Metalloporphyrin Effects on Properties of DNA Polymers

Abstract

Interactions of metallo derivatives of meso-tetrakis(4-N-methylpyridiniumyl)porphyrin [TMpyP(4)] and meso-tetrakis(2-N-methylpyridiniumyl)porphyrin [TMpyP(2)] with several native and synthetic DNAs were studied by a variety of physical techniques: viscosity, flow dichroism (FD), and NMR (31P and1H). The porphyrins were divided into two groups, group I and group III, based on the criteria suggested by Banville et al. Group I porphyrins include NiTMpyP(4) and PdTMpyP(4). Large negative reduced dichroisms (redZ)) observed in FD studies indicated that the group I porphyrins were bound perpendicular to the axis of calf thymus DNA.31P NMR spectra of salmon sperm DNA with the group I porphyrins revealed a small broad downfleld peak centered at ca. -1 ppm. No significant shifts were noted for the imino proton signals (ca. 12–14 ppm) of salmon sperm DNA on addition of the group I porphyrins. Comparison of the effects of the group I porphyrins on DNAs with different GC content revealed larger changes in solution viscosity with increased GC content. Viscosity changes of AT-rich DNAs were dramatically lower, and precipitation of the DNA-porphyrin adducts was often observed. For poly[d(A-C)(G-T)], a large increase was found in the solution viscosity upon treatment with NiTMpyP(4), raising the possibility of mixed GC/AT intercalation sites for NiTMpyP(4). In addition, NiTMpyP(4) increased the viscosity of calf thymus DNA (CT DNA) to a greater extent than PdTMpyP(4) or TMpyP(4). Contrary to previously reported studies, our viscosity data suggest that NiTMpyP(4) is less selective than TMpyP(4) in intercalative type binding to GC base pairs of DNA; i.e., NiTMpyP(4) also binds to mixed AT/GC sites. However, our viscosity results with PdTMpyP(4) are similar to those reported with TMpyP(4), indicating similar binding properties. Group III porphyrins include NiTMpyP(2), CoTMpyP(4), FeTMpyP(4), ZnTMpyP(4), and SnTMpyP(4). No viscometric increase, lowredD values, and the absence of signals shifted downfield or upfield in3IP and1H NMR spectra, respectively, indicate outside, randomly oriented binding. SnTMpyP(4) was shown to nick CCS DNA. This nicking requires the presence of light and may be a radical process dependent on oxidation-reduction of the porphyrin ring system. In any case, it is likely that previously reported changes in the viscosity of CCS DNA with group III porphyrins could be the result of nicking and not intercalation. Thus, all the results we have with group III porphyrins are suggestive of outside binding. The binding of group I porphyrins to DNA is complex, and decreases in signal area in both31P and1H NMR spectra are attributed to outside self-stacking, which leads to aggregation of the polymer at the high concentrations needed for NMR studies. Such signal loss is not usually observed for group III porphyrins, which cannot self-stack. However, in a few cases, some signal loss was evident, and this result was attributed to paramagnetic species. In general, the binding properties of metalloporphyrins appear to reflect those of nonmetalloporphyrins; i.e,, the results with group I species are consistent with intercalation whereas those with group III species are most consistent with electrostatic interactions. However, it is not possible to extrapolate the results found with oligonucleotides to those found with these polymers. © 1987, American Chemical Society. All rights reserved.