Magnetic circular dichroism on oxygen complexes of hemoproteins: correlation between magnetic circular dichroism magnitude and electronic structures of oxygen complexes

The Organism/Organic Exposure to Orbital Stresses (O/OREOS) satellite: radiation exposure in low-earth orbit and supporting laboratory studies of iron tetraphenylporphyrin chloride

We report results from the exposure of the metalloporphyrin iron tetraphenylporphyrin chloride (FeTPPCl) to the outer space environment, measured in situ aboard the Organism/Organic Exposure to Orbital Stresses nanosatellite. FeTPPCl was exposed for a period of 17 months (3700 h of direct solar exposure), which included broad-spectrum solar radiation (∼122 nm to the near infrared). Motivated by the potential role of metalloporphyrins as molecular biomarkers, the exposure of thin-film samples of FeTPPCl to the space environment in low-Earth orbit was monitored in situ via ultraviolet/visible spectroscopy and reported telemetrically. The space data were complemented by laboratory exposure experiments that used a high-fidelity solar simulator covering the spectral range of the spaceflight measurements. We found that thin-film samples of FeTPPCl that were in contact with a humid headspace gas (0.8-2.3% relative humidity) were particularly susceptible to destruction upon irradiation, degrading up to 10 times faster than identical thin films in contact with dry headspace gases; this degradation may also be related to the presence of oxides of nitrogen in those cells. In the companion terrestrial experiments, simulated solar exposure of FeTPPCl films in contact with either Ar or CO2:O2:Ar (10:0.01:1000) headspace gas resulted in growth of a band in the films' infrared spectra at 1961 cm(-1). We concluded that the most likely carriers of this band are allene (C3H4) and chloropropadiene (C3H3Cl), putative molecular fragments of the destruction of the porphyrin ring. The thin films studied in space and in solar simulator-based experiments show qualitatively similar spectral evolution as a function of contacting gaseous species but display significant differences in the time dependence of those changes. The relevance of our findings to planetary science, biomarker research, and the photostability of organic materials in astrobiologically relevant environments is discussed.

The synthesis and characterization of a side-by-side iron phthalocyanine dimer

The QCA paradigm is one of the approaches to decrease the size scale of computing devices. When molecules are used as QCA cells, they may be able to perform computing at room temperature. This paper describes a novel molecular QCA cell candidate which is a side-by-side iron phthalocyanine dimer, and an investigation of its optical and redox properties. The new dodeca(pentyloxy) substituted side-by-side iron phthalocyanine dimer, along with the octa(pentyloxy) iron phthalocyanine monomer, are soluble in non-polar organic solvents. These compounds were isolated by gel permeation chromatography (GPC) and high-performance liquid chromatography (HPLC) to final purities of 98% and 99%, respectively. The NMR spectra of both compounds in CDCl3 are broad due to aggregation, but become well resolved after the addition of the coordinating solvent pyridine-d5. Addition of pyridine also gives changes in the UV-vis spectra and electrochemical peaks of both monomer and dimer in dichloromethane indicative of axial iron coordination. The electrochemical data indicates the loss of pyridine ligands from the oxidized products of both monomer and dimer. The comproportionation constant of side-by-side phthalocyanine dimer shows that its oxidized and reduced mixed-valence complexes are fairly stable. The dimer is thus a candidate for molecular QCA systems.

Highly efficient decomposition of organic dye by aqueous-solid phase transfer and in situ photocatalysis using hierarchical copper phthalocyanine hollow spheres

The hierarchical tetranitro copper phthalocyanine (TNCuPc) hollow spheres were fabricated by a simple solvothermal method. The formation mechanism was proposed based on the evolution of morphology as a function of solvothermal time, which involved the initial formation of nanoparticles followed by their self-aggregation to microspheres and transformation into hierarchical hollow spheres by Ostwald ripening. Furthermore, the hierarchical TNCuPc hollow spheres exhibited high adsorption capacity and excellent simultaneously visible-light-driven photocatalytic performance for Rhodamine B (RB) under visible light. A possible mechanism for the "aqueous-solid phase transfer and in situ photocatalysis" was suggested. Repetitive tests showed that the hierarchical TNCuPc hollow spheres maintained high catalytic activity over several cycles, and it had a better regeneration capability under mild conditions.

Hierarchical nanostructures of copper(II) phthalocyanine on electrospun TiO(2) nanofibers: controllable solvothermal-fabrication and enhanced visible photocatalytic properties

In the present work, 2,9,16,23-tetranitrophthalocyanine copper(II) (TNCuPc)/TiO(2) hierarchical nanostructures were successfully fabricated by a simple combination method of electrospinning technique and solvothermal processing. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), UV-vis diffuse reflectance (DR), Fourier transform infrared spectrum (FT-IR), X-ray photoelectron spectroscopy (XPS), and thermal gravimetric and differential thermal analysis (TG-DTA) were used to characterize the as-synthesized TNCuPc/TiO(2) hierarchical nanostructures. The results showed that the secondary TNCuPc nanostructures were not only successfully grown on the primary TiO(2) nanofibers substrates but also uniformly distributed without aggregation. By adjusting the solvothermal fabrication parameters, the TNCuPc nanowires or nanoflowers were facilely fabricated, and also the loading amounts of TNCuPc could be controlled on the TNCuPc/TiO(2) hierarchical nanostructural nanofibers. And, there might exist the interaction between TNCuPc and TiO(2). A possible mechanism for the formation of TNCuPc/TiO(2) hierarchical nanostructures was suggested. The photocatalytic studies revealed that the TNCuPc/TiO(2) hierarchical nanostructures exhibited enhanced photocatalytic efficiency of photodegradation of Rhodamine B (RB) compared with the pure TNCuPc or TiO(2) nanofibers under visible-light irradiation.

Active site analysis of P450 enzymes: comparative magnetic circular dichroism spectroscopy

Recent structural studies indicate that the substrate- and O2-binding distal pocket of the P450 enzymes are not identical. Thus, P450terp (CYP108) from the alpha-terpineol-metabolizing Pseudomonad differs from P450cam (CYP-101) (C. A. Hasemann et al., J. Mol. Biol. 236, 1169, 1994). In contrast, the distal pockets of P450terp and P450BMP (CYP102 heme domain; Bacillus megaterium) are more closely similar, including novel hydrogen-bonding interactions between the distal H2O ligand and the I helix (C. A. Hasemann et al., Structure, 3, 41-62, 1995). To evaluate the significance of these differences, we have compared solution magnetic circular dichroism (MCD) spectra of P450terp with spectra of other P450 enzymes (e.g., P450cam, P450BMP, P450BM-3holo, and P450BM1), as well as with spectra of chloroperoxidase and NO synthase. Spectra of native P450terp are more similar to those of P450BMP and those of mammalian P450LM-2 than to those of P450cam. Upon substrate-binding, the MCD spectra of ferric P450terp and all other thiolate-ligated heme systems examined to date display a strong Soret band that is distinctly unique relative to the typical Soret MCD pattern(s) of catalases or other 5-coordinate ferric heme systems. This intense negative MCD feature thus appears diagnostic for cysteinate-linked ferric hemes. In the case of ferrous P450s, the intensity of the Soret-region MCD trough varies between substrate-bound and substrate-free enzymes (despite the fact that the substrate is NOT in direct contact with the heme moiety). A novel finding of particular interest is the clear spectral shifts of the Soret MCD band between the substrate-bound and substrate-free forms of ferrous-CO-P450terp. No such observation has been made previously. Furthermore, the band positions for BOTH types of P450terp are red-shifted from known bands of ferrous-CO-P50cam. These data thus indicate a surprising sensitivity of MCD spectra to active-site polarity and to H2O occupancy, concurring with reports of distal pocket effects on CO-binding rates and equilibrium constants. Comparative analysis of the spectral properties of P450terp with MCD spectra of other P450 enzymes, as well as with chloroperoxidase and NO synthase, demonstrates both the expected similarities and the significant differences that reflect active-site structural features. The detailed spectral analysis of P450terp relative to other P450 enzymes presented herein includes the first observation of a substrate-induced spectral shift for a ferrous-CO-P450. Furthermore, testable structural predictions for P450-BM-1 and for the novel NO synthase enzyme (neither of which has been crystallized to date) are made herein. This work thus provides insights into structurally defined P450s and may also lead to understanding of other P450 enzymes.

Cytochrome bo from Escherichia coli: reaction of the oxidized enzyme with hydrogen peroxide

Oxidized cytochrome bo reacts rapidly with micromolar concentrations of H2O2 to form a single derivative. The electronic absorption spectrum of this compound differs from that of the oxidized form of the enzyme reported by this laboratory [Watmough, Cheesman, Gennis, Greenwood and Thomson (1993) FEBS Lett. 319, 151-154]. It is characterized by a Soret maximum at 411 nm, increased absorbance at 555 nm, and reduced intensity at 624 nm. The apparent dissociation constant for this process is of the order of 4 x 10(-6) M, and the bimolecular rate constant for the formation of the new compound is (1.25-1.7) x 10(3) M-1.s-1. Electronic absorption difference spectroscopy shows this product to be identical with the compound formed from the reaction of the mixed-valence form of the enzyme with dioxygen. Investigation of this compound by room-temperature magnetic c.d. spectroscopy shows haem o to be neither high-spin nor low-spin ferric, but to have a spectrum characteristic of an oxyferryl species. There is no evidence for oxidation of the porphyrin ring. Therefore the binuclear centre of this species must consist of an oxyferryl haem (S = 1) coupled to a Cu(II) ion (S = 1/2) to form a new paramagnetic centre. The reaction was also followed by X-band e.p.r. spectroscopy, and this showed the disappearance in parallel with the formation of the oxyferryl species, of the broad g = 3.7, signal which arises from the weakly coupled binuclear centre in the oxidized enzyme. Since no new e.p.r.-detectable paramagnetic species were observed, the Cu(II) ion is presumed to be coupled to another paramagnet, possibly an organic radical. There is no evidence in the electronic absorption spectrum to indicate further reaction of cytochrome bo with H2O2 to form a second species. We argue that the circumstances of formation of this oxyferryl species are the same as those for the P form of cytochrome c oxidase, a species often regarded as containing a bound peroxide ion. The implications of these observations for the reaction mechanism of haem-copper terminal oxidases are discussed.

Magnetic-circular-dichroism studies of Escherichia coli cytochrome bo. Identification of high-spin ferric, low-spin ferric and ferryl [Fe(IV)] forms of heme o

Room-temperature (295 K) magnetic-circular-dichroism spectra at 280-2500 nm have been recorded for Escherichia coli cytochrome bo in its fast form (which has a g = 3.7 EPR signal and reacts rapidly with cyanide) and for its formate, fluoride, cyanide and hydrogen-peroxide derivatives. The spectra of all forms are dominated by signals from low-spin ferric heme b. These include a porphyrin-to-ferric ion charge-transfer transition in the near-infrared region (the near-infrared charge-transfer band) at 1610 nm. High-spin ferric heme o gives rise to a negative magnetic-circular-dichroism feature at 635, 642 and 625 nm (corresponding to a shoulder observed in the electronic absorption spectra) and a derivative charge-transfer feature at 1100, 1180 and 940 nm for the fast, formate and fluoride forms, respectively. The energies of these bands confirm that fluoride and formate are ligands to heme o. The energies of the analogous bands in the spectrum of fast cytochrome bo are typical for high-spin ferric hemes with histidine and water axial ligands. Addition of cyanide ion to fast cytochrome bo causes a red shift in the position of the Soret absorption peak, from 406.5 nm to 413 nm, and results in the loss of the 635-nm feature from the magnetic-circular-dichroism spectrum and of the corresponding shoulder in the electronic absorption spectrum. In the magnetic-circular-dichroism spectrum, the intensities of the Soret and alpha, beta bands are significantly increased. New near-infrared charge-transfer intensity is observed at 1000-2300 nm with a peak near 2050 nm. These changes are interpreted as resulting from a high-spin to low-spin transition at ferric heme o brought about by the binding of cyanide ion. The energy of the near-infrared charge-transfer band suggests that the cyanide ion is bridged to the CuB of the binuclear site. Treatment of fast cytochrome bo with hydrogen peroxide also causes a red shift in the position of the Soret absorbance, to 412 nm, and a loss of the 625-nm absorption shoulder. Changes in the magnetic-circular-dichroism spectrum at 450-600 nm are observed, but there is no significant increase in the intensity of the magnetic-circular-dichroism Soret band and no new near-infrared charge-transfer bands are detected, ruling out a similar high-spin to low-spin transition at heme o.(ABSTRACT TRUNCATED AT 400 WORDS)

The formation of ferric haem during low-temperature photolysis of horseradish peroxidase Compound I

Illumination at low temperature of the peroxide compound of horseradish peroxidase (HRP-I) causes partial conversion of the haem electronic structure from a ferryl-porphyrin radical species into a low-spin ferric state. Magnetic-c.d. (m.c.d.) and e.p.r. spectral features of the photolysis product are almost identical with those of the alkaline form of ferric HRP, proposed on the basis of its near-i.r. m.c.d. spectrum to be a Fe(III)-OH species. The ferric product of HRP-I photolysis also contains free-radical e.p.r. signals. Conversion of HRP-I into the Fe(III)-OH species, which requires transfer of a proton and two electrons from the protein, is shown to be a two-step process.

A comparison by magnetic circular dichroism of compound X and compound II of horseradish peroxidase

The chlorite product of horseradish peroxidase, compound X, is shown by magnetic circular dichroism (MCD) spectroscopy in the temperature range 1.6-50 K to have a very similar haem structure to compound II under the same conditions (pH 10.7). Both are concluded to contain the Fe(IV) = 0 group. The MCD spectrum also detects an unusual species, absorbing at wavelengths between 600 and 750 nm, that has magnetic properties different from those of the ferryl haem group. It is suggested that this is a species at the same oxidation level as ferryl haem but with the porphyrin ring having suffered a one-electron oxidation, i.e. [Fe(III) P.+].

Spectroscopic and equilibrium properties of the indoleamine 2,3-dioxygenase-tryptophan-O2 ternary complex and of analogous enzyme derivatives. Tryptophan binding to ferrous enzyme adducts with dioxygen, nitric oxide, and carbon monoxide

The dioxygen adduct of the heme protein indoleamine 2,3-dioxygenase has been generated at -30 degrees C in mixed solvents, and spectroscopic and equilibrium studies of its L-tryptophan (substrate) binding properties have been carried out for the first time. Comparative studies have also been performed with the NO and CO adducts of the ferrous enzyme. Under the conditions employed (-30 degrees C), both autoxidation and turnover (L-tryptophan + O2----formylkynurenine) of the ternary complex are effectively suppressed. Structural identification of the ternary complex is based on the 1:1 molar stoichiometry for the substrate-oxygenated enzyme adduct formation (Kd approximately 10(-4) M), the time-dependent linear product formation (turnover) at -20 degrees C, and the quantitative conversion of the complex to the ferrous CO derivative by bubbling with CO. Binding of L-tryptophan to the oxygenated enzyme leads to decreases in the intensities of its major absorption bands (lambda max 415, 541, 576 nm) and to a blue shift of its Soret peak. Interestingly, among the ferrous enzyme derivatives examined, only the substrate-bound oxygenated enzyme exhibits solvent-dependent Soret absorption peak positions, e.g., lambda max 411.5 and 413.5 nm in 65% (v/v) aqueous glycerol and ethylene glycol, respectively. In addition, indole binds to the oxygenated enzyme, causing a red shift of its Soret peak in these solvents only in the presence of substrate (411.5----414 nm and 413.5----414.5 nm, respectively), while similar effects of indole are independent of tryptophan for the other ferrous enzyme derivatives.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the spleen green hemeprotein with magnetic and natural circular dichroism spectroscopy: positive evidence for a myeloperoxidase-type active site

The green hemeprotein purified from bovine spleen has been characterized with magnetic and natural circular dichroism (MCD and CD) spectroscopy for the first time. The enzyme derivatives studied include the native high-spin ferric form and its high-spin chloride and low-spin cyanide and nitrite complexes, the ligand-free high-spin ferrous form and its low-spin CO adduct, and Compounds II (ferryl iron species) and III (dioxygen adduct). All these enzyme states exhibit MCD spectra that are considerably different from the spectra of analogous complexes of normal heme iron. In particular, the following distinctions have been observed. The sign of the derivative-shaped MCD bands of the high-spin ferric and Compound II forms in the Soret (380-500 nm) region and of the ferrous low-spin and Compound III forms in both the Soret and visible (500-700 nm) regions are opposite to and, except for the high-spin ferric form, are less symmetric than those seen for normal heme iron systems. MCD intensities in the Soret region for the high-spin ferrous and low-spin ferric derivatives are noticeably smaller than those of normal heme proteins by a factor of up to ten. Prominent MCD bands are seen around 450 and 630 nm for the green hemeprotein derivatives; these features are considerably red-shifted (30-50 nm) relative to the analogous transitions observed for normal heme proteins. In contrast to the aforementioned spectral differences, the MCD and CD spectra of the spleen green hemeprotein derivatives are essentially identical to those previously reported for several derivatives of another spectroscopically anomalous heme-type enzyme, myeloperoxidase. This provides strong evidence that the two enzymes have identical prosthetic groups and endogenous axial ligands coordinated to the central iron. The novel MCD features of the green proteins, taken together with previously reported spectroscopic results, are most consistent with the presence of a chlorin-type prosthetic group in both proteins. In addition, the CD spectral similarities suggest that the two green proteins have nearly identical active-site environments.