Roberto Purrello

Porphyrin assemblies as chemical sensors

Abstract A simple ‘non-covalent’ approach was employed to obtain sensitive and specific sensors for pH, DNA and metal ions, by ‘mixing’ commercially available chemicals such as cationic or anionic water-soluble porphyrins and polypeptides. For example, under the appropriate conditions, the supramolecular complex formed between the anionic meso -tetrakis(4-sulfonatophenyl)porphine (H 2 TPPS) and the protonated form of poly-Lysine can behave as pH-sensor. In fact, H 2 TPPS in the pH range 5.5–12 exists in a monomeric form, and its fluorescence is not pH-dependent. However, at low pH values (≤7), the protonated poly-Lysine promotes porphyrins binding and self-aggregation with consequent strong quenching of their fluorescence, while at pH values higher than 9–10, the porphyrins exist in solution essentially as free monomers and are characterized by an intense fluorescence emission. As a consequence, the H 2 TPPS fluorescence intensity versus pH behavior shows a sigmoidal profile. Interestingly, the molecular recognition processes leading to the formation of these aggregates can be also modulated by using matrices of different nature and length as well as employing porphyrins containing different central metal ions with particular coordination geometries. In such a way we have been able to develop a whole family of sensors covering a wide range of pH. These supramolecular aggregates can also be employed as sensors for DNA. In fact, the addition of DNA (which is a poly-anion) to a preformed H 2 TPPS/poly-Lysine system (pH≤7), causes a displacement of the porphyrin bound to the poly-cationic matrix with consequent increase in the fluorescence intensity of the solution. Therefore, since the variation in fluorescence emission is linearly related to the concentration of DNA added, we have employed such supramolecular system to develop a simple and rapid method for the quantitative determination of DNA in solution. Finally, a remarkable acceleration of the insertion of copper(II) and zinc(II) in cationic porphyrins is observed when these porphyrins are monodispersed on the surface of negatively charged matrices, such as anionic poly-Glutamate. Such peculiarity allowed us to develop a specific fluorescent sensor for both metal ions capable of detecting their presence even at very low concentrations in the nanomolar range.

Supramolecular aggregation/deaggregation in amphiphilic dipolar Schiff-base zinc(II) complexes.

The synthesis, characterization, optical, and fluorescent properties of an amphiphilic Schiff-base bis(salicylaldiminato)zinc(II) complex are reported. Detailed (1)H nuclear magnetic resonance (NMR), diffusion-ordered spectroscopy (DOSY) NMR, optical absorption, and fluorescence spectroscopy studies indicate the existence of aggregate species in noncoordinating solvents. The degree and type of aggregation are related to the concentration and the polarity of the noncoordinating solvent. Dilute solutions are likely characterized by the presence of defined dimers, whereas larger oligomeric aggregates are conceivably formed at higher concentrations. The concentration needed to observe the formation of larger species depends upon solvent polarity. In coordinating solvents or in the presence of coordinating species, a complete deaggregation of the system occurs, because of the axial coordination to the Zn(II) ion, accompanied by considerable changes of (1)H NMR and optical absorption spectra. A dramatic enhancement of fluorescence emission is observed in dichloromethane solutions upon deaggregation with a coordinating agent. The formation of a defined 2:1 supramolecular structure is demonstrated in the case of a ditopic ligand as coordinating species. Overall, these complexes are promising systems for the development of new supramolecular architectures and supramolecular fluorescent probes.

Re-examination of the formation of dinitrosyl–iron complexes during reaction of S-nitrosothiols with Fe(II)

The reaction of S-nitrosothiol compounds with ferrous ions in solution has been investigated and the generated dinitrosyl–iron complexes have been characterized. During the reaction of S-nitrosocysteamine with Fe(II) in water solution in the presence of a twofold excess (with respect to iron) of cysteamine hydrochloride (CSH), an EPR-silent dinuclear iron complex (complex A of formula [Fe2(RS)2(NO)4]) was formed as the major species and was characterized by FAB MS + , UV–Vis, NMR and IR spectroscopies. In the presence of a large excess of CSH (CSH/Fe(II)= 20:1), a green paramagnetic mononuclear complex (complex B of formula [Fe(RS)2(NO)2] − ) was formed. From EPR and UV–Vis data, and also on the basis of the few crystallographic structures known for similar complexes, complex B is proposed to display a distorted tetrahedral geometry (C2), approaching a trigonal bipyramid with a missing ligand, with the unpaired electron mainly localized on the d z 2 orbital of the iron characterized by a d 9 electronic configuration.

Nanoporous crystals of calixarene/porphyrin supramolecular complex functionalized by diffusion and coordination of metal ions

A highly nanoporous material has been obtained by self-assembly of calixarene and porphyrin building blocks. This supramolecular zeolite-like structure was successively functionalized by diffusion and coordination of metal ions to form a new bifunctionalized nanoporous material containing a porphyrinic pigment together with a metal center.

Chiral sign selection on the J‐aggregates of diprotonated tetrakis‐(4‐sulfonatophenyl)porphyrin by traces of unidentified chiral contaminants present in the ultra‐pure water used as solvent

Traces of biological contaminants that cannot be detected, but are expected to be present, in ultra-pure water suffice to select the emerging chiral sign in the spontaneous mirror symmetry breaking that takes place during the formation of the J-aggregates of the amphiphilic diprotonated tetrakis-(4-sulfonatophenyl)porphyrin (H(4)TPPS(4)(2-)). This is demonstrated by competition experiments with a chiral cationic surfactant. The sensitivity of the detection depends on the hierarchical control of the H(4)TPPS(4)(2-) self-aggregation.

Comparison of three fungal laccases from Rigidoporus lignosus and Pleurotus ostreatus: correlation between conformation changes and catalytic activity.

The conformation changes in solution of three fungal laccases in different environmental conditions were examined by circular dichroism (CD) and electron paramagnetic resonance (EPR) spectroscopy. CD measurements indicate that the secondary structure of proteins depends slightly on the pH or ionic strength, though the presence of salt could interfere in the molecular recognition process between substrates and enzymes. The enzymes, however, are highly destabilized by prolonged exposure to low pH or high temperature. The observed unfolding of the proteins coincides with their inactivation and, in some cases, with precipitation. On the other hand, these conditions do not determine the disruption of the geometric arrangement of their metal centres, and this fact suggests that these centres represent the more stable core of the proteins.

Single-stranded nucleic acids as templates for porphyrin assembly formation

Abstract Resonance light scattering (RLS) and circular dichroism (CD) spectroscopies have been used to investigate the interactions of several cationic porphyrins with poly(dA) and poly(rA). Neither tetrakis(N-methylpyridinium-4-yl)porphine nor its zinc(II) derivative (H2T4 and ZnT4) show any tendency to form extended porphyrin assemblies in the presence of poly(dA). However, the poly(dA) complex of CuT4 involves considerable porphyrin self-stacking. In the presence of poly(rA), only ZnT4 of these three porphyrin derivatives, fails to assemble. Differences in the interactions of trans-bis(N-methyl-pyridinium-4-yl)diphenylporphine (t-H2Pagg) with these two single-stranded nucleic acid polymers are described. Whereas the porphyrin is capable of forming extended assemblies with either poly(dA) or poly(rA), increasing the salt concentration in the latter system results in a reversal of the induced circular dichroism spectrum in the Sorel region indicating a conformational change of the porphyrin assembly.

Evidence of induced chirality in stirred solutions of supramolecular nanofibers

Two-modulator generalized ellipsometry is used to determine the spectroscopic Mueller matrix of a solution of porphyrin supramolecular aggregates that have fibrous form. During the measurements the solutions were stirred in clockwise and anticlockwise directions. The pseudopolar decompostion is applied to the experimental Mueller matrices to unveil the birefringent and dichroics properties of the sample. The vortex flow in the stirred solution is found to modify the optical response of the aggregates to polarized light, and, in particular, its chiral signature is determined by the stirring direction in a totally reversible process. The data found show that chirality can be induced by stirring in solutions of supramolecular fibers and that a effective transfer of chirality from a macroscopic phenomenon to the supramolecular structures takes place.

Coordination properties of dialkyltin (IV) in aqueous solution. Thermodynamic study of dimethyltin (IV) complexes with l-amino acids

Abstract The complex formation of dimethyltin (IV) ([SnMe2]2+) with alanine, phenylalanine, triptophane and valine has been investigated by means of potentiometry at 25 °C and I = 0.1 mol dm−3 (KNO3). The selection of the models and the calculation of stability constants have been performed collecting a large amount of data points and using three different computer programs. This procedure has made it possible to detect the species forming in low percentages. A comparison with monodentate and bidentate systems previously studied gives an indication as to the number and the nature of the donor atoms involved in the coordination.

Interaction of Tricationic Corroles with Single/Double Helix of Homopolymeric Nucleic Acids and DNA

In this manuscript a multitechnique approach is proposed to characterize the interaction between new tri-N-methylpyridyl corrole (TMPC) and its germanium(IV) derivative (GeTMPC), with single- and double-stranded nucleic acid homopolymers and calf thymus DNA. The specificity of each spectroscopic technique has been exploited to analyze the different aspects of corrole binding. Noteworthy, this approach allows us to distinguish between H aggregation of TMPC in the presence of polyriboadenilic acid (poly(rA)) and J aggregates in the presence of polyribocytidinic acid (poly(rC)) as well as to identify the formation of GeTMPC dimers in the presence of single-stranded poly(rA) and pseudointercalation with single-stranded poly(rC).