DNA Binding of Porphyrin Conjugates: Characteristics and Consequences

Abstract

Recently cationic porphyrin-peptide conjugates were synthesized to enhance cellular uptake of porphyrins and to deliver peptide moieties to the close vicinity of nucleic acids.The aim of our work was to characterize the binding of porphyrin-peptide conjugates to nucleic acids and nucleoprotein complexes (NP) and describe structural changes induced by the interaction.Porphyrin-peptide conjugates were synthesized, linking tetra-peptides to tris(N-methylpyridyl)carboxyphenyl-porphyrin (TMPCP) and bis(methylpyridyl)carboxyphenyl-porphyrin (BMPCP). These structures can be considered as a typical monomer unit corresponding to the branches of porphyrin-branched chain polypeptide conjugates. DNA was isolated from T7 bacteriophage; as nucleoprotein complex the complete phage particle was used (NP).Binding modes were studied by comprehensive spectroscopic methods such as absorption spectroscopy, CD spectroscopy and fluorescence lifetime measurements. To look for possible structural changes of NP and DNA caused by the porphyrin binding CD spectroscopy was used and the thermal stability of DNA/NP was detected by optical melting method.Our results show that the interaction of porphyrin conjugates with DNA can be either intercalative or external binding. The dominant factor in the interaction with isolated DNA proved to be the charge distribution, with binding affinities higher for tri-cationic molecules than their di-cationic counterparts. Presence of protein capsid opposes but does not inhibit DNA-porphyrin interaction. In the case of NP, binding affinity of conjugates decreases and the mode of interaction is mostly external binding.CD spectra recorded in 200-300 nm range revealed hypochromicity in the DNA bands and alteration of protein bands in the presence of conjugates that can be attributed to the distortion of helical structure and loosening of capsid structure, respectively. Absorption melting showed that the porphyrin binding stabilizes the helical structure, i.e. increases the strand separation temperature of DNA, but does not influence DNA - protein interaction in NP.