Tetrakis(4-7V-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 -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'-deoxyadenosine S'-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.

In the presence of oxygen and visible light, various synthetic water‐soluble porphyrins cleave pBR 322 plasmid supercoiled DNA (form I) producing relaxed (form II) and linear (form III) DNA corresponding to single‐strand and double‐strand breaks respectively. Large variations are observed in the efficiency of the porphyrins containing a diamagnetic metal or no metal at all. Singlet oxygen (1O2) seems to be involved in the mechanism of cleavage consistent with the inhibitory effect of the azide anion, N–3. The higher efficiency of cationic porphyrins (as compared to anionic ones) is due to their greater affinity for DNA as shown by experiments carried out at either high ionic strength or in the presence of the surfactant, sodium dodecyl sulfate.

We present an X-ray absorption spectroscopy study of the local structures of Gd(DTPA)(2)(-) and Gd(DOTA)(-) complexes in the crystalline state (at room and low temperatures) and in aqueous solutions exhibiting various pH values (0.15-7) at different temperatures (25-90 degrees C). Using X-ray absorption fine structure (XAFS) analysis procedures and ab initio multiple scattering calculations of XAFS spectra at the Gd L(3) edge, we reconstructed the Gd(3+) local environment, and compared it with existing structural models. From neutral pH to a value of 1.5, we found that the local environment and complex dynamics around the gadolinium ions were conserved up to 4.5 Å, and the structure agreed well with the known crystallographic data. In these solutions, the gadolinium ions in the complex Gd(DOTA)(-) are bonded to the four carboxylate oxygen atoms [R(Gd-O(av)) 2.38 Å, Debye-Waller (DW) factor 0.006 Å(2)], to the four nitrogen atoms [R(Gd-N(av)) 2.65 Å, DW factor 0.006 Å(2)] and to one water molecule [R(Gd-O(w)) 2.46 Å, DW factor 0.012 Å(2)]. Concerning the complex Gd(DTPA)(2)(-), the gadolinium ions are bonded to the five carbonyl oxygen atoms [R(Gd-O(av)) 2.39 Å, DW factor 0.007 Å(2)], to the three nitrogen atoms [R(Gd-N(av)) 2.64 Å, DW factor 0.006 Å(2)], and to one water molecule [R(Gd-O(w)) 2.47 Å, DW factor 0.018 Å(2)]. In the range of pH (0.15-1.5) for the Gd(DTPA)(2)(-) complexes, thanks to the pH strong dependence of the XAFS signals, we observed a progressive complex dissociation. On the other hand, the XAFS signals of Gd(DOTA)(-) complexes exhibited only a slight pH (1-1.5) dependence. Concerning both complexes, we noted just a slight temperature dependence.