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

Under appropriate conditions of concentration and ionic strength, rans -bis(N-methylpyridinium-4-yl)-diphenylporphine and its copper(II) deri vative produce very large, bisignate circular dichroism signals in the Soret region when bound to DNA. The processes leading to these signal s are shown to be highly cooperative. A new light-scattering experimen tal approach is described for detecting such extended aggregates of ch romophores 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 porp hyrin aggregates form on the DNA, which, in contrast, remains disperse d. A model for these interactions consistent with experimental finding s involves porphyrin organization into long-range chiral structures ha ving antenna-like properties, that is, forming assemblies in which the electric field produced in one oscillating dipole affects the magnitu de of (is coupled to) the other dipoles in the aggregate.