X-ray absorption spectroscopic investigation of Fe(II)-peplomycin and peplomycin derivatives: the effect of axial ligation on Fe-pyrimidine back-bonding

X-ray absorption spectroscopy (XAS) is used to study ferrous complexes of a bleomycin (BLM) congener, peplomycin (PEP), and two of its derivatives, iso-peplomycin (ISO) and depyruvamide peplomycin (DP), in which potential axial ligands have been perturbed and removed, respectively. Application of extended X-ray absorption fine structure analysis shows an elongation of the short-distance component of the first coordination sphere in DP and ISO relative to PEP. The XAS pre-edge intensity concomitantly decreases with increased axial perturbation. The short-distance component of PEP is correlated to the Fe-pyrimidine bond and is related to the amount of pi-back-bonding. Thus, the XAS analysis of these complexes provides structural information relevant to their differences in O2 reactivity.