Journal of Porphyrins and Phthalocyanines

Catalases employ a tyrosinate-ligated ferric heme in order to catalyze the dismutation of hydrogen peroxide to O₂ and water. In the first half of the catalytic cycle, H₂O₂ oxidizes Fe(III) to the formally Fe(V) state commonly referred to as Compound I. The second half of the cycle entails oxidation of a second hydrogen peroxide molecule by Compound I to dioxygen. The present study employs density functional (DFT) calculations to examine the nature of this second step of the catalatic reaction. In order to account for the unusual choice of tyrosinate as an axial ligand in catalases, oxidation of hydrogen peroxide by an imidazole-ligated Compound I is also examined, bearing in mind that imidazole-ligated hemoproteins such as myoglobin or horseradish peroxidase tend to display little, if any, catalatic activity. Furthermore, in order to gauge the importance of the cation radical of Compound I in peroxide activation, the performance of Compound II (the one-electron reduced version of Compound I, formally Fe(IV)), is also examined. It is found that hydrogen peroxide oxidation occurs in a quasi-concerted manner, with two hydrogen-atom transfer reactions, and that the tyrosinate ligand is in no way required at this stage. We propose that the role of the tyrosinate is purely thermodynamic, in avoiding accumulation of the much less peroxide-reactive ferrous form in vivo – all in line with the predominantly thermodynamic role of the cysteinate ligands in enzymes such as cytochromes P450.

meso-Tris(3,5-di-tert-butylphenyl)porphyrin (P-H) is bonded with L-phenylalanine (H-Phe-OH) directly at the unsubstituted meso-position of the former and the p-position of phenyl group of the latter to afford chiral porphyrin-amino acid conjugate H-Phe(p-P)-OH. The N-(9-fluorenyl)-methyloxycarbonyl compound, Fmoc-Phe(p-P)-OH, was synthesized without any loss of enantiomeric purity (based on chiral HPLC analysis) from commercially available L-tyrosine and was useful for preparation of the peptides in both liquid and solid phases. Other meso-tetraarylporphyrins possessing multi-amino acid moieties are reported, as well as achiral porphyrin-amino acids readily prepared and their dipeptidyl porphyrin dyads.

Judiciously designed phthalocyanines (Pcs), such as silicon-Pc bis(3,5-diphenyl)benzoate (1c), with axial substituents which prevent aggregation, can self-assemble to form ordered nanoporous films on electrode surfaces. In this paper, complementary techniques such as Scanning Kelvin Nanoprobe (SKN) microscopy, Atom Force Microscopy (AFM) and electrochemical measurements are used to demonstrate that films formed by silicon-Pc bis(3,5-diphenyl)benzoate allow size- and charge- selective transport of probe molecules through well-defined intermolecular cavities. In contrast, the analogs silicon-Pc bis(4-tert-butylbenzoate) (1a) and silicon-Pc bis(3-thienyl)acetate (1b) have different film morphologies when solvent-cast in the same manner and block the electrode surface. The role of the different axial substituents in orienting the molecules on the substrate is discussed.

We have developed three basic phthalocyanine structures, containing one, two, or four hydroxy groups, which are simple to synthesize and purify, as well as can be characterized well by NMR and MS. These building blocks can easily be further modified to have anchor groups, which make the molecules suitable for attachment to solid substrates. We have used thioacetate and pentafluorophenyl ester moieties, giving target phthalocyanines the ability to self-assemble on gold, metal oxides, and glass. Bonding densities calculated from the absorbances of the layers suggest mean molecular area to be in the range of 1–3 nm², which can be partially controlled by side substituents and the number of linkers.

We report the condensation of 3-chloromethyl-benzoyl chloride with two atropisomers ααββ and αβαβ of meso-5,10,15,20-tetrakis-(2-amino)phenylporphyrin (TAPP), followed by the reaction of the anion of either cyano-acetic acid ethyl ester or (4-nitro-phenyl)-acetic acid ethyl ester to prepare various pre-organized strapped porphyrins. These two reagents were selected as both allow the easy formation of the anion in the α position of the ester group while their electron-withdrawing group (EWG) can be further transformed in a reactive functional group. In the ααββ series, this reaction leads to three isomeric porphyrins differing only by the location of their ethoxycarbonyl groups, oriented either towards the center of the porphyrin or maintained outside of the cavity. In the αβαβ series, as expected, a single porphyrin is obtained in which both straps bear an ethoxycarbonyl group, precursor of a hanging carboxylic function and a cyano or a 4-nitro-phenyl group, which can be reduced to an amine function, suitable for the coupling on a biomolecule.

A whole series of low-symmetry, metal-free tetrapyrrole analogs 1–11 ranging from tetraazaporphyrin H₂TAP(tBu)₄ to napthalocyanine H₂Nc(tBu)₄via H₂Pc(tBu)₄ have been designed and prepared. Their electronic structures have been spectroscopically and electrochemically investigated. Through the preparation of a series of tetrapyrrole compounds with just the same number of four tert-butyl substituents at similar positions of the macrocyclic ring, the effect intrinsic to the enlargement of the ring system has been easily extracted and understood. Along with the increase in the size of central conjugated system, the half-wave potentials of the first oxidation for 1–11 are shifted significantly to the negative direction. This is also true for the half-wave potentials of the first reduction process of this series of tetrapyrrole derivatives but in a slight manner. These results indicate that along with the ring extension, the energy of the HOMO increases significantly while that of the LUMO only slightly increases. The effect of the ring size of these tetrapyrrole compounds on the electronic absorption and fluorescence spectra is also clear: both the Q band and fluorescence shifts to longer wavelength with the increase in the number of fused benzene rings but with a diminishing increase due to the extending of the central conjugated system. These results were reasonably explained by considering the energy levels of frontier molecular orbitals of the series of compounds obtained by the calculations using DFT method at the B3LYP/6-31G(d) level.

Porphyrins with different numbers of β-propionate chains mimicking natural porphyrins were prepared via the 2+2 MacDonald type approach. Photodynamic activity against WiDr colon adenocarcinoma cells showed that activity is related to the number of β-propionate chains, with the derivatives with two carboxylic groups showing higher activity.

Track-etched polymer membranes are typically used as templates in the synthesis of various nanowires or nanotubes arrays. The unique advantages of track-etched membranes, such as uniform pore structure, excellent porosity, easily tailored pore sizes, and a well characterized surface chemistry, may find use in self-assembly strategies where colloidal nanostructures can be tethered to a suitable substrate to produce devices of interest. Meso-tetrakis(4-phenylsulfonicacid)porphyrin dihydrochloride and Sn(IV) tetrakis(4-pyridyl)porphyrin were used to synthesize ionic self-assembled porphyrin nanorods. The track-etched membranes surface charge was changed from negative to positive using polyethyleneimine. The porphyrin nanorods were either filtered through or self-assembled onto the surface of track-etched membranes. Comparisons were made with track-etched membranes modified with, and without, polyethyleneimine. Assembly of the porphyrin nanotubes only occurred on the surface of positively charged track-etched membranes, and filtration of the porphyrin nanorods produced a mesh-like structure on the surface of the membrane irrespective of the track-etched membrane pore diameter. In each case the characteristic absorbance profiles of the porphyrin nanorods was maintained. Transmission electron microscopy, scanning electron microscopy, atomic force microscopy, and UV-vis spectroscopy were used to characterize the various systems.

Porphyrin-functionalized cup-shaped nanocarbons (CNC-H₂P) have been assembled onto nanostructured SnO₂ films using an electrophoretic deposition method to examine the photoelectrochemical properties. The obtained CNC-H₂P nanohybrid films were examined by a series of steady-state and time-resolved spectroscopic measurements and photoelectrochemical measurements. The resulting nanohybrid film afforded drastic enhancement in the photoelectrochemical performance as well as broader photoresponse in the visible region as compared with the reference CNC system without porphyrins. The enhancement of photocurrent generation may be caused by the efficient electron injection from the long-lived charge-separated state of CNC-H₂P upon photoexcitation. This feature makes cup-shaped nanocarbon materials a useful candidate for developing efficient photoelectrochemical and photovoltaic cells.

Regioselective synthesis of 2,3-dibromo-12-formyl-5,10,15,20-tetraphenylporphyrin, H₂TPPBr₂(CHO) was achieved by bromination of H₂TPP(CHO) as the precursor. The Suzuki/Stille coupling reaction of H₂TPPBr₂(CHO) produced good yields of various H₂TPPR₂(CHO) (R = phenyl, 2-thienyl and phenylethynyl) derivatives. These porphyrins and their metal complexes were characterized by electronic absorption, ¹H NMR and mass spectroscopic methods. MTPPR₂(CHO) derivatives showed marked change in their absorption spectral features in contrast to their corresponding MTPP(CHO) complexes. The crystal structures of ZnTPP(CHO)(CH₃OH)·0.5(C₇H₈) and H₂TPP(Ph)₂(CHO) exhibited more of saddle distortion combined with varying degree of ruffled and wave distortions of their macrocyclic rings as evidenced from the normal coordinate structure decomposition analysis.