Long-range assemblies on poly(dG-dC)2 and poly(dA-dT)2: phosphonium cationic porphyrins and the importance of the charge.
The present paper describes synthesis and spectroscopic properties of novel cationic meso-tetraphenylporphyrins bearing two (trans) (P2) or three (P3) triphenylphosphonium substituents. The porphyrin aggregation in aqueous solutions is discussed in detail. Porphyrin binding to and self-organization onto long-range assemblies on poly(dA-dT)2 or poly(dG-dC)2 were probed by combination of absorption, fluorescence, circular dichroism (CD), transient and resonance light-scattering (RLS) techniques. The higher hydrophobicity of P2 is manifested by more extensive self-organization. Induced CD and intensive RLS indicate binding to the chiral environment on the nucleic acids exterior and exciton coupling between adjacent porphyrin moieties. The CD spectra of P2 on poly(dG-dC), and poly(dA-dT)2 suggest that the binding geometry is essentially independent of the base sequence. The fluorescence lifetime of about 4 ns was attributed to the long-range assembly. In the case of P3 the distinctly different CD spectra induced by GC or AT base-pair regions reveal that the number of the substituents determines how closely the porphyrin can approach the specific electronic environment on the nucleic acid exterior. The fluorescence lifetime of the P3 assembly is about 2 ns.