Oligonucleotide analogues with integrated bases and backbone (ONIB). Part 31: Aminomethylene-linked CG and GC dinucleosides of the ONIB Type: Formation of duplexes and quadruplexes

The self-complementary guanosine- and cytidine-derived aminomethylene- linked C[n]G dinucleoside 9 was synthesized by reductive amination of aldehyde 3 with an iminophosphorane derived from azide 7. Deacylation of 9 gave the isopropylidene-protected dinucleoside 10. The sequence-isomeric G[n]C dinucleoside 11 was similarly prepared from aldehyde 8 and azide 5, and deacylated to 12. The association of 10 and 12 in CHCl₃ or in CHCl₃/DMSO mixtures, and the structure of the associates were studied by ¹H-NMR, ESI-MS, CD, and vapor pressure osmometry (VPO). Broad ¹H-NMR signals of dinucleosides 10 and 12 evidence an equilibrium between duplexes and quadruplexes (Hoogsteen base pairing between the Watson-Crick base-paired duplexes). The quadruplex dominates for the G[n]C dinucleoside 12 between -50° and room temperature. The sequence-isomeric C[n]G 10 forms mostly only a cyclic duplex in CDCl₃ and in CDCl ₃/(D₆)DMSO 9 : 1.