DNA-PORPHYRIN ADDUCTS - 5-COORDINATION OF DNA-BOUND VOTMPYP(4) IN AN AQUEOUS ENVIRONMENT - NEW PERSPECTIVES ON THE V=O STRETCHING FREQUENCYAND DNA INTERCALATION

VOTMpyP(4) (vanadyl meso-tetrakis(4-N-methylpyridiniumyl)porphyrin) ex ists as five-coordinate (5C) and six-coordinate (6C) forms in nonaqueo us solvents with V=O stretching bands at 996-1005 and 957-977 cm-1, re spectively. The V=O bands provided two types of information about addu cts formed between VOTMpyP(4) and biopolymers, particularly nucleic ac ids, in aqueous solution. First, the appearance of two V=O bands (at 9 89-993 and 956-964 cm-1) attributable to the 5C and 6C forms, respecti vely, allowed an assessment of the amount of each form. Second, the fr equency dependence of the forms allowed us to conclude that the solven t environment around polymers such as DNA has an acceptor number simil ar to that of water and that both the 5C and 6C forms were bound. Thes e results provide the first direct observation of 5C VOTMpyP(4) in an aqueous environment and the first strong evidence that a 6C form of a coordinatively fluxional metalloporphyrin can bind to DNA. The V=O str etching frequency of approximately 996 cm-1 estimated from acetonitril e/water and dimethylformamide/water mixtures for 5C VOTMpyP(4) is rath er different from the 970 cm-1 previously predicted by extrapolation o f data in methanol/water mixtures. However, the value of 989-993 cm-1 found for the DNA-bound species was similar to the 996-cm-1 value foun d here. The DNA binding interactions were also probed by visible spect roscopy in the Soret region and by CD spectroscopy of porphyrin and DN A bands. These methods were used to compare the DNA binding of VOTMpyP (4) with the binding of porphyrins with known, but diverse, binding pr operties, namely TMpyP(4), NiTMpyP(4), and ZnTMpyP(4). These studies c learly showed that VOTMpyP(4) and ZnTMpyP(4) bind in a similar manner. ZnTMpyP(4) is an established outside binder which does not increase D NA viscosity and which selectively binds to AT over GC regions. Likewi se, VOTMpyP(4) demonstrated no increase in DNA viscosity and an AT pre ference. In particular, the visible absorption and CD spectra of VOTMp yP(4) in the presence of 1:1 mixtures of poly(dA-dT).poly(dA-dT) ([pol y(dAdT)]2) and poly(dG-dC).poly(dG-dC) ([poly(dGdC)]2) are nearly iden tical to those with [poly(dAdT)12 alone and very different from those of [poly(dGdC)]2 alone. Thus, VOTMpyP(4) is not an intercalator, and t here is no evidence to suggest that it is a partial intercalator. The species is thus a useful paramagnetic probe selective for AT regions o f DNA.