Bridged cobalt amine complexes induce DNA conformational changes effectively

Cobalt(III) hexaamine ion [Co(NH3)(6)](3+) is known to facilitate the transition 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(lll)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(lll)hexaamine binding sites. Therefore, the imidazole-bridged Co(lll)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(A(2)C(15)G(15)T(2)) with [(NH3)(5)Co(Im)Co(NH3)(5)]Br-5 and [{(NH3)(5)Co(Im)}(2)Co(NH3)(4)]Br-7 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(m(5)C-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. (C) 1997 Elsevier Science Inc.