Anation Reactions of Cobalt(III) complexes

Assessing the sequence specificity in the binding of Co(III) to DNA via a thermodynamic approach

The interaction specificities of Co(III) with DNA were investigated via consideration of thermodynamic characteristics of the duplex to single strand transition for DNA oligomers incubated in the presence of [Co(NH3)5(OH2)](ClO4)3. It has previously been demonstrated that incubation of the DNA oligomer [(5medC-dG)4]2 with this cobalt complex leads to coordination of the cobalt center to the DNA, presumably at N7 of guanine bases [D. C. Calderone, E. J. Mantilla, M. Hicks, D. H. Huchital, W. R. Murphy, Jr. and R. D. Sheardy, (1995) Biochemistry 34, 13841]. In this report, DNA oligomers of different sequence were incubated with [Co(NH3)5(OH2)](ClO4)3 via protocols developed previously and the treated oligomers were subjected to thermal denaturation for comparison to the untreated oligomers. The DNA oligomers were designed in order to investigate the sequence specificity, if any, in the reaction of the cobalt complex with DNA. The values of Tm, delta HvH, and delta n (the differential ion binding term) obtained from the thermal denaturations were used to assess the sequence specificity of the interaction. For all oligomers, treated or untreated, Tm and delta HvH vary linearly with log [Na+] and hence the value of delta n is a function of the Na+ concentration. The results indicate no significant reaction between the cobalt complex and oligomers possessing isolated -GA- or -CG- sites; however, the thermodynamic characteristics of DNA oligomers possessing either an isolated -GG- site or an isolated -GC- site were altered by the treatment. Atomic absorption studies of the treated oligomers demonstrate that only the DNA oligomers possessing isolated -GG- or -GC- sites bind cobalt. Hence, the changes in the thermodynamic properties of these oligomers are a result of cobalt binding with a remarkable sequence specificity.

NMR Spectroscopic Characterization of Methylcobalt(III) Compounds with Classical Ligands. Crystal Structures of [Co(NH(3))(5)(CH(3))]S(2)O(6), trans-[Co(en)(2)(NH(3))(CH(3))]S(2)O(6) (en = 1,2-Ethanediamine), and [Co(NH(3))(6)]-mer,trans-[Co(NO(2))(3)(NH(3))(2)(CH(3))](2)-trans- [Co(NO(2))(4)(NH(3))(2)]

A series of methylcobalt(III) compounds with solely classical ligands such as NH(3), H(2)O, NO(2)(-), CN(-), and 1,2-ethanediamine (en) has been prepared by ligand substitution of the pentaamminemethylcobalt(III) cation. The new compounds have been characterized by (1)H, (13)C, and (59)Co nuclear magnetic resonance spectroscopy and by absorption spectroscopy. Single-crystal X-ray structure determinations at 122.0(5) K were performed on [Co(NH(3))(5)(CH(3))]S(2)O(6) (1), trans-[Co(en)(2)(NH(3))(CH(3))]S(2)O(6) (2), and [Co(NH(3))(6)]-mer,trans-[Co(NO(2))(3)(NH(3))(2)(CH(3))](2)-trans-[Co(NO(2))(4)(NH(3))(2)] (3). Compound 1 is monoclinic, P2(1)/n, with cell dimensions a = 6.4856(9) Å, b = 18.0720(11) Å, c = 9.7854(14) Å, and beta = 98.280(13) degrees. Compound 2 is orthorhombic, P2(1)2(1)2, with cell dimensions a = 12.211(3) Å, b = 10.107(3) Å, and c = 11.514(3) Å. Compound 3 is monoclinic, P2(1)/n, with cell dimensions a = 8.390(2) Å, b = 8.970(3) Å, c = 20.650(9) Å, and beta = 91.29(2) degrees. Virtually identical Co-C distances in the range 1.980-1.984 Å are found in the three compounds. The methyl group exerts a strong trans influence. The largest relative elongation of the bond to the trans ligand is 0.170 Å in the trans-[Co(en)(2)(NH(3))(CH(3))](2+) cation. The structural trans influence appears to decrease with the overall charge of the coordination compound. Estimates of the scalar one-bond cobalt-carbon coupling constants, (1)J(Co)(-)(C), in the pentacyanomethylcobalt(III) anion was obtained by line shape analysis, yielding values of 127, 95, and 62 Hz for the cis cyano groups, the trans cyano group, and the methyl group, respectively.