Molecular Complexes Of Nucleosides And Nucleotides With A Monomeric Cationic Porphyrin And Some Of Its Metal Derivatives

Tetrakis(4-N-methylpyridyl)porphine (H₂TMpyP) and a number of its metal derivatives interact extensively with mononucleotides and mononucleosides in aqueous solution. The complexes formed are of a stacking-type involving extensive overlap of the T-systems of the porphyrin and purine or pyrimidine bases. Coulombic attractions help stabilize the complexes but there is no evidence for ligation of the bases to axial sites of the metalloporphyrins. Stability constants determined via NMR and spectrophotometric titrations are larger for purine bases than pyrimidines with a given porphyrin derivative. More dramatic influences on stability result from changing porphyrins. Porphyrins having no axial ligands (e.g., metal-free copper(II), palladium(II), and nickel(II) derivatives) or one axial ligand (Zn(II)) produce much larger interactions with a given nucleotide or nucleoside than do metalloporphyrins having two axial ligands (e.g., Mn(III), Fe(III), or Co(III)). The kinetics of the interaction of H₂TMpyP with 2\u27-deoxyadenosine 5\u27-monophosphate (dAMP) were studied via the laser raman temperature-jump method. The measured rate constants are consistent with a simple stacking model for the interaction