STUDIES OF THE 2 1 CHROMOMYCIN-A3-MG2+ COMPLEX IN METHANOL - ROLE OF THE CARBOHYDRATES IN COMPLEX-FORMATION/

Chromomycin A3 (CRA3) is a glycosylated antitumor antibiotic that bind s to DNA as a 2:1 drug-Mg2+ complex. We have previously shown that the carbohydrates of CRA3 stabilize the 2:1 drug-Mg2+ complex in methanol . In the present study we investigate the structure of the 2:1 drug-Mg 2+ complex in methanol. The 2:1 CRA3-Mg2+ complex is octahedral in met hanol, with the possibility of forming eight diastereoisomers. A combi nation of conformational analysis and DNMR indicates that only two of these isomers are formed. In both isomers the CDE trisaccharide of one CRA3 molecule contacts the chromophore of the other CRA3 molecule, st abilizing the 2:1 complex relative to the 1:1 complex. The major isome r in methanol is identical with the 2:1 complex that binds to DNA, rep orted by Patel and co-workers (J. Mol. Biol. 1992, 223, 259-279). In a ddition, studies of degradation products of CRA3 show that the entire CDE trisaccharide is critical for stable dimer formation. The AB disac charide and the acyclic side chain do not appear to play major roles i n dimer formation. Taken together, our results show that the ability t o form stable dimers is critical for DNA binding and the CDE trisaccha ride specifically stabilizes the dimeric complex with the appropriate shape to bind to the right-handed DNA duplex. The results reported her e provide a starting point for designing synthetic model compounds bas ed on CRA3. Studies on such model compounds will establish minimal str uctural requirements for dimer formation and DNA binding and may lead to the development of new antitumor agents.