Spectroscopic Investigation and Molecular Modeling on Porphyrin/PAMAM Supramolecular Adduct Maria Angela Castriciano* 1 , Andrea Romeo 2 , Nicola Angelini 3 , Norberto Micali 4 , Salvatore Guccione 5 and Luigi Monsu ` Scolaro 2 1 CNR-ISMN, Vill. S.Agata, Messina, Italy 2 Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, University of Messina, and C.I.R.C.M.S.B., Vill. S.Agata, Messina, Italy 3 CNR-ISM, Monterotondo Scalo, (RM), Italy 4 CNR-IPCF, Vl. Ferdinando Stagno d’Alcontres n.39, Contrada Papardo, Faro Superiore, Messina, Italy 5 Dipartimento di Scienze Farmaceutiche, University of Catania, Catania, Italy Received 1 September 2010, accepted 24 November 2010, DOI: 10.1111/j.1751-1097.2010.00872.x ABSTRACT Noncovalent adducts (TPPC@PAMAM) between meso-tetra- kis(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 spectro- scopic 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 spectro- scopic and photophysical techniques. All the experimental results point to the formation of a ternary complex between the binary adduct and the DNA backbone. INTRODUCTION Porphyrins are an important class of natural and synthetic compounds having manifold implications in different research and application fields. In particular, water soluble porphyrins have been largely investigated for their capability of interacting with biomolecules, i.e. nucleic acids (1–7), polypeptides (7–14), and proteins (15,16). During the last decades many reports have been focused on the application of porphyrins and their metal-derivatives as biological markers and active principle for the medical treatment of different type of diseases: (1) in spongiform encephalopathy or prion diseases by inhibiting the conversion of normal protease-sensitive prion protein to the abnormal protease-resistant form (17,18) and (2) as in vitro inhibitors of b-amyloid peptides aggregation, responsible for the Alzheimer disease (19,20). Furthermore, due to their ability to produce singlet oxygen ( 1 O 2 ) and localize into tumor cells, porphyrins are widely applied as photosensitizers in photody- namic therapy (PDT) (21,22). Although several potential PDT agents are actually under investigation, Photofrin II, a complex mixture of porphyrin oligomers (23), is the only one regularly approved and used for the photodynamic treatment of esophageal and endobronchial carcinoma (24). A crucial factor for the application of many water soluble porphyrins in biomedical field is their propensity to self-aggregate in aqueous solutions above a critical concentration, compromising their activity. Indeed, the aggregation state affects the photoproduc- tion of singlet oxygen, whose oxidative effects are responsible for PDT action (25). As far as aggregation is concerned, very few investigations have been devoted to study the meso- tetrakis(4-carboxyphenyl)porphyrin (TPPC) both in neutral and acidic environments. Due to the protonation of the carboxylate groups (the highest pK a value is 6.6) (26), TPPC is present in neutral aqueous solution as monomeric species only at very low concentration (<2 lM). In acidic condition (pH <1) the protonation of the nitrogen atoms of the porphyrin core occurs yielding a dicationic species, which is able to self- assemble into J-aggregates, whose spectroscopic features are dependent on the counteranion nature (27,28). Under mild acidic condition (pH 3.5) this porphyrin tends to self- assemble into H-aggregates with an average size of about 20 A ˚ (29). In order to stabilize TPPC in aqueous solution up to millimolar concentration, we recently reported a simple procedure to obtain a supramolecular adduct between TPPC and a biocompatible amino-terminated poly (propylene glycol) (Jeffamine M-600). Due to the partial hydrophobic character of the Jeffamine chains, the aggregation state of this complex can be modulated by changing the polarity of the solvent (30). Furthermore, through a similar supramolecular approach, we reported an artificial peroxidase system soluble in aqueous solution easily obtained by interacting poly (amidoamine) *Corresponding author email: castriciano@pa.ismn.cnr.it (Maria Angela Castriciano) Ó 2011 The Authors Photochemistry and Photobiology Ó 2011 The American Society of Photobiology 0031-8655/11 Photochemistry and Photobiology, 2011, 87: 292–301 292