NMR studies of the interaction of the antibiotic nogalamycin with the hexadeoxyribonucleotide duplex d(5'-GCATGC)2

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Abstract
1H resonance assignments in the NMR spectra of the self-complementary hexadeoxyribonucleoside pentaphosphate d(5''-GCATGC)2 and its complex with the antibiotic nogalamycin, together with interproton distance constraints obtained from two-dimensional nuclear Overhauser effect (NOE) spectra, have enabled us to characterize the three-dimensional structure of these species in solution. In the complex described, two drug molecules are bound per duplex, in each of two equivalent binding sites, with full retention of the dyad symmetry. Twenty-eight NOE distance constraints between antibiotic and nucleotide protons define the position and orientation of the bound drug molecule. Nogalamycin intercalates at the 5''-CA and 5''-TG steps with the major axis of the anthracycline chromophore aligned approximately at right angles to the major axes of the base pairs. The nogalose sugar occupies the minor groove of the helix and makes many contacts with the deoxyribose moieties of three nucleotides along one strand of the duplex in the 5''-TGC segment. The charged dimethylamino group and hydroxyl functions of the bicyclic sugar lie in the major groove juxtaposed to the guanine base, the bridging atoms of the bicyclic sugar making contacts with the methyl group of the thymine. Thus the antibiotic is not symmetrically disposed in the intercalation site but is in close contact in both grooves with atoms comprising the 5''-TGC strand. The intercalation cavity is wedge-shaped, the major axes of the base pairs forming the site being tilted with respect to one another. All base-pair hydrogen-bonding interactions are maintained in the complex, and there is no evidence for Hoogsteen pairing. The free duplex adopts a regular right-handed B-type conformation in which all glycosidic bond angles are anti and all sugar puckers lie in the C2''-endo range. In the complex the glycosidic bond angles and the sugar puckers deviate little from those observed for the duplex alone. The presence of two bound nogalamycin molecules substantially slows the "breathing" motions of the base pairs forming the intercalation cavity, and the observation of two downfield-shifted resonances in the 31P NMR spectrum of the complex suggests a pronounced local helix unwinding at the drug binding site. The footprinting data of Fox and Waring [Fox, K. R., and Waring, M. J. (1986) Biochemistry 25, 4349-4356] imply that the highest affinity binding sites of nogalamycin have the sequence 5''-GCA (or 5''-TGC). Our findings show that the major determinants of specificity appear to be hydrogen-bonding interactions between the O6 and N7 atoms of the guanine in the intercalation site and the two hydroxyl groups of the bicyclic sugar of the antibiotic, coupled with hydrogen-bonding/electrostatic interaction between the protonated dimethylamino group and the O6 carbonyl of the terminal guanine.