Helix–coil transition of the dG‐dC‐dG‐dC self‐complementary duplex and complex formation with daunomycin in solution

The duplex‐to‐strand transition of the self‐complementary sequence dG‐dC‐dG‐dC has been probed at the exchangeable and nonexchangeable protons and backbone phosphates by high‐resolution nmr spectroscopy. The Watson‐Crick imino and amino hydrogen‐bonded protons, as well as the exposed amino protons, could be followed through the duplex‐to‐strand transition and provide information on base‐pair stability at the tetranucleotide duplex level. The magnitudes of the experimental upfield nonexchangeable base‐proton chemical shifts on duplex formation are consistent with calculations based on base‐pair overlap geometries of the B‐DNA type. The variation of the 31 P chemical shifts in dG‐dC‐dG‐dC with temperature appear to monitor changes in the ω,ω′ rotation angles about the OP bonds in the postmelting transition temperature region. The complex formed between the antitumor anthracycline antibiotic daunomycin and the dG‐dC‐dG‐dC duplex was probed at the nucleic acid and the antibiotic resonances as a function of temperature. The experimental complexation shifts of the observable daunomycin resonances have put constraints on possible overlap geometries between the intercalating anthracycline ring and adjacent base pairs.