5‐Aza‐7‐deazaguanine–Isoguanine and Guanine–Isoguanine Base Pairs in Watson–Crick DNA: The Impact of Purine Tracts, Clickable Dendritic Side Chains, and Pyrene Adducts

The Watson–Crick coding system depends on the molecular recognition of complementary purine and pyrimidine bases. Now, the construction of hybrid DNAs with Watson–Crick and purine–purine base pairs decorated with dendritic side chains was performed. Oligonucleotides with single and multiple incorporations of 5‐aza‐7‐deaza‐2′‐deoxyguanosine, its tripropargylamine derivative, and 2′‐deoxyisoguanosine were synthesized. Duplex stability decreased if single modified purine–purine base pairs were inserted, but increased if pyrene residues were introduced by click chemistry. A growing number of consecutive 5‐aza‐7‐deazaguanine–isoguanine base pairs led to strong stepwise duplex stabilization, a phenomenon not observed for the guanine–isoguanine base pair. Spacious residues are well accommodated in the large groove of purine–purine DNA tracts. Changes to the global helical structure monitored by circular dichroism spectroscopy show the impact of functionalization to the global double‐helix structure. This study explores new areas of molecular recognition realized by purine base pairs that are complementary in hydrogen bonding, but not in size, relative to canonical pairs.