Synthesis and DNA Binding of Novel Water-Soluble Cationic Methylcobalt Porphyrins

Organocobalt derivatives of tetracationic water-soluble porphyrins are difficult to prepare via the typical reductive alkylation of the Co(II)(por) (porH2 = porphyrin ligand). None have been reported. The problem may arise because the porphyrin core is made relatively electron poor by the positively charged peripheral groups. We have circumvented this problem by using the [Co(III)(NH3)5CH3]2+ reagent, which inserts the Co(III)−CH3 moiety directly into porH2 in water under basic conditions. The method afforded two new [CH3Co(por)]4+ derivatives, [CH3CoTMpyP(4)]4+ and [CH3CoTMAP]4+, where [TMpyP(4)]4+ and [TMAP]4+ are the coordinated, NH-deprotonated forms of meso-tetrakis(N-methyl-4-pyridiniumyl)porphyrin and meso-tetrakis(N,N,N-trimethylaniliniumyl)porphyrin, respectively. The binding of the two new [CH3Co(por)]4+ cations to DNA and to the synthetic DNA polymers [poly(dA−dT)]2 and [poly(dG−dC)]2 was studied. Using published criteria by which changes in DNA viscosity and in the visible and CD spectra in the Soret region can be used to assess DNA binding, we conclude that both are outside binders. A large hypochromicity of the Soret bands of the [CH3Co(por)]4+ cations observed upon outside binding to DNA may indicate a high degree of self-stacking. The visible absorption and CD spectra of the [CH3Co(por)]4+ cations in the presence of 1:1 mixtures of [poly(dA−dT)]2 and [poly(dG−dC)]2 are nearly identical to those with [poly(dA−dT)]2 alone and are very different from those of [poly(dG−dC)]2 alone. Thus, both cations show a high preference for outside binding at AT-rich over GC-rich DNA sites. Upon binding of each of the [CH3Co(por)]4+ cations to all of the DNA polymers, the Soret bands exhibit blue shifts, whereas the Soret bands of the corresponding [(H2O)2Co(por)]5+ cations exhibit red shifts. The blue shifts strongly suggest that the [CH3Co(por)]4+ cations, particularly [CH3CoTMAP]4+, become five-coordinate forms to some extent on DNA binding; this result is the first good evidence for the presence at equilibrium of five-coordinate CH3Co(III)(N4) forms in water.