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 binds 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-Mg2+ complex in methanol. The 2:1 CRA3-Mg2+ complex is octahedral in methanol, with the possibility of forming eight diastereoisomers. A combination 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, stabilizing the 2:1 complex relative to the 1:1 complex. The major isomer in methanol is identical with the 2:1 complex that binds to DNA, reported by Patel and co-workers (J. Mol. Biol. 1992, 223, 259-279). In addition, studies of degradation products of CRA3 show that the entire CDE trisaccharide is critical for stable dimer formation. The AB disaccharide and the acyclic side chain do not appear to play major roles in dimer formation. Taken together, our results show that the ability to form stable dimers is critical for DNA binding and the CDE trisaccharide specifically stabilizes the dimeric complex with the appropriate shape to bind to the right-handed DNA duplex. The results reported here provide a starting point for designing synthetic model compounds based on CRA3. Studies on such model compounds will establish minimal structural requirements for dimer formation and DNA binding and may lead to the development of new antitumor agents.