Synthesis and evaluation of potential Nπ and Nσ metal chelation sites within the β-hydroxy-l-histidine subunit of bleomycin A2: Functional characterization of imidazole Nπ metal complexation Original Research Article

The synthesis and evaluation of 4 and 5, fully functionalized deglycobleomycin A2 (2) analogues incorporating an oxazole and a pyrrole in place of the β-hydroxy-l-histidine imidazole, are detailed. The oxazole agent is only capable of Nπ metal complexation through a form related to the N1-H imidazole tautomer of bleomycin A2 (1) while the pyrrole agent may potentially mimic the Nσ metal complexation capabilities of the imidazole N3-H tautomer. Metal complexes (Fe-II, Fe-III) of 4 and 5 were found to cleave duplex DNA in the presence of O2 (Fe-II) or H2O2 (Fe-III). The oxazole agent 4 which is incapable of Nσ metal chelation was found to behave analogous to, albeit slightly less effectively than, deglycobleomycin A2 resulting in the characteristic 5′-GC/5′-GT sequence selective cleavage of duplex DNA directly confirming that imidazole/oxazole Nπ metal chelation is sufficient for functional reactivity. Importantly, the effective substitution of the oxazole O-1 for the histidine N-1 further illustrates that this group does not require deprotonation upon metal complexation, oxygen activation, or the ensuing oxidation reactions, that the functional bleomycin A2 tautomer is the imidazole N1-H tautomer, and that the imidazole N1-H functionality is not contributing to the polynucleotide recognition through H-bonding to the phosphate backbone or nucleotide bases. In contrast, the pyrrole agent 5 which is incapable of Nπ metal chelation, but possesses the capabilities of functioning as a Nσ metal donor was also found to cleave duplex DNA, but does so in a nonsequence selective fashion with a significantly reduced efficiency and a diminished double to single strand cleavage ratio both only slightly above that of background iron itself. These observations are analogous to those made with 3 which lacks the imidazole altogether and further support the observations that Nπ coordination, not Nσ coordination, of the imidazole is required for the functional activity of bleomycin A2.