Porphyrin

About: Porphyrin is a research topic. Over the lifetime, 22226 publications have been published within this topic receiving 510695 citations. The topic is also known as: porphyrin.

Porphyrin-sensitized solar cells

Abstract: Nature has chosen chlorophylls in plants as antennae to harvest light for the conversion of solar energy in complicated photosynthetic processes. Inspired by natural photosynthesis, scientists utilized artificial chlorophylls – the porphyrins – as efficient centres to harvest light for solar cells sensitized with a porphyrin (PSSC). After the first example appeared in 1993 of a porphyrin of type copper chlorophyll as a photosensitizer for PSSC that achieved a power conversion efficiency of 2.6%, no significant advance of PSSC was reported until 2005; beta-linked zinc porphyrins were then reported to show promising device performances with a benchmark efficiency of 7.1% reported in 2007. Meso-linked zinc porphyrin sensitizers in the first series with a push–pull framework appeared in 2009; the best cell performed comparably to that of a N3-based device, and a benchmark 11% was reported for a porphyrin sensitizer of this type in 2010. With a structural design involving long alkoxyl chains to envelop the porphyrin core to suppress the dye aggregation for a push–pull zinc porphyrin, the PSSC achieved a record 12.3% in 2011 with co-sensitization of an organic dye and a cobalt-based electrolyte. The best PSSC system exhibited a panchromatic feature for light harvesting covering the visible spectral region to 700 nm, giving opportunities to many other porphyrins, such as fused and dimeric porphyrins, with near-infrared absorption spectral features, together with the approach of molecular co-sensitization, to enhance the device performance of PSSC. According to this historical trend for the development of prospective porphyrin sensitizers used in PSSC, we review systematically the progress of porphyrins of varied kinds, and their derivatives, applied in PSSC with a focus on reports during 2007–2012 from the point of view of molecular design correlated with photovoltaic performance.

Heme Enzyme Structure and Function

Abstract: Metalloporphyrins are widely used throughout the biosphere and of these heme (iron protoporphyrin IX, Fig. 1) is one of the most abundant and widely used. Heme shuttles electrons between proteins as in mitochondrial respiration or transports and stores O2 as with the globins. The role of heme in more active enzymatic chemical transformation began to be appreciated just after the discovery by Mason1 and Hayaishi2 that O2 O atoms can be enzymatically incorporated into organic substrates which represented the seminal discovery of oxygenases. While the enzymes used in these studies did not contain heme, it was not too long before heme-containing oxygenases also were discovered. In 1958 Klingenberg3 and Garfinkel4 found an unusual pigment in microsomes that when reduced in the presence of CO generated a spectrum with a peak at 450 nm instead of the expected 420 nm peak. Hence the name P450 was born. In 1964 Omura and Sato5,6 showed that this “pigment” is actually a protein and the function of this strange heme protein became clear in a seminal study by Estabrook et al.7 that demonstrated the involvement of the 450 nm pigment in steroid hydroxylation. Thus by the mid-1960s it was established that heme plays an active role in biology by somehow catalyzing the hydroxylation of organic substrates. While these discoveries certainly mark the beginning of modern approaches to studying heme enzyme oxygenases, the enzymatic role of heme dates much earlier to 1903 when horseradish peroxidase (HRP) was described.8 Indeed, owing to the ease of purification and stability of the various intermediates, HRP dominated heme enzyme studies until P450 was discovered. Figure 1 Structure of iron protoporphyrin IX. Heme enzymes can catalyze both reductive and oxidative chemistry but here we focus on those that catalyze oxidation reactions, and especially those for which crystal structures are available. There are two broad classes of heme enzyme oxidants: oxygenases that use O2 to oxidize, usually oxygenate, substrates and peroxidases that use H2O2 to oxidize, but not normally oxygenate, substrates. Of the two oxidants molecular oxygen is the most unusual because even though the oxidation of nearly all biological molecules by O2 is a thermodynamically favorable process, O2 is not a reactive molecule. The reason, of course, is that there is a large kinetic barrier to these reactions owing to O2 being a paramagnetic molecule so the reaction between a majority of biological molecules that have paired spins is a spin forbidden process. Overcoming this barrier is why Nature recruited transition metals and heme into enzyme active sites. As shown in Fig. 2, heme oxygenases bind O2 and store the O2 oxidizing equivalents in the iron, porphyrin, and/or amino acid side chains for further selective oxidation of substrates. Peroxidases use H2O2 as the oxidant and while not having the O2 spin barrier, H2O2 presents its own problems. The reaction between H2O2 and transition metals generates toxic hydroxyl radicals in the well known Fenton chemistry9 which would be highly destructive to enzyme active sites. As illustrated in Fig. 2, all heme oxidases are at some point in the catalytic cycle peroxidases. Molecular oxygen must first be reduced by two electrons to the peroxide level before the interesting chemistry starts: cleavage of the O-O bond. This bond can cleave either homolytically, which gives two hydroxyl radicals, or heterolytically to effectively give H2O and a naked O atom with only 6 valence electrons. Since the release of hydroxyl radicals in the active site must, in most cases, be avoided Nature has engineered heme enzyme active sites to ensure that the heterolytic pathway dominates. Figure 2 Oxygen and peroxide activation by heme enzymes. Oxygenases like P450 must have the iron reduced to ferrous (Fe(II) or Fe2+) before O2 can bind. The oxy complex is best described as ferric-superoxide, Fe(III)-OO−. A second electron transfer results ... The list of heme enzymes is substantial and thus it is necessary to be selective on which to discuss in detail. It may appear that a disproportionate amount of space is devoted to peroxidases and P450s. This is true and admittedly reflects the author’s own interests and area of expertise. Additionally, however, peroxidases are the most extensively studied heme enzymes and have provided fundamental insights into the chemistry and structure shared by many other enzymes. The other enzymes to be discussed were selected owing to both subtle variations on common themes and novel features that Nature selected for specific biological function.

High-valent iron(IV)-oxo complexes of heme and non-heme ligands in oxygenation reactions.

Abstract: High-valent iron(IV)-oxo species have been implicated as the key reactive intermediates in the catalytic cycles of dioxygen activation by heme and non-heme iron enzymes. Our understanding of the enzymatic reactions has improved greatly via investigation of spectroscopic and chemical properties of heme and non-heme iron(IV)-oxo complexes. In this Account, reactivities of synthetic iron(IV)-oxo porphyrin pi-cation radicals and mononuclear non-heme iron(IV)-oxo complexes in oxygenation reactions have been discussed as chemical models of cytochrome P450 and non-heme iron enzymes. These results demonstrate how mechanistic developments in biomimetic research can help our understanding of dioxygen activation and oxygen atom transfer reactions in nature.

Probing structure-function relations in heme-containing oxygenases and peroxidases

Abstract: Structural factors that influence functional properties are examined in the case of four heme enzymes: cytochrome P-450, chloroperoxidase, horseradish peroxidase, and secondary amine mono-oxygenase. The identity of the axial ligand, the nature of the heme environment, and the steric accessibility of the heme iron and heme edge combine to play major roles in determining the reactivity of each enzyme. The importance of synthetic porphyrin models in understanding the properties of the protein-free metal center is emphasized. The conclusions described herein have been derived from studies at the interface between biological and inorganic chemistry.