCTTTCTTCC) oli gomers have been designed such that they are complementary in parallel orientation. When mixed in a 1:1 molar ratio, the system adopts an an tiparallel duplex at neutral pH with three mismatched base pairs, On l owering the pH below 5.5, a new complex is formed, The NMR results sho w the coexistence of a intermolecular pyrimidine.purine:pyrimidine DNA tripler and a single stranded oligopurine at this PH. The tripler is stabilized by five T.A:T, four C+.G:C and two mismatched triads, namel y, C+.G-T and T.A-C. This tripler is further stabilized by a Hoogsteen C+.G base-pair on one end, Temperature dependence of the imino proton resonances reveals that the triplex dissociates directly into single strands around 55 degrees C, without duplex intermediates. parallel du plexes are not formed under any of the conditions employed in this stu dy.