Cryogenic photolysis of activated bleomycin to ferric bleomycin

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. Light was shown to trigger ABLM attack on DNA in liquid at -30 degrees C, so ABLM was irradiated (at its 350 nm ligand-to-metal charge-transfer transition) at 77 K to stabilize possible intermediates. ABLM photolysis (quantum yield, Phi = 0.005) generates two kinds of product: Fe(III).BLM (with no detectable intermediate) and one or more minor (1-2%) radical O-Fe-BLM byproduct, photostable at 77 K. Adding DNA, even without its target H4', increases the quantum yield of ABLM conversion >10-fold while suppressing the observed radical yield. Since cryogenic solid-phase reactions can entail only constrained local rearrangement, the reaction(s) converting ABLM to Fe(III).BLM must be similarly constrained.