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.
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