A SPECTROSCOPIC AND THERMODYNAMIC STUDY OF PORPHYRIN DNA SUPRAMOLECULAR ASSEMBLIES/

Assemblies of trans-bis(N-methylpyridinium-4-yl)diphenylporphine ions on the surface of calf thymus DNA have been studied using several spec troscopic techniques: absorbance, circular dichroism, and resonance li ght scattering. The aggregation equilibrium can be treated as a two-st ate system-monomer and assembly-each bound to the nucleic acid templat e. The aggregate absorption spectrum in the Soret region is resolved i nto two bands of Lorentzian line shape, while the DNA-bound monomer sp ectrum in this region is composed of two Gaussian bands. The Beer-Lamb ert law is obeyed by both porphyrin forms. The assembly is also charac terized by an extremely large, bisignate induced circular dichroism (C D) profile and by enhanced resonance light scattering (RLS). Both the CD and RLS intensities depend linearly on aggregate concentration. The RLS result is consistent with a model for the aggregates as being eit her of a characteristic size or of a fixed distribution of sizes, inde pendent of total porphyrin concentration or ionic strength. Above thre shold values of concentration and ionic strength, the mass action expr ession for the equilibrium has a particularly simple form: K' = cac(-1 ); where cac is defined as the ''critical assembly concentration.'' Th e dependence of the cac upon temperature and ionic strength (NaCl) has been investigated at a fixed DNA concentration. The value of the cac scales as the inverse square of the sodium chloride concentration and, from temperature dependence studies, the aggregation process is shown to be exothermic.