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

VOTMpyP(4) (vanadyl meso-tetrakis(4-N-methylpyridiniumyl)porphyrin) exists as five-coordinate (5C) and six-coordinate (6C) forms in nonaqueous solvents with V=O stretching bands at 996-1005 and 957-977 cm-1, respectively. The V=O bands provided two types of information about adducts formed between VOTMpyP(4) and biopolymers, particularly nucleic acids, in aqueous solution. First, the appearance of two V=O bands (at 989-993 and 956-964 cm-1) attributable to the 5C and 6C forms, respectively, allowed an assessment of the amount of each form. Second, the frequency dependence of the forms allowed us to conclude that the solvent environment around polymers such as DNA has an acceptor number similar to that of water and that both the 5C and 6C forms were bound. These 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 stretching frequency of approximately 996 cm-1 estimated from acetonitrile/water and dimethylformamide/water mixtures for 5C VOTMpyP(4) is rather different from the 970 cm-1 previously predicted by extrapolation of 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 found here. The DNA binding interactions were also probed by visible spectroscopy in the Soret region and by CD spectroscopy of porphyrin and DNA bands. These methods were used to compare the DNA binding of VOTMpyP(4) with the binding of porphyrins with known, but diverse, binding properties, namely TMpyP(4), NiTMpyP(4), and ZnTMpyP(4). These studies clearly showed that VOTMpyP(4) and ZnTMpyP(4) bind in a similar manner. ZnTMpyP(4) is an established outside binder which does not increase DNA viscosity and which selectively binds to AT over GC regions. Likewise, VOTMpyP(4) demonstrated no increase in DNA viscosity and an AT preference. In particular, the visible absorption and CD spectra of VOTMpyP(4) in the presence of 1:1 mixtures of poly(dA-dT).poly(dA-dT) ([poly(dAdT)]2) and poly(dG-dC).poly(dG-dC) ([poly(dGdC)]2) are nearly identical 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 there is no evidence to suggest that it is a partial intercalator. The species is thus a useful paramagnetic probe selective for AT regions of DNA.