Two-dimensional homonuclear and heteronuclear NMR and minimized potential energy calculations have been combined to define the structure of the antitumor agent mitomycin C (MC) cross-linked to deoxyguanosines on adjacent base pairs in the d(Tl-A2-C3-G4-T5-A6)-d(T7-A8-C9-G10-Tl 1-A12) duplex. The majority of the mitomycin and nucleic acid protons in the MC-X 6-mer complex have been assigned from through-bond and through-space two-dimensional proton NMR studies in aqueous solution at 5 and 20 °C. The C3•G10 and G4-C9 base pairs are intact at the cross-link site and stack on each other in the complex. The amino protons of G4 and G10 resonate at 9.36 and 8.87 ppm and exhibit slow exchange with solvent H20. The NMR experimental data establish that the mitomycin is cross-linked to the DNA through the amino groups of G4 and G10 and is positioned in the minor groove. The conformation of the cross-link site is defined by a set of NOEs between the mitomycin HI″ and H2″ protons and the nucleic acid imino and amino protons of G4 and the H2 proton of A8 and another set of NOEs between the mitomycin geminal H10″ protons and the nucleic acid imino and amino protons of G10 and the H2 proton of A2. Several phosphorus resonances of the d(T-A-C-G-T-A) duplex shift dramaticaly on mitomycin cross-link formation and have been assigned from proton-detected phosphorus-proton two-dimensional correlation experiments. The proton chemical shifts and NOEs establish fraying at the ends of the d(T-A-C-G-T-A) duplex, and this feature is retained on mitomycin cross-link formation. The base-base and base-sugar NOEs exhibit similar patterns for symmetry-related steps on the two nucleic acid strands in the MC-X 6-mer complex, while the proton and phosphorus chemical shifts are dramatically perturbed at the G10-T11 step on cross-link formation. The NMR distance constraints have been included in minimized potential energy computations on the MC-X 6-mer complex. These computations were undertaken with the nonplanar five-membered ring of mitomycin in each of two pucker orientations. The resulting low-energy structures MX1 and MX2 have the mitomycin cross-linked in a widened minor groove with the chromophore ring system in the vicinity of the G10-T11 step on one of the two strands in the duplex. The experimental evidence supports this model since the H1′ of G10 is shifted dramatically upfield and the H1′ proton of T11 is shifted dramatically downfield on complex formation. The unconstrained final minimum energy conformations MX1 and MX2, which differ in the pucker of the five-membered ring of mitomycin, exhibit similar torsion angles at the bonds involved in covalent linkage of the antitumor agent to the DNA. However, there are large differences in several nucleic acid backbone torsion angles between conformations MX1 and MX2. A choice between them cannot be currently made since both MX1 and MX2 satisfy the available experiment NMR parameters for the MC-X 6-mer complex in solution.
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