Spectroscopic investigation and molecular modeling on porphyrin/PAMAM supramolecular adduct

Noncovalent adducts (TPPC@PAMAM) between meso-tetrakis(4-carboxyphenyl)porphyrin (TPPC) and polyamidoamine PAMAM dendrimer (generation 2.0) have been obtained by simply mixing the two components at different stoichiometric amount. The resulting species are readily soluble and stable in aqueous solution up to millimolar concentration. Electrostatic interactions between the anionic carboxylate groups of TPPC and the protonated amino groups of the PAMAM dendrimer play an important role in the stabilization of these adducts. UV/Vis absorption, steady state and time-resolved fluorescence emission and anisotropy measurements suggest the presence of equilibria involving different species as function of the [PAMAM]/[TPPC] ratio. At low ratios the observed spectroscopic behavior evidence the presence of H-aggregates, while at higher ratios well-defined species containing monomeric TPPC strongly interacting with the charged dendrimer are formed. Docking of the binary supramolecular adduct further supports the experimental results showing a favorable interaction with the porphyrin being completely included in the dendrimer. The interaction of the binary TPPC@PAMAM adduct (1/1 ratio) with calf-thymus DNA has been investigated through spectroscopic and photophysical techniques. All the experimental results point to the formation of a ternary complex between the binary adduct and the DNA backbone.