Crystal and molecular structure of the DNA-binding antitumour antibiotic triostin A

Crystals of the polar conformer of the quinoxaline antibiotic triostin A are orthorhombic, space group P212121, with a= 20.940(5), b= 18.528(4), and c= 18.795(4)Å. There are two molecules of isoamyl acetate solvent per triostin A molecule. The cyclic octadepsipeptide has approximate two-fold symmetry. The quinoxaline chromophores and the disulphide cross-bridge project from opposite sides of the peptide ring. The conformation is compared with that found for the synthetic analogue TANDEM, and with the results of n.m.r. investigation. The observed structure accounts for the higher binding constants of the natural antibiotic, as well as its capacity to bind at GC-rich sequences in DNAs.