Structures of Cobalt(III)‐Pepleomycin and Cobalt(III)‐Deglycopepleomycin (green forms) Determined by NMR Studies

Pepleomycin (PEP) is a metalloglycopeptide that has stronger anticancer activity and less pulmonary toxicity than bleomycin (BLM). PEP, like BLM, exerts its action by binding to and degrading DNA in the presence of oxygen and certain metals. Obtaining detailed structural information of PEP and PEP‐DNA complexes is crucial to understanding its anticancer activity. The structures of two green forms of cobalt‐PEP species, HO 2 − ‐Co(III)‐PEP (denoted CoPEP) and deglycosylated HO 2 − ‐Co(III)‐PEP (denoted CodPEP) have been obtained by NOE restrained refinements. Earlier studies of the related HO 2 − ‐Co(III)‐BLM A 2 proposed that two chiral conformers (form A or B) could exist with either the β‐aminoalanine primary amine (A,NH 2 ) or the mannose carbamoyl nitrogen (M,NH 2 ) as the axial ligand. Analysis of our NOESY data shows convincingly that form A is the most probable conformer with the mannose carbamoyl M,NH 2 and the β‐aminoalanine primary amine A,NH 2 as the axial ligands in CoPEP and CodPEP, respectively. The NOE cross‐peaks resulting from the interactions between the N‐terminus (i.e., the metal‐binding domain) and the C‐terminus of CoPEP and CodPEP have similar patterns, suggesting that they both adopt compact structures with the bithiazole group folded back over the N‐terminus.