Exchangeable proton ENDOR as a probe of the redox-active iron center in activated bleomycin and ferric bleomycinElectronic supplementary information (ESI) available: Scheme 1S showing the dispersion rapid passage Q-band EPR spectra of ABLM and Fe(iii)·BLM. See DOI: 10.1039/c7dt01354j

Activated bleomycin (ABLM) is a drug-Fe( iii )-hydroperoxide complex kinetically competent in DNA attack ( via H4′ abstraction). This intermediate is relatively stable, but its spontaneous conversion to ferric bleomycin (Fe( iii )·BLM) is poorly characterized because no observable intermediate product accumulates. The Fe( iii )·BLM formed cryophotolytically from ABLM and kept at 77 K was remarkably similar by EPR and ENDOR criteria to Fe( iii )·BLM formed from Fe( iii ) + BLM solution. The notable ENDOR criteria were the ENDOR frequencies and features of orientation-selected, strongly hyperfine-coupled, exchangeable protons associated with the environs of the iron within