Unravelling the mechanisms of reactive oxygen species formation in nanohybrid systems of porphyrins and enriched (6,5) single-walled carbon nanotubes for photosensitization

Two different porphyrins inside the exciton volume of a carbon nanotube with charged N either in or out.

Unravelling the molecular structure and packing of a planar molecule by combining nuclear magnetic resonance and scanning tunneling microscopy

The determination of the molecular structure of a porphyrin is achieved by using nuclear magnetic resonance (NMR) and scanning tunneling microscopy (STM) techniques. Since macroscopic crystals cannot be obtained in this system, this combination of techniques is crucial to solve the molecular structure without the need for X-ray crystallography. For this purpose, previous knowledge of the flatness of the reagent molecules (a porphyrin and its functionalizing group, a naphthalimide) and the resulting molecular structure obtained by a force-field simulation are used. The exponents of the I-V curves obtained by scanning tunneling spectroscopy (STS) allow us to check whether the thickness of the film of molecules is greater than a monolayer, even when there is no direct access to the exposed surface of the metal substrate. Photoluminescence (PL), optical absorption, infrared (IR) reflectance and solubility tests are used to confirm the results obtained here with this NMR/STM/STS combination.

Nanometrological porphyrins

A new cationic silver N-alkylpyridylporphyrin complex is able to 'sense' nanometric conductive particles with a diameter below 10 nm. The luminescence of the molecule changes its maximum from red to blue when it embraces a conductive (metallic or semiconducting) nanoparticle. The change is explained on the basis of a charge transfer between the molecule and the conductive nanoparticle along with a geometrical distortion of the porphyric ring and pyridinium substituents. This new molecule could be used to sense nanoparticle contamination in the environment, in the industry of heterogeneous catalysis and many other branches of nanometrological applications.

Thermal stability of the prototypical Mn porphyrin-based superoxide dismutase mimic and potent oxidative-stress redox modulator Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin chloride, MnTE-2-PyP(5+)

Cationic Mn porphyrins are among the most potent catalytic antioxidants and/or cellular redox modulators. Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin chloride (MnTE-2-PyPCl(5)) is the Mn porphyrin most studied in vivo and has successfully rescued animal models of a variety of oxidative stress-related diseases. The stability of an authentic MnTE-2-PyPCl(5) sample was investigated hereon by thermogravimetric, derivative thermogravimetric, and differential thermal analyses (TG/DTG/DTA), under dynamic air, followed by studies at selected temperatures to evaluate the decomposition path and appropriate conditions for storage and handling of these materials. All residues were analyzed by thin-layer chromatography (TLC) and UV-vis spectroscopy. Three thermal processes were observed by TG/DTG. The first event (endothermic) corresponded to dehydration, and did not alter the MnTE-2-PyPCl(5) moiety. The second event (endothermic) corresponded to the loss of EtCl (dealkylation), which was characterized by gas chromatography-mass spectrometry. The residue at 279°C had UV-vis and TLC data consistent with those of the authentic, completely dealkylated analog, MnT-2-PyPCl. The final, multi-step event corresponded to the loss of the remaining organic matter to yield Mn(3)O(4) which was characterized by IR spectroscopy. Isothermal treatment at 188°C under static air for 3h yielded a mixture of partially dealkylated MnPs and traces of the free-base, dealkylated ligand, H(2)T-2-PyP, which reveals that dealkylation is accompanied by thermal demetallation under static air conditions. Dealkylation was not observed if the sample was heated as a solid or in aqueous solution up to ∼100°C. Whereas moderate heating changes sample composition by loss of H(2)O, the dehydrated sample is indistinguishable from the original sample upon dissolution in water, which indicates that catalytic activity (on Mn basis) remains unaltered. Evidently, dealkylation at high temperature compromises sample activity.

Measuring the electronic properties of single-walled carbon nanotubes with adsorbed porphyrins using optical transitions

The dielectric constants of diverse media surrounding single-walled carbon nanotubes (SWCNTs) were probed using photoluminescence (PL) excitation maps of porphyrin/SWCNT aqueous suspensions. The excitation and emission maxima of the nanotubes in these maps were used to probe the dielectric constant variation and doping originated from the porphyrin molecules. The net dielectric constant was calculated for the surrounding medium for each nanotube index and porphyrin isomer. The spread of the dielectric constant values calculated from the data for each (n, m) nanotube chiral index is interpreted on the basis of selective adsorption by each (n, m) nanotube, for each porphyrin isomer. Ultraviolet (UV) Raman spectroscopy corroborates the doping process through the shift of a G band around 1608 cm-1

Perhalogenated 2-pyridylporphyrin complexes: synthesis, self-coordinating aggregation properties, and catalytic studies

The synthesis of 2, 3, 7, 8, 12, 13, 17, 18-octabromo-meso-tetrakis(2-pyridyl)porphyrin, H 2 Br 8 T 2 PyP , is described, including the comparison of four attempted methods for the demetallation of ZnBr 8 T 2 PyP . One of the methods represents a strategy of demetallation based on the acid-base properties of the macrocycle, the solvent-dependent kinetics of metal insertion into porphyrins and the pH -dependent solubility of the 2-pyridylporphyrin derivatives in water. Self-coordinating aggregation of ZnBr 8 T 2 PyP in non-coordinating solvents was verified by 1 H NMR spectroscopy. The Mn(III)/Mn(II) redox potential for MnBr 8 T 2 PyPCl is 0.38 V higher than the reduction potential measured for its first-generation analogue, MnT 2 PyPCl . Cyclohexane hydroxylation by iodosylbenzene was performed in CH3 CN catalyzed by MnBr 8 T 2 PyPCl and MnT 2 PyPCl . MnBr 8 T 2 PyPCl was highly active, even at low concentration (5 × 10−5 M ), but perhalogenation did not account for oxidative robustness. At such a low catalyst concentration, MnT 2 PyPCl exhibited no activity as inferred by comparison to blank experiments.

SOD-like activity of Mn(II) beta-octabromo-meso-tetrakis(N-methylpyridinium-3-yl)porphyrin equals that of the enzyme itself

Mn porphyrins are among the most efficient SOD mimics with potency approaching that of SOD enzymes. The most potent ones, Mn(III) N-alkylpyridylporphyrins bear positive charges in a close proximity to the metal site, affording thermodynamic and kinetic facilitation for the reaction with negatively charged superoxide. The addition of electron-withdrawing bromines onto beta-pyrrolic positions dramatically improves thermodynamic facilitation for the O2*- dismutation. We have previously characterized the para isomer, Mn(II)Br(8)TM-4-PyP(4+) [Mn(II) beta-octabromo-meso-tetrakis(N-methylpyridinium-4-yl)porphyrin]. Herein we fully characterized its meta analogue, Mn(II)Br(8)TM-3-PyP(4+) with respect to UV/vis spectroscopy, electron spray mass spectrometry, electrochemistry, O2*- dismutation, metal-ligand stability, and the ability to protect SOD-deficient Escherichia coli in comparison with its para analogue. The increased electron-deficiency of the metal center stabilizes Mn in its +2 oxidation state. The metal-centered Mn(III)/Mn(II) reduction potential, E((1/2))=+468 mV vs NHE, is increased by 416 mV with respect to non-brominated analogue, Mn(III)TM-3-PyP(5+) and is only 12 mV less positive than for para isomer. Yet, the complex is significantly more stable towards the loss of metal than its para analogue. As expected, based on the structure-activity relationships, an increase in E((1/2)) results in a higher catalytic rate constant for the O2*- dismutation, log k(cat)> or =8.85; 1.5-fold increase with respect to the para isomer. The IC(50) was calculated to be < or =3.7 nM. Manipulation of the electron-deficiency of a cationic porphyrin resulted, therefore, in the highest k(cat) ever reported for a metalloporphyrin, being essentially identical to the k(cat) of superoxide dismutases (log k(cat)=8.84-9.30). The positive kinetic salt effect points to the unexpected, unique and first time recorded behavior of Mn beta-octabrominated porphyrins when compared to other Mn porphyrins studied thus far. When species of opposing charges react, the increase in ionic strength invariably results in the decreased rate constant; with brominated porphyrins the opposite was found to be true. The effect is 3.5-fold greater with meta than with para isomer, which is discussed with respect to the closer proximity of the quaternary nitrogens of the meta isomer to the metal center than that of the para isomer. The potency of Mn(II)Br(8)TM-3-PyP(4+) was corroborated by in vivo studies, where 500 nM allows SOD-deficient E. coli to grow >60% of the growth of wild type; at concentrations > or =5 microM it exhibits toxicity. Our work shows that exceptionally high k(cat) for the O2*- disproportionation can be achieved not only with an N(5)-type coordination motif, as rationalized previously for aza crown ether (cyclic polyamines) complexes, but also with a N(4)-type motif as in the Mn porphyrin case; both motifs sharing "up-down-up-down" steric arrangement.

Impact of electrostatics in redox modulation of oxidative stress by Mn porphyrins: protection of SOD-deficient Escherichia coli via alternative mechanism where Mn porphyrin acts as a Mn carrier

Understanding the factors that determine the ability of Mn porphyrins to scavenge reactive species is essential for tuning their in vivo efficacy. We present herein the revised structure-activity relationships accounting for the critical importance of electrostatics in the Mn porphyrin-based redox modulation systems and show that the design of effective SOD mimics (per se) based on anionic porphyrins is greatly hindered by inappropriate electrostatics. A new strategy for the beta-octabromination of the prototypical anionic Mn porphyrins Mn(III) meso-tetrakis(p-carboxylatophenyl)porphyrin ([Mn(III)TCPP](3-) or MnTBAP(3-)) and Mn(III) meso-tetrakis(p-sulfonatophenyl)porphyrin ([Mn(III)TSPP](3-)), to yield the corresponding anionic analogues [Mn(III)Br(8)TCPP](3-) and [Mn(III)Br(8)TSPP](3-), respectively, is described along with characterization data, stability studies, and their ability to substitute for SOD in SOD-deficient Escherichia coli. Despite the Mn(III)/Mn(II) reduction potential of [Mn(III)Br(8)TCPP](3-) and [Mn(III)Br(8)TSPP](3-) being close to the SOD-enzyme optimum and nearly identical to that of the cationic Mn(III) meso-tetrakis(N-methylpyridinium-2-yl)porphyrin (Mn(III)TM-2-PyP(5+)), the SOD activity of both anionic brominated porphyrins ([Mn(III)Br(8)TCPP](3-), E(1/2)=+213 mV vs NHE, log k(cat)=5.07; [Mn(III)Br(8)TSPP](3-), E(1/2)=+209 mV, log k(cat)=5.56) is considerably lower than that of Mn(III)TM-2-PyP(5+) (E(1/2)=+220 mV, log k(cat)=7.79). This illustrates the impact of electrostatic guidance of O(2)(-) toward the metal center of the mimic. With low k(cat), the [Mn(III)TCPP](3-), [Mn(III)TSPP](3-), and [Mn(III)Br(8)TCPP](3-) did not rescue SOD-deficient E. coli. The striking ability of [Mn(III)Br(8)TSPP](3-) to substitute for the SOD enzymes in the E. coli model does not correlate with its log k(cat). In fact, the protectiveness of [Mn(III)Br(8)TSPP](3-) is comparable to or better than that of the potent SOD mimic Mn(III)TM-2-PyP(5+), even though the dismutation rate constant of the anionic complex is 170-fold smaller. Analyses of the medium and E. coli cell extract revealed that the major species in the [Mn(III)Br(8)TSPP](3-) system is not the Mn complex, but the free-base porphyrin [H(2)Br(8)TSPP](4-) instead. Control experiments with extracellular MnCl(2) showed the lack of E. coli protection, indicating that "free" Mn(2+) cannot enter the cell to a significant extent. We proposed herein the alternative mechanism where a labile Mn porphyrin [Mn(III)Br(8)TSPP](3-) is not an SOD mimic per se but carries Mn into the E. coli cell.

Partially and fully β-brominated Mn-porphyrins in P450 biomimetic systems: Effects of the degree of bromination on electrochemical and catalytic properties

Beta-hexabromo-5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrinatomanganese(III) chloride (Mn(III)(Br6TCMPP)Cl) was prepared by selective Br2-hexabromation of its parent non-brominated manganese complex (Mn(III)(TCMPP)Cl), whereas the octabrominated analogue beta-octabromo-5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrinatomanganese(III) chloride (Mn(III)(Br8TCMPP)Cl) was synthesized via metallation of the corresponding free-base. Beta-octabromo-5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrin was obtained by demetallation of its brominated Cu(II) derivative, which, in its turn, was prepared by either a Br2 or an N-bromosuccinimide protocol. Relative to Mn(III)(TCMPP)Cl (E(1/2) = -0.16 V vs. normal hydrogen electrode, CH2Cl2), the Mn(III)/Mn(II) reduction potential of Mn(III)(Br8TCMPP)Cl and Mn(III)(Br6TCMPP)Cl showed anodic shifts of 0.43 and 0.33 V, respectively, which corresponded to a linear shift of 0.05 V per bromine added. These manganese complexes were evaluated as cytochrome P450 mimics in catalytic iodosylbenzene (PhIO)-oxidations of cyclohexane and cyclohexene. In aerobic PhIO-oxidation of cyclohexene, epoxidation and allylic autoxidation reactions were inversely related, competitive processes; the most efficient P450-mimics were the least effective autoxidation catalysts. Mn(III)(Br6TCMPP)Cl was more efficient as epoxidation or hydroxylation catalyst than both its fully and non-beta-brominated counterparts were. There was no linear relationship between the catalytic efficiency and both the number of bromine substituents and the Mn(III)/Mn(II) potential; these observations were compared to Lyons system literature data and discussed. Analogously to enzymatic optimum pH effects, an optimum redox potential effect is suggested as relevant in designing and understanding cytochrome P450 biomimetic catalysts.

Iodo(phthalocyaninato)chromium(III)

A new chromium(III)-phthalocyanine complex with the formula [Cr(C(32)H(16)N(8))I], or CrPcI where Pc is phthalocyanate(2-), has been obtained by the reaction of pure Cr powder with phthalonitrile under a stream of iodine vapour. The five-coordinate Cr atom is bonded to the four isoindole N atoms of the phthalocyaninate(2-) ligand and to one apical iodine ligand, and has a square-pyramidal coordination geometry. The Cr(III) cation is significantly displaced [0.456 (2) A] from the N(4)-isoindole plane towards the I atom. The Cr-I bond is tilted 2.51 (4) degrees to the N(4)-isoindole plane.