Synthesis and evaluation of potential N-pi and N-sigma metal chelation sites within the beta-hydroxy-L-histidine subunit of bleomycin A(2): Functional characterization of imidazole N-pi metal complexation

The synthesis and evaluation of 4 and 5, fully functionalized deglycobleomycin A(2) (2) analogues incorporating an oxazole and a pyrrole in place of the beta-hydroxy-L-histidine imidazole, are detailed, The oxazole agent is only capable of N-pi metal complexation through a form related to the N-1-H imidazole tautomer of bleomycin A(2) (1) while the pyrrole agent may potentially mimic the N-sigma metal complexation capabilities of the imidazole N-3-H tautomer. Metal complexes (Fe-II, Fe-III) of 4 and 5 were found to cleave duplex DNA in the presence of O-2 (Fe-II) or H2O2 (Fe-III). The oxazole agent 4 which is incapable of N-sigma metal chelation was found to behave analogous to, albeit slightly less effectively than, deglycobleomycin A(2) resulting in the characteristic 5'-GC/5'-GT sequence selective cleavage of duplex DNA directly confirming that imidazole/oxazole N-pi 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 A(2) tautomer is the imidazole N-1-H tautomer, and that the imidazole N-1-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 N-pi metal chelation, but possesses the capabilities of functioning as a N-sigma 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 N-pi coordination, not N-sigma coordination, of the imidazole is required for the functional activity of bleomycin A(2).