Hemoglobin has previously been shown to display ascorbate peroxidase and urate peroxidase activity, with measurable Michaelis-Menten parameters that reveal a particularly low Km for ascorbate as well as for urate - lower than the respective in vivo concentrations of these antioxidants in blood. Also, direct detection of a hemoglobin-ascorbate interaction was possible by monitoring the 1H-NMR spectrum of ascorbate in the presence of hemoglobin. The relative difference in structures between ascorbate and urate may raise the question as to exactly what the defining structural features would be, for a substrate that binds to hemoglobin with high affinity.
View Article and Find Full Text PDFThe structures and energetics of the tetracarbaboranes CBH (n = 6 to 13) have been investigated by density functional theory and coupled cluster calculations. In general, the lowest energy structures of the tetracarbaboranes CBH minimize the number of C-C polyhedral edges as well as the degrees of the carbon vertices. For the CBH and CBH systems the lowest energy structures are pyramidal structures having all four carbon atoms located on the base of the pyramid.
View Article and Find Full Text PDFThe putative initial adduct of ferrous superoxide reductase (SOR) with superoxide has been alternatively formulated as ferric-peroxo or ferrous-superoxo. The ~600-nm UV-vis absorption band proposed to be assigned to this adduct (either as sole intermediate in the SOR catalytic cycle, or as one of the two intermediates) has recently been interpreted as due to a ligand-to-metal charge transfer, involving thiolate and superoxide in a ferrous complex, contrary to an alternative assignment as a predominantly cysteine thiolate-to-ferric charge transfer in a ferric-peroxo electromer. In an attempt to clarify the electromeric formulation of this adduct, we report a computational study using a multiconfigurational complete active space self-consistent field (MC-CASSCF) wave function approach as well as modelling the UV-vis absorption spectra with time-dependent density functional theory (TD-DFT).
View Article and Find Full Text PDFThe ability of mononuclear non-heme iron complexes to function as molecular oxygen transporters is investigated by density functional theory. The factors governing the efficiency of the reversible binding of dioxygen at the active site of the dinuclear non-heme iron enzyme hemerythrin, including antiferromagnetic coupling and the conversion of dioxygen to hydroperoxo by a proton coupled 2-electron transfer mechanism, are revisited and considered as possible tools in mononuclear non-heme complexes. Several mononuclear non-heme model complexes, including active sites of enzymes already known to interact with dioxgenic ligands, are constructed and the molecular oxygen transportation capabilities of these complexes are examined computationally.
View Article and Find Full Text PDFUhl and coworkers have synthesized and characterized structurally the 11-vertex tetracarbalanes (AlMe)7(CEt)4(μ-H)2 and (AlEt)7(CCH2Ph)4(C[triple bond, length as m-dash]CPh)(μ-H). In order to understand the nature of the unusual C4Al7 polyhedra in such systems, the complete series of permethylated tetracarbalanes C4Aln-4Men (n = 6 to 14) as model compounds have been investigated using density functional theory. An overriding factor in determining the polyhedra for the lowest energy structures of a wide range of tetracarbalanes C4Aln-4Men (n = 6 to 14) is the availability of vertices of degrees 3 and 4 for all four carbon atoms.
View Article and Find Full Text PDFConsideration of the well-known very stable icosahedral B12H12(2-) as a three-dimensional analogue of benzene was extended by the recent synthesis of the biicosahedral B21H18(-) as a three-dimensional analogue of naphthalene. The preferred structures of metallaboranes derived from B21H18(-) have now been examined by density functional theory. The isoelectronic species CpNiB20H17 and CpCoCB19H17 have the 46 skeletal electrons expected by the Wade-Mingos and Jemmis rules for a structure consisting of two face-sharing fused icosahedra.
View Article and Find Full Text PDFTheoretical studies on the dicarbalanes C2Al(n-2)Men (n = 7-14; Me = methyl) predict both carbon atoms to be located at degree 4 vertices of a central C2Al(n-2) deltahedron in the lowest energy structures. As a consequence, deltahedra having two degree 4 vertices, two degree 6 vertices, and eight degree 5 vertices rather than the regular icosahedron having exclusively degree 5 vertices are found for the 12-vertex dicarbalane C2Al10Me12. However, the lowest energy C2Al(n-2)Men (n = 7-11) structures are based on the same most spherical (closo) deltahedra as the corresponding deltahedral boranes.
View Article and Find Full Text PDFBacterial nitric oxide reductase (cNOR) is an important binuclear iron enzyme responsible for the reduction of nitric oxide to nitrous oxide in the catalytic cycle of bacterial respiration. The reaction mechanism of cNOR as well as the key reactive intermediates of the reaction are still under debate. Here, we report a computational study based on ONIOM (DFT:MM) calculations aimed at investigating the reaction mechanism of cNOR.
View Article and Find Full Text PDFReported here is a computational study based on density functional theory that presents the first attempt to investigate the 2-electron 2-proton reaction of Fe(III)-H2NOH to Fe(III)-HNO in the catalytic cycle of hydroxylamine oxidoreductase-a multiheme-containing enzyme that catalyzes the conversion of hydroxylamine (HA) to nitrite in nitrifying bacteria. Two subsequent protonation events are proposed to initiate the process, of which the second is suggested to be concerted with a one-electron oxidation. The final one-electron oxidation is further proposed to be accompanied by a third deprotonation process, suggesting that Fe(III)-HNO may not be an isolable intermediate in the HAO catalytic cycle.
View Article and Find Full Text PDFThe reaction between nitrite and the oxy forms of globins has complex autocatalytic kinetics with several branching steps and evolves through chain reactions mediated by reactive species (including radicals) such as hydrogen peroxide, ferryl and nitrogen dioxide, starting with a lag phase, after which it proceeds onto an autocatalytic phase. Reported here are UV-Vis spectra collected upon stopped-flow mixing of myoglobin with a supraphysiological excess of nitrite. The best fit to the experimental data follows an A → B → C reaction scheme involving the formation of a short-lived intermediate identified as ferryl.
View Article and Find Full Text PDFIt is generally accepted that the catalytic cycles of superoxide reductases (SORs) and cytochromes P450 involve a ferric hydroperoxo intermediate at a mononuclear iron center with a coordination sphere consisting of four equatorial nitrogen ligands and one axial cysteine thiolate trans to the hydroperoxide. However, although SORs and P450s have similar intermediates, SORs selectively cleave the Fe-O bond and liberate peroxide, whereas P450s cleave the O-O bond to yield a high-valent iron center. This difference has attracted the interest of researchers, and is further explored here.
View Article and Find Full Text PDFObjective: To assess the epidemiological and clinico-pathological features, surgical and reconstructive techniques, adjuvant treatments and clinical outcome of breast carcinoma in males (BCM) at the Egyptian National Cancer Institute (NCI).
Patients And Methods: Thirty-two males with breast carcinoma presented to NCI between January 2000 and December 2002. They were evaluated by complete history, physical examination, laboratory and radiological investigations.
Purpose: The aim of this study was to reevaluate the role and effectiveness of pedicled myocutanous and myofascial flaps in reconstruction after resection of head and neck cancer.
Patients And Methods: This study represents the authors own experience using pedicled myocutanous and myofascial flaps in reconstruction after resection of malignant tumors of different sites in the head and neck. The study included 121 patients with head and neck cancer operated upon at the National Cancer Institute, Cairo University and Alminia Cancer Center over 3 years duration, between July 2005 and the end of July 2008.