PORPHYRIN ASSEMBLIES ON DNA AS STUDIED BY A RESONANCE LIGHT-SCATTERING TECHNIQUE

Under appropriate conditions of concentration and ionic strength, trans-bis(N-methylpyridinium-4-yl)-diphenylporphine and its copper(II) derivative produce very large, bisignate circular dichroism signals in the Soret region when bound to DNA. The processes leading to these signals are shown to be highly cooperative. A new light-scattering experimental approach is described for detecting such extended aggregates of chromophores in which the radiation used is within an absorption maximum, specifically in the porphyrin Soret absorption region. Results from such resonance light-scattering experiments confirm that extended porphyrin aggregates form on the DNA, which, in contrast, remains dispersed. A model for these interactions consistent with experimental findings involves porphyrin organization into long-range chiral structures having antenna-like properties, that is, forming assemblies in which the electric field produced in one oscillating dipole affects the magnitude of (is coupled to) the other dipoles in the aggregate.