Bridged cobalt amine complexes induce DNA conformational changes effectively

Cobalt(III) hexamine ion [Co(NH₃)₆]³⁺ is known to facilitate the transmission of B- to Z-DNA or B- to A-DNA depending on the DNA sequences. Specific interactions are found between the amines of [Co(NH₃)₆]³⁺ and DNA atoms of A-DNA or Z-DNA. Bridged Co(III)pentaamine complexes, with multiple amine groups arranged in a rigid framework, may enhance the effectiveness of the conformational transition by occupying simultaneously two of the Co(III)hexaamine binding sites. Therefore, the imidazole-bridged Co(III)pentaamine complexes have been synthesized and their interactions with DNA oligonucleotides investigated by circular dichroism (CD) and NMR spectroscopy. CD studies of the titrations of d(A₂C₁₅G₁₅T₂) with [(NH₃)₅Co(Im)Co(NH₃)₅]Br₅ and [(NH₃)₅Co(Im)]₂Co(NH₃)₄]Br₇ showed that the former metal compound indeed is more effective than [Co(NH₃)₆]³⁺ in inducing the transition from B- to A-DNA. The conversion of B- to A-DNA was also supported by one- and two-dimensional NMR studies. Similarly for the titrations of poly(dC-dG)·poly(dC-dG) and d(m⁵C-G)₁₅ with these two bridged Co(III) complexes, efficient induction of Z-DNA was observed. Our studies suggest that bridged Co(III)pentaamine complexes may be useful agents for studying nucleic acid structures.