Low-temperature UV-vis Research Articles

Salinibacter Sensory Rhodopsin: SENSORY RHODOPSIN I-LIKE PROTEIN FROM A EUBACTERIUM

Halobacterium salinarum sensory rhodopsin I (HsSRI), a dual receptor regulating both negative and positive phototaxis in haloarchaea, transmits light signals through changes in protein-protein interactions with its transducer, halobacterial transducer protein I (HtrI). Haloarchaea also have another sensor pigment, sensory rhodopsin II (SRII), which functions as a receptor regulating negative phototaxis. Compared with HsSRI, the signal relay mechanism of SRII is well characterized because SRII from Natronomonus pharaonis (NpSRII) is much more stable than HsSRI and HsSRII, especially in dilute salt solutions and is much more resistant to detergents. Two genes encoding SRI homologs were identified from the genome sequence of the eubacterium Salinibacter ruber. Those sequences are distantly related to HsSRI ( approximately 40% identity) and contain most of the amino acid residues identified as necessary for its function. To determine whether those genes encode functional protein(s), we cloned and expressed them in Escherichia coli. One of them (SrSRI) was expressed well as a recombinant protein having all-trans retinal as a chromophore. UV-Vis, low-temperature UV-Vis, pH-titration, and flash photolysis experiments revealed that the photochemical properties of SrSRI are similar to those of HsSRI. In addition to the expression system, the high stability of SrSRI makes it possible to prepare large amounts of protein and enables studies of mutant proteins that will allow new approaches to investigate the photosignaling process of SRI-HtrI.

Fourier Transform Infrared Characterization of a CuB−Nitrosyl Complex in Cytochromeba3fromThermus thermophilus: Relevance to NO Reductase Activity in Heme−Copper Terminal Oxidases

The two heme-copper terminal oxidases of Thermus thermophilus have been shown to catalyze the two-electron reduction of nitric oxide (NO) to nitrous oxide (N2O) [Giuffre, A.; Stubauer, G.; Sarti, P.; Brunori, M.; Zumft, W. G.; Buse, G.; Soulimane, T. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 14718-14723]. While it is well-established that NO binds to the reduced heme a3 to form a low-spin heme {FeNO}7 species, the role CuB plays in the binding of the second NO remains unclear. Here we present low-temperature FTIR photolysis experiments carried out on the NO complex formed by addition of NO to fully reduced cytochrome ba3. Low-temperature UV-vis, EPR, and RR spectroscopies confirm the binding of NO to the heme a3 and the efficiency of the photolysis at 30 K. The nu(NO) modes from the light-induced FTIR difference spectra are isolated from other perturbed vibrations using 15NO and 15N18O. The nu(N-O)a3 is observed at 1622 cm-1, and upon photolysis, it is replaced by a new nu(N-O) at 1589 cm-1 assigned to a CuB-nitrosyl complex. This N-O stretching frequency is more than 100 cm-1 lower than those reported for Cu-NO models with three N-ligands and for CuB+-NO in bovine aa3. Because the UV-vis and RR data do not support a bridging configuration between CuB and heme a3 for the photolyzed NO, we assign the exceptionally low nu(NO) to an O-bound (eta1-O) or a side-on (eta2-NO) CuB-nitrosyl complex. From this study, we propose that, after binding of a first NO molecule to the heme a3 of fully reduced Tt ba3, the formation of an N-bound {CuNO}11 is prevented, and the addition of a second NO produces an O-bond CuB-hyponitrite species bridging CuB and Fea3. In contrast, bovine cytochrome c oxidase is believed to form an N-bound CuB-NO species; the [{FeNO}7{CuNO}11] complex is suggested here to be an inhibitory complex.

Heterobimetallic dioxygen activation: synthesis and reactivity of mixed Cu-Pd and Cu-Pt bis(mu-oxo) complexes.

Heterobimetallic CuPd and CuPt bis(mu-oxo) complexes have been prepared by the reaction of (PPh3)2MO2 (M=Pd, Pt) with LCu(I) precursors (L=beta-diketiminate and di- and triamine ligands) and characterized by low-temperature UV-vis, resonance Raman, and 1H and 31P[1H] NMR spectroscopy in conjunction with DFT calculations. The complexes decompose upon warming to yield OPPh3, and in one case this was shown to occur by an intramolecular process through crossover experiments using double-labeling (oxo and phosphine). The reactivity of one of the complexes, LMe2Cu(mu-O)2Pt(PPh3)2 (LMe2 = beta-diketiminate), with a variety of reagents including CO2, 2,4-di-tert-butylphenol, 2,4-di-tert-butylphenolate, [NH4][PF6], and dihydroanthracene, was compared to that of homometallic Pt2 and Cu2 counterparts. Unlike typical [Cu2(mu-O)2]2+ cores which have electrophilic oxo groups, the oxo groups in the [Cu(mu-O)2Pt]+ core behave as bases and nucleophiles, similar to previously described Pt2 compounds. In addition, however, the [Cu(mu-O)2Pt]+ core is capable of oxidatively coupling 2,4-di-tert-butylphenol and 2,4-di-tert-butylphenolate. Theoretical evaluation of the electron affinities, basicities, and H-atom transfer kinetics and thermodynamics of the Cu2 and CuM (M=Pd, Pt) cores showed that the latter are more basic and form stronger O-H bonds.

Pre-steady state kinetic studies on the microsecond time scale of the laccase from Trametes versicolor

Laccase from Trametes versicolor reduces dioxygen to water. The enzyme is used in green chemistry applications such as the selective oxidation of alcohols in the presence of a suitable mediator (TEMPO) or in biofuel cells. We studied the catalytic mechanism of the enzyme by the stopped-flow and our newly developed rapid-mixing rapid sampling method, which has an experimental dead time of 75 ± 15 μs. Equilibrium and kinetic analyses yielded a reduction potential of 717 ± 5 mV for Type 1 copper center. EPR and low-temperature UV–Vis spectroscopy indicate that oxidation of the blue copper center and O O bond splitting occur within 100 μs, without detectable formation of a peroxide intermediate. These results indicate a rapid internal electron transfer between the various copper centers (>25.000/s) and rapid binding of O 2 ( k on > 5 × 10 7 M −1 s −1). Mechanistic aspects of the catalytic cycle are shortly discussed.

Oxygen binding and activation by the complexes of PY2- and TPA-appended diphenylglycoluril receptors with copper and other metals

The copper(I) complexes of diphenylglycoluril basket receptors and , appended with bis(2-ethylpyridine)amine (PY2) and tris(2-methylpyridine)amine (TPA), respectively, and their dioxygen adducts were studied with low-temperature UV-vis and X-ray absorption spectroscopy (XAS). The copper(I) complex of, [.Cu(I)2] or, forms a micro-eta2:eta2 dioxygen complex, whereas the copper(I) complex of, [.Cu(I)2] or, does not form a well defined dioxygen complex, but is oxidized to Cu(II). Dioxygen is bound irreversibly to and the formed complex is stable over time. The coordination geometries of the above complexes were determined by XAS, which revealed that pyridyl groups and amine N-donors participate in the coordination to Cu(I) ions in the complexes of both receptors. The catalytic activities of various metal complexes of and , that were designed as mimics of dinuclear copper enzymes that can activate dioxygen, were investigated. Phenolic substrates that were expected to undergo aromatic hydroxylation, showed oxidative polymerization without insertion of oxygen. The mechanism of this polymerization turns out to be a radical coupling reaction as was established by experiments with the model substrate 2,4-di-tert-butylphenol. In addition to Cu(II), the Mn(III) complex of and the Fe(II) complex of were tested as oxidation catalysts. Oxidation of catechol was observed for the Cu(II) complex of receptor but the other metal complexes did not lead to oxidation.

An oxo-ferryl tryptophan radical catalytic intermediate in cytochrome c and quinol oxidases trapped by microsecond freeze-hyperquenching (MHQ)

The pre-steady state reaction kinetics of the reduction of molecular oxygen catalyzed by fully reduced cytochrome oxidase from Escherichia coli and Paracoccus denitrificans were studied using the newly developed microsecond freeze-hyperquenching mixing-and-sampling technique. Reaction samples are prepared 60 and 200 μs after direct mixing of dioxygen with enzyme. Analysis of the reaction samples by low temperature UV–Vis spectroscopy indicates that both enzymes are trapped in the P M state. EPR spectroscopy revealed the formation of a mixture of two radicals in both enzymes. Based on its apparent g-value and lineshape, one of these radicals is assigned to a weakly magnetically coupled oxo-ferryl tryptophan cation radical. Implications for the catalytic mechanism of cytochrome oxidases are discussed.

How can the metal affect the proton transfer to the dihydrides [{P(CH2CH2PPh2)3}MH2] (M = Fe, Ru, Os)? A low-temperature electronic spectroscopy study

According to low-temperature UV-Vis spectroscopy data, the two-step protonation of iron subgroup metal hydrides [{P(CH2CH2PPh2)3}MH2] (M = Fe, Ru, Os) with p-nitrophenol includes the formation of ion pairs stabilized by a hydrogen bond between the cationic dihydrogen complex and the phenolate anion. The trend of the extent of proton transfer appeared to be aperiodic, FeI ≪ OsI < RuI, in contrast to the previously obtained sequence of the proton-acceptor capacity of the hydride ligand, which increases down the group.

Low-temperature UV-visible and NMR spectroscopic investigations of O(2) binding to ((6)L)Fe(II), a ferrous heme bearing covalently tethered axial pyridine ligands.

In this report, we describe the reversible dioxygen reactivity of ((6)L)Fe(II) (1) [(6)L = partially fluorinated tetraphenylporphyrin with covalently appended TMPA moiety; TMPA = tris(2-pyridylmethyl)amine] using a combination of low-temperature UV-vis and multinuclear ((1)H and (2)H) NMR spectroscopies. Complex 1, or its pyrrole-deuterated analogue ((6)L-d(8))Fe(II) (1-d(8)), exhibits downfield shifted pyrrole resonances (delta 28-60 ppm) in all solvents utilized [CH(2)Cl(2), (CH(3))(2)C(O), CH(3)CN, THF], indicative of a five-coordinate high-spin ferrous heme, even when there is no exogenous axial solvent ligand present (i.e., in methylene chloride). Furthermore, ((6)L)Fe(II) (1) exhibits non-pyrrolic upfield and downfield shifted peaks in CH(2)Cl(2), (CH(3))(2)C(O), and CH(3)CN solvents, which we ascribed to resonances arising from the intra- or intermolecular binding of a TMPA-pyridyl arm to the ferrous heme. Upon exposure to dioxygen at 193 K in methylene chloride, ((6)L)Fe(II) (1) [UV-vis: lambda(max) = 433 (Soret), 529 (sh), 559 nm] reversibly forms a dioxygen adduct [UV-vis: lambda(max) = 422 (Soret), 542 nm], formulated as the six-coordinate low-spin [delta(pyrrole) 9.3 ppm, 193 K] heme-superoxo complex ((6)L)Fe(III)-(O(2)(-)) (2). The coordination of the tethered pyridyl arm to the heme-superoxo complex as axial base ligand is suggested. In coordinating solvents such as THF, reversible oxygenation (193 K) of ((6)L)Fe(II) (1) [UV-vis: lambda(max) = 424 (Soret), 542 nm] also occurs to give a similar adduct ((6)L)Fe(III)-(O(2)(-)) (2) [UV-vis: lambda(max) = 418 (Soret), 537 nm. (2)H NMR: delta(pyrrole) 8.9 ppm, 193 K]. Here, we are unable to distinguish between a bound solvent ligand or tethered pyridyl arm as axial base ligand. In all solvents, the dioxygen adducts decompose (thermally) to the ferric-hydroxy complex ((6)L)Fe(III)-OH (3) [UV-vis: lambda(max) = 412-414 (Soret), 566-575 nm; approximately delta(pyrrole) 120 ppm at 193 K]. This study on the O(2)-binding chemistry of the heme-only homonuclear ((6)L)Fe(II) (1) system lays the foundation for a more complete understanding of the dioxygen reactivity of heterobinuclear heme-Cu complexes, such as [((6)L)Fe(II)Cu(I)](+), which are models for cytochrome c oxidase.

A study on the vibrational structure of poly(phenylenevinylene)s via low-temperature UV-vis and fluorescence spectroscopy

UV-vis and fluorescence spectroscopy of poly(phenylenevinylene) derivatives were acquired at low temperature (−108 °C). Noticeable spectroscopic red-shifts (as large as 17 nm) were detected from both polymers and model compounds in solutions when the temperature was decreased from 25 °C to −108 °C, indicating that the molecules adopted a more planar conformation at the low temperature. The low temperature spectra revealed the hidden structured absorption and emission. The vibrational structures in the fluorescence spectra were analyzed by using the obtained spectroscopic data at the low temperature. The very similar spectroscopic properties between the polymers (1 and 2) and their model compounds (3 and 4) strongly suggested that the observed spectroscopic red-shift from the polymers at the low temperature was an intrinsic property of the chromophore.

Synthesis, characterization and reactivity of a series of dinuclear copper complexes bearing the ligand bis[3-(2-hydroxybenzylideneamino)phenyl] sulfone and derivatives

The ligands bis[3-(X-2-hydroxybenzylideneamino)phenyl] sulfones (X=none: BHBAPS, X=3- tert-butyl: BH(t-Bu)BAPS and X=3,5-dichloro: BHCl 2BAPS) were prepared. These, together with Cu(OAc) 2 were used in the syntheses of the dinuclear copper complexes Cu 2(BBAPS)(μ-OMe) 2 ( 1), Cu 2[B(t-Bu)BAPS] (μ-OH) 2 ( 2), and Cu 2[BCl 2BAPS] (μ-OMe) 2 ( 3). Complex 1 was crystallographically characterized. The structures of 2 and 3 are similar to 1 by comparison of IR, UV–Vis, FAB-MS and elemental analyses results. Complexes 1– 3 (1 mol%) were used in the oxidation of 2,4- and 2,6-di-t-butylphenol (dtbp) at −50 °C with H 2O 2. The results show that (1) the coupling products are preferred when CH 2Cl 2 is used; and (2) the quinone yield increases when THF is utilized. In CH 2Cl 2 with 2,4-dtbp, the yield of the coupling product based on the complex amount, is in the order 2, 1, and 3 with yields of 6300, 4700 and 200%, respectively. Low temperature UV–Vis results of the reaction of 1 with H 2O 2 showed the growth of peaks at 390 and 580 nm indicative of a μ-η 2:η 2-peroxo or μ-η 1:η 1-hydroperoxo intermediate. At −50 °C, this spectrum does not change. But when warmed to 0 °C, a spectrum similar to the original spectrum was obtained. This probably indicates hydrogen radical abstractions of the peroxo intermediate from solvents, and if excess H 2O 2 is present, the peroxo intermediate may again be formed. This reusability of the complex explains the high yield using 1 and 2.

Reactions of Solid Organic Compounds with Metal Species

The results on chemical reactions of metal atoms, small clusters and nanoparticles with different organic and inorganic substances in the temperature range 12–300 K are presented. Complexation and reactions of atoms and clusters of magnesium, samarium and silver with alkyl halides, carbon dioxide, ethylene and some mesogenic cyanophenyls were studied by the technique of matrix isolation and low temperatrure co-condensation of metal and ligands vapours, low temperature UV-Vis, IR-and ESR-spectroscopy in the combination with quantum chemistry calculations. In some cases the reaction products were analysed by high performance gas chromatography. It was shown that cryochemical reactions of metal particles of different size reflected the system redundant energy.

Enhanced Adsorption Capacity and Photo-Catalytic Oxidative Activity of Dyes in Aqueous Medium by Hydrothermally Treated Titania Pillared Clay

Titania pillared montmorillonite clay was prepared by two different routes viz. 1) conventional ion exchange method and 2) modified method wherein the post hydrothermal treatment after ion-exchange was employed. The influence of the post hydrothermal treatment on the textural properties of titania-pillared clays was studied. The calcined clay was characterized by different physico-chemical techniques such as XRD, EDX, low temperature (77 K) nitrogen adsorption and UV-Vis diffuse reflectance spectroscopy. The content of pillared titania remained unchanged irrespective of the method of preparation. The method of preparation and severity of the conditions employed for the hydrothermal treatment resulted in alteration of the crystallinity and crystallite size of the anatase. The changes in the average pore diameter was found to comensurate with the changes in crystallite size of anatase phase. The increase in total pore volume as a function of the severity of the post hydrothermal treatment resulted in the decrease in micropore volume. The severity of the post hydrothermal treatment governed the extent of the blue shift in UV-Vis DRS spectra.

Spectroscopic study of low temperature interactions in metal-organic co-condensates

The results on spectroscopic study of low temperature interactions of metal atoms, small clusters and nanoparticles with different organic and inorganic substances in the temperature range 12–300 K are presented. Complexation and reactions of atoms and clusters of magnesium, samarium and silver with carbon dioxide, ethylene and some mesogenic cyanophenyls were studied by the technique of matrix isolation and low temperature co-condensation of metal and ligand vapors, low temperature UV–Vis, IR- and ESR-spectroscopy in combination with quantum chemistry calculations. It was shown that cryochemical reactions of metal particles of different sizes reflected the system's redundant energy.

The photoreactions of recombinant phytochrome from the cyanobacterium Synechocystis: a low-temperature UV-Vis and FT-IR spectroscopic study.

The interconvertible photoreactions of recombinant phytochrome from Synechocystis reconstituted with phycocyanobilin were investigated by light-induced optical and Fourier-transform infrared (FT-IR) difference spectroscopy at low temperatures for the first time. The photochemistry was found to be deferred below -100 degrees C for the transformation of red-absorbing form of phytochrome (Pr)-->far-red-absorbing form of phytochrome (Pfr), and no formation of an intermediate similar to the photoproduct of phytochrome A obtained at -140 degrees C (lumi-R) was observed. Two intermediates could be stabilized below -40 degrees C and between -40 and -20 degrees C, and were denoted as meta-Ra and meta-Rc, respectively. Above -20 degrees C Pfr was obtained. In the reverse reaction two intermediates could be stabilized below -60 degrees C (lumi-F) and between -60 and -40 degrees C (meta-F). The FT-IR difference spectra of the late Pr-->Pfr photoreaction show great similarities to the spectra obtained from oat phytochrome A suggesting similar conformation of the chromophore and interactions with its protein environment, whereas deviations in the spectra of meta-Ra were observed. A large band around 1700 cm-1 in the difference spectra between the intermediates and Pr which is tentatively assigned to the C19=O group of the prosthetic group indicates the Z,E isomerization around the C15=C16-methine bridge of the chromophore during the formation of meta-Ra. In the difference spectra of the parent states only small differences are observed in this region suggesting that the frequency of the carbonyl group is similar in Pr and Pfr. Since the FT-IR difference spectra between lumi-F and Pfr show great similarities to the spectra of the parent states, it is assumed that during the formation of lumi-F the chromophore largely returns into the primary Pr conformation. The FT-IR spectra recorded in a medium of 2H2O generally show a downshift of the significant bands due to the isotope effect. The appearance of a characteristic band around 935 cm-1 in all 2H2O spectra suggests an assignment to an N-2H bending vibration of the chromophore.

Low-temperature photochemistry of nitro-substituted aromatic azides; subsequent reactions of intermediates

Triplet 4-nitrophenylnitrene ( 1B) and triplet 4-nitrene-4′-nitrostilbene ( 2B) are characterized by their low-temperature UV-Vis and ESR spectra as well as by their consecutive thermal and photochemical reactions. The main thermal reaction of 1B and 2B between 77 and 90 K is a stepwise, twofold hydrogen abstraction from the methyltetrahydrofuran (2-MTHF) matrix solvent, leading to the corresponding primary amines. The activation energy of the first hydrogen abstraction of 1B and 2B is not influenced by their structure and is about 30 kJ mol −1. A similar amount of energy is required for the second hydrogen abstraction. Since the activation energy required for diffusion in a solid 2-MTHF matrix is about 87 kJ mol −1, bimolecular processes that require diffusion are less efficient between 77 and 87 K. While 2B is photochemically stable, 1B reacts after optical excitation under hydrogen abstraction from the solvent to form an imino radical 1C. The photochemically induced hydrogen abstraction after the excitation of 1B and the absence of the analogous reaction after the excitation of 2B can be explained by means of two models; either as a hot ground state reaction or as a reaction starting from an electronically excited state of 1B. In the latter model, it is assumed that the activation energy of the hydrogen abstraction is lower in the first excited state than in the electronic ground state of 1B. Both models predict that the hydrogen abstraction is more efficient if the difference in energy between the electronic ground and electronically excited states is high enough. This is the case in 1B but not in 2B. The experimental results are discussed in connection with theoretical statements arising from quantum chemical calculations.

Direct Detection of the Radical Cation 2,6-Di-tert-butyl-4-methylphenol Generated by Electron-Transfer Oxidation with Matrix Alkyl Halide Cation Radicals. A Low-Temperature EPR and UV-VIS Optical Absorption Study.

Direct Detection of the Radical Cation 2,6-Di-tert-butyl-4-methylphenol Generated by Electron-Transfer Oxidation with Matrix Alkyl Halide Cation Radicals. A Low-Temperature EPR and UV-VIS Optical Absorption Study.

Characterization of High Valent Iron Porphyrin in Catalytic Reaction by Iron(III) Tetrapentafluorophenylporphyrin

Abstract High valent species formed from iron(III) meso-tetra-(pentafluorophenyl)porphyrin was first prepared in dichloromethane and characterized by low-temperature UV-vis absorption spectra, proton NMR and ESR. These measurements showed the formation of oxo iron(IV) porphyrin π-cation radical, not iron(V) porphyrin.