The interactions between the water-soluble tentacle porphyrin meso-tetrakis[4-[(3-(trimethylammonio)propyl)- oxy]phenyl]porphine (T theta OPP) and calf thymus (CT) DNA had been found to exhibit unusual features : (a) At pH 7.0, T theta OPP underwent extensive self-stacking along the DNA surface (Marzilli, et al. J. Am. Chern. Soc. 1992, 114, 7575); (b) the DNA-bound form of T theta OPP was extensively protonated even at pH 7.0 (Petho, et al. J. Chem. Soc., Chem. Commun. 1993, 1547). In the studies presented here, the formation of the T theta OPP-DNA adduct was investigated under various conditions of pH, salt concentration, [porphyrin]/[DNA base pair] ratio (R), and absence/presence of buffer. Under all conditions studied, T theta OPP was an outside binder. Methods employed included circular dichroism (CD), UV-visible absorbance, and fluorescence spectrophotometry. Time-dependence experiments were also conducted in order to detect any change in binding characteristics over time. Evidence for outside-bound, self-stacked forms of T theta OPP included a large conservative induced visible CD feature, whereas the evidence for DNA-bound, protonated T theta OPP included a Soret band red-shifted to 451 nm, a single positive induced CD band at 451 nm, and a red-shifted fluorescence band at 740 nm. At least two forms of the outside-bound, self-stacked porphyrin were evident. At high R (0.25), hypochromicity of the Soret band and a conservative type CD feature indicated that T theta OPP was extensively stacked. At lower R (0.05), increases in the Soret band intensity and changes in the band shape suggested a less extensively stacked form, but this form of T theta OPP had a larger conservative feature in the CD spectrum. The intensity of the conservative band suggested that the less stacked form is very stable at R = 0.05. The CD data are consistent with a model in which the interacting T theta OPP in the less stacked form has a different relative orientation than the more stacked e the outside-bound, self-stacked form of T theta OPP. At R = 0.01, a decrease in CD band intensity suggested that the porphyrin was less stacked than at R = 0.05; this change in stacking occurred over 1-2 h. : (a) At pH 7.0, T theta OPP underwent extensive self-stacking along the DNA surface (Marzilli, et al. J. Am. Chern. Soc. 1992, 114, 7575); (b) the DNA-bound form of T theta OPP was extensively protonated even at pH 7.0 (Petho, et al. J. Chem. Soc., Chem. Commun. 1993, 1547). In the studies presented here, the formation of the T theta OPP-DNA adduct was investigated under various conditions of pH, salt concentration, [porphyrin]/[DNA base pair] ratio (R), and absence/presence of buffer. Under all conditions studied, T theta OPP was an outside binder. Methods employed included circular dichroism (CD), UV-visible absorbance, and fluorescence spectrophotometry. Time-dependence experiments were also conducted in order to detect any change in binding characteristics over time. Evidence for outside-bound, self-stacked forms of T theta OPP included a large conservative induced visible CD feature, whereas the evidence for DNA-bound, protonated T theta OPP included a Soret band red-shifted to 451 nm, a single positive induced CD band at 451 nm, and a red-shifted fluorescence band at 740 nm. At least two forms of the outside-bound, self-stacked porphyrin were evident. At high R (0.25), hypochromicity of the Soret band and a conservative type CD feature indicated that T theta OPP was extensively stacked. At lower R (0.05), increases in the Soret band intensity and changes in the band shape suggested a less extensively stacked form, but this form of T theta OPP had a larger conservative feature in the CD spectrum. The intensity of the conservative band suggested that the less stacked form is very stable at R = 0.05. The CD data are consistent with a model in which the interacting T theta OPP in the less stacked form has a different relative orientation than the more stacked e the outside-bound, self-stacked form of T theta OPP. At R = 0.01, a decrease in CD band intensity suggested that the porphyrin was less stacked than at R = 0.05; this change in stacking occurred over 1-2 h.