title: Synthesis and evaluation of potential N pi and N sigma metal chelation sites within the beta-hydroxy-L-histidine subunit of bleomycin A2: functional characterization of imidazole N pi metal complexation.
abstract: The synthesis and evaluation of 4 and 5, fully functionalized deglycobleomycin A2 (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 Npi metal complexation through a form related to the N1-H imidazole tautomer of bleomycin A2 (1) while the pyrrole agent may potentially mimic the Nsigma 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 Nsigma 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 Npi 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 N'-H tautomer, and that the imidazole N'-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 Npi metal chelation, but possesses the capabilities of functioning as a Nsigma 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 Nsigma coordination, not Npi coordination, of the imidazole is required for the functional activity of bleomycin A2.
authors.affiliation: Department of Chemistry, Scripps Research Institute, La Jolla, CA 92037, U.S.A.
authors.forename: D L
authors.initials: DL
authors.lastname: Boger
authors.forename: T M
authors.initials: TM
authors.lastname: Ramsey
authors.forename: H
authors.initials: H
authors.lastname: Cai
mesh_terms: D000964:Antimetabolites, Antineoplastic / Q000737:chemistry* / Q000378:metabolism; D001345:Autoradiography; D001483:Base Sequence; D001665:Binding Sites; D001761:Bleomycin / Q000737:chemistry* / Q000378:metabolism; D002614:Chelating Agents / Q000737:chemistry* / Q000378:metabolism; D003001:Cloning, Molecular; D004247:DNA / Q000737:chemistry / Q000378:metabolism; D004278:DNA, Superhelical / Q000737:chemistry / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D006639:Histidine / Q000737:chemistry*; D006900:Hydroxylation; D008969:Molecular Sequence Data; D013329:Structure-Activity Relationship
chemical_list: D000964:Antimetabolites, Antineoplastic; D002614:Chelating Agents; D004278:DNA, Superhelical; D001761:Bleomycin; D006639:Histidine; D004247:DNA