Recombination R-triplex: H-bonds contribution to stability as revealed with minor base substitutions for adenine

Several cellular processes involve alignment of three nucleic acids strands, in which the third strand (DNA or RNA) is identical and in a parallel orientation to one of the DNA duplex strands. Earlier, using 2-aminopurine as a fluorescent reporter base, we demonstrated that a self-folding oligonucleotide forms a recombination-like structure consistent with the R-triplex. Here, we extended this approach, placing the reporter 2-aminopurine either in the 5'- or 3'-strand. We obtained direct evidence that the 3'-strand forms a stable duplex with the complementary central strand, while the 5'-strand participates in non-Watson-Crick interactions. Substituting 2,6-diaminopurine or 7-deazaadenine for adenine, we tested and confirmed the proposed hydrogen bonding scheme of the A*(T.A) R-type triplet. The adenine substitutions expected to provide additional H-bonds led to triplex structures with increased stability, whereas the substitutions consistent with a decrease in the number of H-bonds destabilized the triplex. The triplex formation enthalpies and free energies exhibited linear dependences on the number of H-bonds predicted from the A*(T.A) triplet scheme. The enthalpy of the 10 nt long intramolecular triplex of -100 kJ x mol(-1) demonstrates that the R-triplex is relatively unstable and thus an ideal candidate for a transient intermediate in homologous recombination, t-loop formation at the mammalian telomere ends, and short RNA invasion into a duplex. On the other hand, the impact of a single H-bond, 18 kJ x mol(-1), is high compared with the overall triplex formation enthalpy. The observed energy advantage of a 'correct' base in the third strand opposite the Watson-Crick base pair may be a powerful mechanism for securing selectivity of recognition between the single strand and the duplex.