Synthesis and evaluation of deglycobleomycin A2 analogues containing a tertiary N-methyl amide and simple ester replacement for the L-histidine secondary amide: Direct functional characterization of the requirement for secondary amide metal complexation

Article abstract of DOI:10.1016/0968-0896(95)00183-2

The synthesis and comparative examination of 3-5, analogues of deglycobleomycin A₂ (2) which address the inferred importance of the L-histidine secondary amide directly, are detailed. The agent 3 lacks only the L-histidine β-hydroxy group of deglycobleomycin A₂ and the corresponding agents 4 and 5 incorporate a tertiary N-methyl amide and simple ester in place of the L-histidine secondary amide. The DNA cleavage properties of 3 proved essentially indistinguishable from those of deglycobleomycin A₂ (2) confirming that the distinctions between bleomycin A₂ (1) and deglycobleomycin (2) are due to the removal of the disaccharide and not the introduction of the L-histidine free β-hydroxy group. The agents 4 and 5 containing a tertiary N-methyl amide and ester in place of the L-histidine secondary amide were found to cleave duplex DNA but to do so in a nonsequence selective fashion with a substantially reduced efficiency and a diminished double to single strand cleavage ratio that are only slightly greater than that of free iron itself. These latter observations establish the functional requirement for the L-histidine secondary amide and are consistent with the proposals that the L-histidine deprotonated secondary amide is required for functional metal chelation and activity.