NMR investigation of mithramycin A binding to d(ATGCAT)2: a comparative study with chromomycin A3.

The binding of mithramycin A to the d(A1T2G3C4A5T6) duplex was investigated by 1H NMR and found to be similar to that of its analogue chromomycin A3. In the presence of Mg2+, mithramycin binds strongly to d(ATGCAT)2. On the basis of the two-dimensional NOESY spectrum, the complex formed possesses C2 symmetry at a stoichiometry of two drugs per duplex (2:1) and is in slow chemical exchange on the NMR time scale. NOESY experiments reveal contacts from the E-pyranose of mithramycin to the terminal and nonterminal adenine H2 proton of DNA and from the drug hydroxyl proton to both G3NH2 protons, C4H1' proton, and A5H1' proton. These data place the drug chromophore and E pyranose on the minor groove side of d(ATGCAT)2. NOE contacts from the A-, B-, C-, and D-pyranoses of mithramycin to several deoxyribose protons suggest that the A- and B-rings are oriented along the sugar-phosphate backbone of G3-C4, while the C- and D-rings are located along the sugar-phosphate backbone of A5-T6. These drug-DNA contacts are very similar to those found for chromomycin binding to d(ATGCAT)2. Unlike chromomycin, the NOESY spectrum of mithramycin at the molar ratio of one drug per duplex reveals several chemical exchange cross-peaks corresponding to the drug-free and drug-bound proton resonances. From the intensity of these cross-peaks and the corresponding diagonal peaks, the off-rate constant was estimated to be 0.4 s-1. These data suggest that the exchange rate of mithramycin binding to d(ATGCAT)2 is faster than that of chromomycin.