Structural Characterization of Iron-Bleomycin by Resonance Raman Spectroscopy

Resonance enhanced modes in the Raman spectra of the ferric, ferrous, and CO-bound ferrous forms of iron bleomycin A₂ (Fe-BLM) have been detected. Lines in the high-frequency region of the spectrum of the ferric complex are assigned to the amide I (1608 cm^-1) and amide II (1478 cm^-1) modes of the β-hydroxyhistidine amide moiety. Two other lines in the spectrum (1388 and 1371 cm^-1) are assigned to modes of the pyrimidine ring. All of these modes are sensitive to the redox state of the iron and the presence of exogenous ligands. The data indicate that the nitrogen atom of the β-hydroxyhistidine amide and N1 of the pyrimidine ring are two of the ligands that coordinate iron. The valence- and ligand-dependent frequency changes of the amide I mode are similar to the valence- and ligand-dependent changes in the v₄ mode of iron porphyrins. This suggests the involvement of a delocalized π-electron system in Fe-BLM analogous to that in iron porphyrins. The first experimental evidence for hydroxide ligation to iron in Fe(III)-BLM is demonstrated by the presence of an Fe-OH stretching mode at 561 cm^-1. The C-O and the Fe-CO stretching modes of CO-Fe(II)-BLM are located at 1980 and 511 cm^-1, respectively. The frequencies of these modes, which are typical of those for iron porphyrin and heme protein CO complexes, indicate that the electronic properties of CO-Fe(II)-BLM are similar to those of CO-bound iron porphyrins.