Porphyrin-like Structure Research Articles

Facilely synthesized nitrogen-doped reduced graphene oxide functionalized and/or co-doped with metal ions as electrocatalyst for oxygen reduction reaction

<h2>Abstract</h2> Due to fossil fuels depletion and environmental pollution, clean and sustainable energy technologies, e.g. fuel cells and metal-air batteries, have attracted extensive attention. To push further the research on these electrochemical devices, low-cost, durable and efficient electrocatalysts alternative to platinum are required, to boost the oxygen reduction reaction (ORR). A microwave-assisted method has been optimized, to obtain effective heterogeneous catalyst for ORR, starting from graphene oxide (GO), urea and a transition metal (e.g. Mn and Cu) precursor. We have proved that our synthetic method originates porphyrin-like structures containing pyrrole rings within the reduced GO (rGO) basal plane which coordinate the Mn²⁺. In the case of copper, however, Cu²⁺ forms an ionic tetra coordinated structure anchored at the rGO surface via residual oxygen containing functional groups. In both cases, metal complex acts as an ORR highly efficient catalytic reaction center and their identification were strongly supported by several characterization techniques, such as X-ray Photoelectron Spectroscopy (XPS), X-ray absorption spectroscopies (XAS) and Transmission Electron Microscopy (TEM), together with Density Functional Theory (DFT) simulations. All synthesized materials exhibit outstanding catalytic properties toward ORR, as evidenced by electron transfer numbers larger than 3.8 and peroxide percentages lower than 7%, similar to Pt/C reference electrode.

Accumulation mechanisms of radiocaesium within lichen thallus tissues determined by means of in situ microscale localisation observation.

Many lichens are well known to accumulate radiocaesium and, thus acting as biomonitors of contamination levels. However, the actual localisation and chemical forms of radiocaesium in contaminated lichens have not yet been elucidated because, despite their high radioactivity, these forms are present in trace amounts as chemical entities. Here, we use autoradiography and demonstrate for the first time in situ microscale localisation of radiocaesium within thallus tissues to investigate the radiocaesium forms and their accumulation mechanism. Radiocaesium distributions showed similar trends in lichen tissues collected two and six years after the Fukushima nuclear accident. The radiocaesium was localised in the brown pigmented parts i.e., melanin-like substances, in the lower cortex of lichen thallus. Quantum chemical calculations showed that functional group of melanin-like substances can chelate Cs+ ion, which indicates that the Cs+ ions form complexes with the substances. Based on these findings, we suggest that radiocaesium ions may be retained stably in melanin-like substances for long periods (two to six years) due to steric factors, such as those seen in porphyrin-like structures and via multimer formation in the lower cortex. In addition, electron microscopy and autoradiography were used to observe radiocaesium-bearing microparticles (CsMPs) on/in the upper cortex and around the medullary layer. Micron-sized particles appeared to adhere to the surface tissue of the thallus, as shown by electron microscopy, suggesting that the particles were trapped by development of an adhesive layer; that is, CsMPs were trapped both physically and physiologically. These findings provide information on in situ localisation of two chemical forms of radiocaesium, cations and particles, in lichen thallus tissues and their accumulation mechanisms.

Searching for correlations between geometric and spectroscopic parameters of intramolecular hydrogen bonds in porphyrin-like macrocycles.

Chemical bond lengths and angles are characteristic structural parameters of a molecule. Similarly, the frequencies of the vibrational modes and the NMR chemical shifts are unique "chemical fingerprints" specific to a compound. These are the basic parameters describing newly obtained compounds and enabling their identification. Intramolecular hydrogen bonding significantly influences the physicochemical properties of macrocyclic compounds with a porphyrin-like structure. This work presents the verification for correlations between geometric and spectroscopic parameters related to hydrogen bonds in this type of macrocyclic compounds. In particular, such relationships were investigated for a large group of porphyrin, porphycene, and dibenzotetraaza[14]annulene derivatives and a group of other macrocycles with similar structure. A very strong linear correlation was found only between the vibrational frequencies of the NH groups involved in a hydrogen bond and the length of this bond, which applied to all macrocyclic compounds of this type. Several other relationships were found between spectroscopic (IR, Raman, NMR) and geometric (X-ray) parameters, highlighting differences and similarities between different families of macrocycles. Apart from providing a better understanding of the nature of hydrogen bonds and their characteristics in porphyrin-like macrocyclic compounds, these relationships will facilitate the identification of new macrocycles and the extrapolation of their spectroscopic properties.

Covalently Linked Porphyrins as One-Dimensional Conductors.

We apply first-principles calculations to address the problem of the formation and characterization of covalently linked porphyrin-like structures. We show that upon pressure a rehybridization process takes place which leads to one-dimensional compounds resembling nanothreads, in which carbon atoms are all 4-fold coordinated. We also show that the resulting nanostructures have metallic character and possess remarkable mechanical properties. Moreover, in the case of porphyrin-metal complexes, we find that the covalently linked structures may be a platform for the stabilization of straight metallic wires.

Recent Progress in (Photo-)-Electrochemical Conversion of CO2 With Metal Porphyrinoid-Systems.

Since decades, the global community has been facing an environmental crisis, resulting in the need to switch from outdated to new, more efficient energy sources and a more effective way of tackling the rising carbon dioxide emissions. The activation of small molecules such as O2, H+, and CO2 in a cost—and energy-efficient way has become one of the key topics of catalysis research. The main issue concerning the activation of these molecules is the kinetic barrier that has to be overcome in order for the catalyzed reaction to take place. Nature has already provided many pathways in which small molecules are being activated and changed into compounds with higher energy levels. One of the most famous examples would be photosynthesis in which CO2 is transformed into glucose and O2 through sunlight, thus turning solar energy into chemical energy. For these transformations nature mostly uses enzymes that function as catalysts among which porphyrin and porphyrin-like structures can be found. Therefore, the research focus lies on the design of novel porphyrinoid systems (e.g. corroles, porphyrins and phthalocyanines) whose metal complexes can be used for the direct electrocatalytic reduction of CO2 to valuable chemicals like carbon monoxide, formate, methanol, ethanol, methane, ethylene, or acetate. For example the cobalt(III)triphenylphosphine corrole complex has been used as a catalyst for the electroreduction of CO2 to ethanol and methanol. The overall goal and emphasis of this research area is to develop a method for industrial use, raising the question of whether and how to incorporate the catalyst onto supportive materials. Graphene oxide, multi-walled carbon nanotubes, carbon black, and activated carbon, to name a few examples, have become researched options. These materials also have a beneficial effect on the catalysis through for instance preventing rival reactions such as the Hydrogen Evolution Reaction (HER) during CO2 reduction. It is very apparent that the topic of small molecule activation offers many solutions for our current energy as well as environmental crises and is becoming a thoroughly investigated research objective. This review article aims to give an overview over recently gained knowledge and should provide a glimpse into upcoming challenges relating to this subject matter.

How does the porphyrin-like vacancy affect the spectral properties of graphene quantum dots? A theoretical study

By processing graphene quantum dot, an ideal semiconductor material with suitable band gap and higher electron mobility can be obtained. Thus, it has a broad prospect in the application of photoelectric response materials. Here, a graphene defect with porphyrin-like structure is selected to achieve the controlable light absorption. The double five-membered-ring parallel vacancy are based on self-healing properties of popular graphene defects. Aimed to separate exciton and hole more effectively and achieve higher photoelectric conversion efficiency, the occupied orbital and unoccupied orbital of the quantum dot with objected defect structure is taken as orderly dispersion to form an obvious charge separation state under the demonstration with first principles calculation. Most importantly, a real time real space charge separation is calculated by time-dependent ab-initio quantum dynamics based on numerical atomic basis sets. The result shows the specific graphene defects can form an efficient pure graphene photoelectric response medium like porphyrin skeleton, and the vacancy will induce to adjust and control the specific wavelength of the response light and charge separated state manipulably with odevity of number of peripheral carbon rings by the calculation of ultrafast process.

Proving the existence of Mn porphyrin-like complexes hosted in reduced graphene oxide with outstanding performance as oxygen reduction reaction catalysts

The lack of low-cost, durable and highly efficient electrocatalysts to perform oxygen reduction reaction (ORR) still represents a significant challenge for the development of fuel cells cathodes. Recently, Pt-free metalloporphyrins supported on N-doped reduced graphene oxide (N-rGO) have been proposed as innovative ORR catalysts characterized by notable stability and superior performance. In this paper, we propose a fast and green microwave-assisted method to synthetize manganese-nitrogen co-doped rGO, characterized by catalytic activity comparable to Pt-containing materials and improved long-term stability. Theoretical calculations and experimental results showed that our synthetic method produces N-rGO matrices which coordinate Mn2+ ions in porphyrin-like structures. These Mn/N-rGO complexes are responsible for the outstanding performance of the proposed catalyst, being able to weaken O–O molecular bond, thus effectively reducing the activation energy towards oxygen catalytic reaction. Our findings open the path to a green co-doping process of graphene for the production of low-cost and efficient alternatives to noble metals-based ORR catalysts.

Nanoparticles exhibit enzyme-like activity

Advancing the trend to use metal-organic frameworks (MOFs) in medical applications, researchers in China report that an inexpensive MOF-derived nanomaterial exhibits antibacterial properties and promotes wound healing in mice (Angew. Chem., Int. Ed. 2019, DOI: 10.1002/anie.201813994). Huiyu Liu of Beijing University of Chemical Technology and colleagues previously developed a procedure for making nanospheres from ZIF-8, a zinc-based MOF with a porphyrin-like structure, and reported that they exhibit antitumor properties when triggered by ultrasound. Now, her team has explored the material’s potential in biocatalysis. The motivation stems from metal-N-C bonding motifs that the nanomaterial shares with metalloenzymes. First, the researchers tested the nanospheres’ ability to mediate oxidation of organic compounds in the presence of hydrogen peroxide. They found that the performance was similar to that of natural horseradish peroxidase and was due to catalytically active Zn-N4 moieties. Then, through a serie...

Kinetics of the Thermal Decomposition of 2,4,6-Triazido-3,5-Difluoropyridine

The kinetics and products of the thermal decomposition of 2,4,6-triazido-3,5-difluoropyridine in melt at temperatures of 120–160°C have been studied using pressure measurements, differential thermal analysis, and IR spectroscopy. The reaction occurs in two macroscopic steps, each described by a first-order kinetic equation. In the first, the activation energy and the common logarithm of the pre-exponential factor are 35.6 ± 1.2 kcal/mol and 15.1 ± 0.6 s–1, respectively. For the studied compound (and certain other heterocyclic azides, such as 2,4,6-triazido-1,3,5-triazine and 2,4,6-triazidopyrimidine), the pre-exponential factor is anomalously high. This is due to the fact that the 2,4,6-triazido-3,5-difluoropyridine molecule contains no hydrogen atoms. For such azides, the usual decomposition mechanism (cleavage of the azide group with subsequent attack of nitrene on a hydrogen atom of a neighboring molecule) is impossible, which favors the stability of the formed nitrene. In this case, the reaction occurs through a complex chain polymerization mechanism, which leads to the formation of specific condensed products: packs of planar polyconjugate carbon–nitrogen networks with a porphyrin-like structure.

Highly luminescent N-doped carbon quantum dots from lemon juice with porphyrin-like structures surrounded by graphitic network for sensing applications

Green synthetic approach to synthesizing highly luminescent N-doped carbon quantum dots with porphyrin like moiety for sensitive and selective detection of Fe3+.

Expanded Porphyrin-like Structures Based on Twinned Triphenylenes

Triphenylene twins are intriguing structures, and those bridged through their 3,6-positions by dipyrromethene units give a new class of macrocycles that can be viewed as rigid, expanded porphyrin derivatives in which coplanarity is enforced in a formally antiaromatic π system. Somewhat surprisingly, however, macrocyclization leads to significant overall stabilization of the dipyrromethene chromophores.

Photochemical Effect of Humic Acid Components Separated Using Molecular Imprinting Method Applying Porphyrin-like Substances as Templates in Aqueous Solution

To elucidate the relationship between photochemical functions with the structure of humic acids (HA), we developed a molecular imprinting method to separate the substances with given structure and investigated their photochemical behavior in aqueous solution. The substances with porphyrin-like core structure, such as chlorophyll or heme, were employed as template substances for preparing molecular imprinting polymers (MIP). The polymers were used to separate the substances with porphyrin-like structure from HA. Photochemical experiments were conducted to evaluate effects of the separated HA fractions on the photodegradation of coexisting organic pollutant. The results showed that all fractions bound by MIP accelerated photochemical degradation of coexisting 2,4-dichlorophenoxyacetic acid (2,4-D) under simulated sunlight (lambda>290 nm) irradiation, indicating that HA with porphyrin-like structure possesses better photoactivity than ones without the structure. The photochemical degradation of 2,4-D was enhanced when Fe(III), the ubiquitous element in natural aquatic systems, was added owing to the formation of Fe(III) complex with the HA. Electron paramagnetic resonance (EPR) spectra indicated that OH* and 1O2 radicals were generated in the solutions of HA fractions bound by MIP under simulated sunlight irradiation, implying that 2,4-D degradation could be related to oxidation reactions caused by reactive oxygen species (ROS).

Sequential Leaching of Coal to Investigate the Elution of Inorganic Elements Into Coal Extract (HyperCoal)

Two Argonne premium coal samples, Illinois No. 6 (IL) and Wyodak−Anderson (WY), were extracted by 1-methynaphthalene for 1 h at 360 °C and under 1 MPa nitrogen (cold) protection. Elution of inorganic elements into coal extracts as well as their chemistry has been mainly investigated. An indirect method for metallic speciation was employed by initially washing coal with a variety of acids. Subsequently, the washed coals as well as the respective raw coal were extracted. For a given metal, elution of its ion-exchangeable fraction was defined as the difference between its amounts eluted into the extracts of raw coal and acetic acid-washed coal. Elution of submicrometer discrete particles was defined as the difference between the extracts of acetic acid-washed coal and nitric acid-washed coal. Elution of its fraction insoluble in nitric acid was assigned as organometals which are chemically associated with coal carbonaceous matrix and/or those incorporated into fine clay minerals. About 822 and 1110 ppm inorganic elements were eluted into the extracts of IL and WY coals, respectively. Fe was the most prevalent. The transition metals including Cr, Ni, Mn, Co, Cu, and Zn were also abundant. These metals were mostly nitric-acid insoluble. Electron spin resonance spectroscopy characterization suggested the high-spin Fe 3+ state for Fe that is virtually totally associated with coal functional groups. A portion of it may have a porphyrin-like structure that is soluble in sulfuric acid (7% in methanol, v/v). While the left Fe can be high-spin Fe 3+ stored in the organometallic complex having a distorted axial symmetry. The other transition metals mainly partitioned between organometallic complex and cations incorporated into fine clay minerals. Regarding the remaining metals in coal extracts, they are mainly submicrometer discrete particles in IL extract. Elution of the ion-exchangeable carboxylates was however prominent during WY coal extraction.

Necrosis Avid Contrast Agents

Two categories of necrosis-avid contrast agents (NACAs), namely porphyrin- and nonporphyrin-based complexes, have thus far been discovered as necrosis-targeting markers for noninvasive magnetic resonance imaging (MRI) identification of acute myocardial infarction, assessment of tissue or organ viability, and therapeutic evaluation after interventional therapies. In addition to necrosis labeling, other less-specific functions, such as first-pass perfusion, blood pool contrast effect, hepatobiliary contrast enhancement (CE), adrenal and spleen CE, and renal functional imaging, also are demonstrated with NACAs. Despite various investigations with a collection of clues in favor of certain hypotheses, the mechanisms of such a unique targetability for NACAs still remain to be elucidated. However, a few things have become clear that porphyrin-like structures are not necessary for necrosis avidity and the albumin binding is not the supposed driving force but only a parallel nonspecific feature shared by both NACAs and non-NACA substances. Although the research and development of NACAs still remain in preclinical stage at a relatively small scale, their significance rests upon striking enhancement effects, which may warrant their eventual versatile clinical applications. The present review article is intended to summarize the cumulated facts about the evolving research on this topic, to demonstrate experimental observations for better understanding of the mechanisms, to trigger broader public interests and more intensive research activities, and to advocate, toward both academics and industries, further promotion of preclinical and clinical development of this unique and promising class of contrast agents.

Synthesis and structure of supramolecular polyoxothiometalate nanocluster containing 52 metal atoms [{Mo3S4(H2O)5}4(γ-SiW10O36)4]16–

The polyoxothiometalate complex [{Mo3S4(H2O)5}4(γ-SiW10O36)4]16– was prepared by the reaction of the cationic triangular molybdenum aqua complex [Mo3S4(H2O)9]4+ (electrophile) with the lacunary polyoxotungstate complex [γ-SiW10O36]8– (nucleophile). According to the results of single-crystal X-ray diffraction analysis of the dimethylammonium salt, the anionic nanocluster contains 52 molybdenum and tungsten atoms and has a cyclic porphyrin-like structure.

INHIBITION OF NITRIC OXIDE SYNTHASE ISOFORMS BY IMMUNOSUPPRESSIVE PEPTIDES

527 Peptides derived from the HLA class I heavy chain (aa 75-84) have immunosuppressive properties as documented in several rodent transplantation models. Based on in vivo data and other physicochemical properties of active peptide derivatives, computer modeling and single amino acid substitutions generated a new set of peptides with 100-fold higher immunosuppressive activity. These peptides were shown to modulate heme oxygenase-1 (HO-1) protein and activity levels in a specific manner. Computer modeling further suggested that these peptide may mimic a porphyrin-like structure and may modify, besides HO-1, the activity of other heme enzymes like nitric oxide synthase (NOS). Peptides RDP1258, DRDP1258, D2, D2RP (negative control) and 2702 were analyzed in enzyme activity assays of all three NOS isoforms in vitro - neuronal NOS (nNOS), endothelial NOS (eNOS) and cytokine-inducible NOS (iNOS). Inhibition of enzyme activity was observed in the presence of peptides with the following IC50 values (in µM) obtained. Inhibitory activity also correlated with immunosuppressive activity observed in vivo.Table With affinity-purified iNOS, inhibition by RDP1258 was non-competitive with both arginine (substrate) and tetrahydrobiopterin (co-factor). The inhibition of iNOS by RDP1258 displayed a positive cooperativity as indicated by a Hill coefficient of 1.8. Using affinity-purified nNOS, RDP1258 displayed an even higher degree of cooperativity with a Hill coefficient of 3.7. Both, cytochrome c reductase activity and NADPH oxidase activity of nNOS were inhibited by RDP1258. Addition of RDP1258, DRDP1258 and D2 dramatically increased the concentration of calmodulin necessary to activate nNOS. These observations are compatible with the proposal that inhibition of NOS by these immunosuppressive peptides may be a result of multi-site effects with binding sites present on both NOS and calmodulin. In conclusion, these findings support the hypothesis that the investigated immunosuppressive peptides mimic a porphyrin-like structure that particularly influences HO-1 and NOS and possibly other heme-containing enzymes in its immunomodulatory activity.

Possible Intermediates in Biological Metalloporphyrin Oxidative Degradation. Nickel, Copper, and Cobalt Complexes of Octaethylformybiliverdin and Their Conversion to a Verdoheme

Formylbiliverdin and related chlorophyll-derived molecules are possible products of heme catabolism and other biologically important oxidative processes and are likely to be initially formed as metal complexes. To explore the properties of the formylbiliverdin moiety bound to transition metal ions, complexes of octaethylformylbiliverdin (H2OEFB) with Cu(II), Ni(II), and Co(II) have been prepared, since attempts to prepare an iron complex have produced only an unstable material. Transmetalation of MgII(OEFB), made by photooxidation of MgII(octaethyl-porphyrin), with a metal(II) acetate yields the low-spin complexes: CuII(OEFB), NiII(OEFB), and CoII(OEFB). Single-crystal X-ray diffraction of CuII(OEFB) reveals that it consists of a four-coordinate copper(II) center which is bound to the four nitrogen atoms in distorted planar coordination. The tetrapyrrole ligand has a helical geometry. The structure of the solid is complicated by the existence of three molecules in the asymmetric unit and C−H···O hydrogen bonding between pairs of these in the tab/slot arrangement seen in complexes of octaethylbiliverdin. Both CuII(OEFB) and CoII(OEFB) can be converted to the verdoheme analogues, [CuII(OEOP)]+ and [CoII(OEOP)]+, where OEOP is the anion of octaethyl-5-oxaporphyrin, by the addition of hydrogen peroxide. Additionally, [CuII(OEOP)]+ can be produced by heating a toluene solution CuII(OEFB) in the presence of trifluoroacetic acid under dioxygen. Carbon monoxide is produced when CuII(OEFB) is converted to [CuII(OEOP)]+ by either method. [CuII(OEOP)](PF6) has been characterized by single-crystal X-ray diffraction which shows that the cation has a planar, porphyrin-like structure. The room-temperature EPR spectrum of this complex shows that the copper is four-coordinate with four nitrogen based ligands, but frozen solutions of [CuII(OEOP)]+ show a triplet EPR spectrum indicative of a dimeric species much like that in the X-ray crystal structure. The 1H NMR spectrum of diamagnetic NiII(OEFB) has been shown to be consistent with the helical structure through the use of lanthanide and chiral lanthanide shift reagents. The EPR spectra of CoII(OEFB) show that it forms a low-spin adduct with pyridine and that this adduct acts as a reversible dioxygen carrier. The geometric and electronic structural properties of these complexes of formylbiliverdin are compared to those of analogous compounds of biliverdin and of porphyrins.

Adsorption of mutagens to chlorophyllin–chitosan, an insoluble form of chlorophyllin

Chlorophyllin, a water soluble derivative of chlorophyll, is known to suppress the mutagenic and carcinogenic actions of compounds having polycyclic structures, e.g. heterocyclic amines and aflatoxin B 1. There is evidence that this suppressing effect arises, at least in part, by a complex formation between the porphyrin-like structure of chlorophyllin and the planar molecular surfaces of these compounds. We report here that chlorophyllin can form an insoluble salt-like material when mixed with chitosan, a polyglucosamine, and that the solid chlorophyllin–chitosan thus prepared can efficiently trap polycyclic mutagenic compounds. The adsorbed polycyclic mutagens were elutable with buffers of acidic pH, but only to small extents. Chlorophyllin–chitosan may be expected to be useful as an intercepting agent against polycyclic mutagens and carcinogens.

Bilirubin conformational analysis and circular dichroism

Typical linear and porphyrin-like structure representations of bilirubin give an incorrect impression of its actual shape and expected solution properties. Conformational alnalysis of bilirubin, assisted by molecular dynamics computations, indicates that (i) nonbonded intramolecular steric interactions are minimized in a ridge-tile shape conformation lying at a global energy minimum on the conformational energy map and (ii) considerable additional stabilization is achieved though a network of intramolecular hydrogen bonds. The linear and porphyrin-like conformations are computed to lie some 37-48 kcal/mol above the isoenergetic global minimum energy conformations, which correspond to superimposable (identical) or to nonsuperimposable (enantiomeric) mirror image intramolecularly hydrogen-bonded ridge-tile conformers

Experiences with tritiated compounds prepared by exposure to tritium gas.

The Random Tritiation of organic compounds by contact with curie quantities of tritium gas [1] has greatly simplified the labeling of many organic compounds. Despite the absence of an adequate theory for predicting the specific activity attainable with a particular substance, the method has proven successful in labeling a great variety of compounds, including biologically active materials such as ribonucleic acid, lysozyme [2], and insulin [3]. In the Merck Sharp & Dohme Research Laboratories, this technique has been employed for the tritium labeling of a steroid, many substituted aromatic compounds, a partly saturated polycyclic structure, a poly-basic amino acid, a porphyrin-like structure, a sugar phosphate, and, in confirmation of an earlier report, [1] sucrose. Conditions prevailing during the tritiation of these compounds, as well as specific activities attained, are listed in Table I.