DNA-TENTACLE PORPHYRIN INTERACTIONS - AT OVER GC SELECTIVITY EXHIBITED BY AN OUTSIDE BINDING SELF-STACKING PORPHYRIN

The binding characteristics of the tentacle porphyrin T theta OPP [(3- (trimethylammonio)propyl)oxy]phenyl]porphyrin) to [poly(dA-dT)](2) and to [poly(dG-dC)](2) were investigated. T theta OPP differs from other porphyrins studied with DNA in having the positive charges on a flexi ble group, i.e, the tentacle. We assessed whether the DNA binding of s uch a species could be influenced by the sequence. In previous studies with calf thymus (CT) DNA, T theta OPP was found to bind in a noninte rcalative outside binding mode with self-stacking increasingly importa nt at higher ratios of T theta OPP to base pairs. Decreases in absorba nce and strong induced conservative CD signals in the Soret region cha racteristic of outside self-stacking were found herewith [poly(dA-dT)] (2) and [poly(dG-dC)](2); thus, the mode of binding was outside self-s tacking to both polymers. The resulting spectra for the two polymers w ere found to be sufficiently different to allow competitive binding st udies, which showed preferential binding of T theta OPP to [poly(dA-dT )](2) over [poly(dG-dC)](2). The visible and induced CD spectra of T t heta OPP bound to [poly(dA-dT)](2) were similar to those reported for T theta OPP bound to CT DNA, indicating that binding to CT DNA may be at AT sites. With [poly(dG-dC)](2), self-stacking was more extensive t han with [poly(dA-dT)](2) or with CT DNA, as indicated by broad Soret bands and low-intensity CD signals. Furthermore, in high salt (100 mM NaCl), some of the spectral differences between the T theta OPP-[poly( dG-dC)](2) adduct and the other DNA adducts were even more pronounced. Unlike in previous examples of AT binding selectivity by porphyrins, the UV CD spectra of the DNA polymers did not change significantly. As found previously for CT DNA, the bound, protonated form of T theta OP P has a positive, nonconservative CD band with [poly(dA-dT)](2), indic ative of non-self-stacked outside binding. However, the induced CD ban d associated with the [poly(dG-dC)](2)-bound, protonated form of T the ta OPP was conservative, indicating an unusual, possibly self-stacked, outside bound species. Our results demonstrate that even for a tentac le porphyrin and its protonated form, sequence has a significant effec t on binding affinity and binding mode. Unlike in-previous studies wit h metalloporphyrins that are more rigid than T theta OPP and distort D NA, this selectivity probably arises from differences in the surface a nd hydration state of relatively undistorted DNA. Thus, T theta OPP is a better probe of DNA features.