Low-temperature Electronic Absorption Research Articles

Second-Coordination-Sphere Effects Reveal Electronic Structure Differences between the Mitochondrial Amidoxime Reducing Component and Sulfite Oxidase.

A combination of X-ray absorption and low-temperature electronic absorption spectroscopies has been used to probe the geometric and electronic structures of the human mitochondrial amidoxime reducing component enzyme (hmARC1) in the oxidized Mo(VI) and reduced Mo(IV) forms. Extended X-ray absorption fine structure analysis revealed that oxidized enzyme possesses a 5-coordinate [MoO2(SCys)(PDT)]- (PDT = pyranopterin dithiolene) active site with a cysteine coordinated to Mo. A 5-coordinate geometry is retained in the reduced state, with the equatorial oxo being protonated. Low-temperature electronic absorption spectroscopy of hmARC1 reveals a spectrum for the oxidized enzyme that is significantly different from what has been reported for sulfite oxidase family enzymes. Time-dependent density functional theory computations on oxidized and reduced hmARC1, and a small molecule analogue for hmARC1ox, have been used to assist us in making detailed band assignments and developing a greater understanding of enzyme electronic structure contributions to reactivity. Our understanding of the hmARCred HOMO and the LUMO of the benzamidoxime substrate reveal a potential π-bonding interaction between these redox orbitals, with two-electron occupation of the substrate LUMO along the reaction coordinate activating the O-N bond for cleavage and promoting oxygen atom transfer to the Mo site.

Electrochemical Formation and Reactivity of a Mn‐Peroxo Complex Bearing an Amido N5 Ligand

AbstractHerein we report the electrochemical generation of a mononuclear MnIII(OO) (peroxo) complex supported on a dpaq ligand (dpaq=2‐(bis(pyridin‐2‐ylmethyl)amino)‐N‐(quinolin‐8‐yl)acetamide) for the first time, and its reactivity in N,N‐dimethylformamide. The formation of the MnIII(dpaq)(OO) complex is probed by low temperature electronic absorption spectro‐electrochemistry experiments. An analysis of the reduction of the MnIII(dpaq)(OO) complex is carried out combining cyclic voltammetry and simulations. The involvement of a MnII(dpaq)(OOH) complex is proposed based on CV data and is corroborated by DFT computations.

Characterization and Subsequent Reactivity of an Fe-Peroxo Porphyrin Generated by Electrochemical Reductive Activation of O2.

Reductive activation of O2 is achieved by using the [FeIII(F20TPP)Cl] (F20TPP = 5,10,15,20-tetrakis(pentafluorophenyl) porphyrinate) porphyrin through electrochemical reduction of the [FeIII(F20TPP)(O2•-)] superoxo complex. Formation of the [FeIII(F20TPP)(OO)]- peroxo species is monitored by using low-temperature electronic absorption spectroscopy, electron paramagnetic resonance, and cyclic voltammetry. Its subsequent protonation to yield the [FeIII(F20TPP)(OOH)] hydroperoxo intermediate is probed using low-temperature electronic absorption spectroscopy and electron paramagnetic resonance.

Electrochemical formation of MnIII-peroxo complexes supported by pentadentate amino pyridine and imidazole ligands

A novel and efficient method for preparing [Mn(III)(O2)(L)](+) complexes using electrochemically generated superoxide is reported, with the reaction probed by low temperature electronic absorption and electron paramagnetic resonance spectroscopic techniques.

Spectroscopic and DFT studies of second-sphere variants of the type 1 copper site in azurin: covalent and nonlocal electrostatic contributions to reduction potentials.

The reduction potentials (E(0)) of type 1 (T1) or blue copper (BC) sites in proteins and enzymes with identical first coordination spheres around the redox active copper ion can vary by ~400 mV. Here, we use a combination of low-temperature electronic absorption and magnetic circular dichroism, electron paramagnetic resonance, resonance Raman, and S K-edge X-ray absorption spectroscopies to investigate a series of second-sphere variants--F114P, N47S, and F114N in Pseudomonas aeruginosa azurin--which modulate hydrogen bonding to and protein-derived dipoles nearby the Cu-S(Cys) bond. Density functional theory calculations correlated to the experimental data allow for the fractionation of the contributions to tuning E(0) into covalent and nonlocal electrostatic components. These are found to be significant, comparable in magnitude, and additive for active H-bonds, while passive H-bonds are mostly nonlocal electrostatic in nature. For dipoles, these terms can be additive to or oppose one another. This study provides a methodology for uncoupling covalency from nonlocal electrostatics, which, when coupled to X-ray crystallographic data, distinguishes specific local interactions from more long-range protein/active interactions, while affording further insight into the second-sphere mechanisms available to the protein to tune the E(0) of electron-transfer sites in biology.

Steric and Electronic Influences on the Structures of Peroxomanganese(III) Complexes Supported by Tetradentate Ligands

AbstractWe report the generation and characterization of two peroxomanganese(III) complexes supported by the L7py24‐Cl and L7iso‐q2 ligands {L7py24‐Cl = 1,4‐bis[(4‐chloro‐2‐pyridyl)methyl]‐1,4‐diazepane and L7iso‐q2 = 1,4‐bis[(2‐isoquinolinyl)methyl]‐1,4‐diazepane} and describe X‐ray structures of corresponding manganese(II) compounds. Ground and excited state properties of these peroxomanganese(III) complexes, as well as previously reported [MnIII(O2)(L7py24‐Me)]+ and [MnIII(O2)(L7q2)]+ species {L7py24‐Me = 1,4‐bis[(4‐methyl‐2‐pyridyl)methyl]‐1,4‐diazepane and L7q2 = 1,4‐bis[(2‐quinolinyl)methyl]‐1,4‐diazepane} were probed using low‐temperature electronic absorption, magnetic circular dichroism (MCD), and variable‐temperature, variable‐field MCD spectroscopy. These data sets afford electronic transition energies and estimates of ground‐state zero‐field splitting parameters, which permit a detailed comparison of electronic structure. These data support the proposal that all complexes share a similar geometry, consisting of a side‐on peroxomanganese(III) moiety coordinated by the tetradentate ligand in a trans fashion. However, differences in d‐d transition energies offer conclusive evidence that, among this series of complexes, it is the location, and thus the steric influence, of the pyridine substituent that modulates the electronic and geometric structure of the MnIII‐O2 unit. Within this series, perturbations in electronic properties of the supporting ligand have little impact on d‐d transition energies. Models of peroxomanganese(III) complexes developed using density functional theory (DFT) computations support this proposal, and time‐dependent DFT computations qualitatively reproduce the experimental trend in d‐d transition energies.

Giant Excitonic Zeeman Splittings in Colloidal Co2+-Doped ZnSe Quantum Dots

Colloidal Co(2+):ZnSe diluted magnetic semiconductor quantum dots (DMS-QDs) were prepared by the hot injection method and studied spectroscopically. Ligand-field electronic absorption and magnetic circular dichroism (MCD) spectra confirm homogeneous substitutional speciation of Co(2+) in the ZnSe QDs. Absorption spectra collected at various times throughout the syntheses reveal that dopants are absent from the central cores of the QDs but are incorporated at a constant concentration during nanocrystal growth. The undoped cores are associated with dopant exclusion from the ZnSe critical nuclei. Analysis of low-temperature electronic absorption and MCD spectra revealed excitonic Zeeman splitting energies (DeltaE(Zeeman)) of these Co(2+):ZnSe QDs that were substantially smaller than anticipated from bulk Co(2+):ZnSe data. This reduction in DeltaE(Zeeman) is explained quantitatively by the absence of dopants from the QD cores, where dopant-exciton overlap would be greatest. Since dopant exclusion from nucleation appears to be a general phenomenon for DMS-QDs grown by direct chemical methods, we propose that DeltaE(Zeeman) will always be smaller in colloidal DMS-QDs grown by such methods than in the corresponding bulk materials.

Electronic Spectra and Configuration Interaction of Tm3+ in TmCl63-

Low-temperature electronic absorption and emission data are reported for Tm 3 + at the octahedral site in crystals of Cs 2 NaTmCl 6 . Thirty-seven crystal field levels (total degeneracy 88) of the f 1 2 configuration (totaldegeneracy 91) have been assigned, and in several cases the levels are split due to electron-phonon coupling interactions. The fitting of the energy levels, using the conventional f 1 2 analysis with 12 variable parameters, gives a mean deviation of 53.3 cm - 1 . This is reduced to 9.3 cm - 1 by including the 4f 1 2 np 6 /4f 1 3 np 5 configuration interaction, using 16 variable parameters. The results indicate a tendency for the early members in the series of Ln 3 + ions to interact with the p-electron, and the later members with the p-hole, configurations, following the redox properties of the ions. The interacting configuration is charge-transfer (n = 3) rather than metal ion (n = 5), and the mixing of ligand wave functions with those of the metal ion may be responsible for the unusually strong electron-phonon coupling identified for several electronic states of Tm 3 + in TmCl 6 3-.

Magnetic quantum dots: synthesis, spectroscopy, and magnetism of Co2+ - and Ni2+-doped ZnO nanocrystals.

We report a method for the preparation of colloidal ZnO-diluted magnetic semiconductor quantum dots (DMS-QDs) by alkaline-activated hydrolysis and condensation of zinc acetate solutions in dimethyl sulfoxide (DMSO). Mechanistic studies reveal that Co(2+) and Ni(2+) dopants inhibit nucleation and growth of ZnO nanocrystals. In particular, dopants are quantitatively excluded from the critical nuclei but are incorporated nearly isotropically during subsequent growth of the nanocrystals. The smaller nanocrystal diameters that result upon doping are explained by the Gibbs-Thompson relationship between lattice strain and crystal solubility. We describe methods for cleaning the nanocrystal surfaces of exposed dopants and for redispersion of the final DMS-QDs. Homogeneous substitutional doping is verified by high-resolution low-temperature electronic absorption and magnetic circular dichroism (MCD) spectroscopies. A "giant Zeeman effect" is observed in the band gap transition of Co(2+):ZnO DMS-QDs. MCD and Zeeman spectroscopies are used to quantify the magnitude of the p-d exchange interaction (N(0)beta) that gives rise to this effect. N(0)beta values of -2.3 +/- 0.3 eV (-18 500 cm(-1)) for Co(2+):ZnO and -4.5 +/- 0.6 eV (-36 300 cm(-1)) for Ni(2+):ZnO have been determined. Ligand-to-metal charge-transfer transitions are observed in the MCD spectra of both Co(2+):ZnO and Ni(2+):ZnO DMS-QDs and are analyzed in the context of an optical electronegativity model. The importance of these charge-transfer states in determining N(0)beta is discussed. Ferromagnetism with T(C) > 350 K is observed in aggregated nanocrystals of Co(2+):ZnO that unambiguously demonstrates the existence of intrinsic high-T(C) ferromagnetism in this class of DMSs.

Probing the binding sites of exchanged chlorophyll a in LH2 by Raman and site-selection fluorescence spectroscopies

In this work we have selectively released the 800 nm absorbing bacteriochlorophyll a molecules of the LH2 protein from the photosynthetic bacterium Rhodopseudomonas acidophila, strain 10050, and replaced them with chlorophyll a (Chl a). A combination of low-temperature electronic absorption, resonance Raman and site-selection fluorescence spectroscopies revealed that the Chl a pigments are indeed bound in the B800 binding site; this is the first work that formally proves that such non-native chlorins can be inserted correctly into LH2.

Why is there an "inert" metal center in the active site of nitrile hydratase? Reactivity and ligand dissociation from a five-coordinate Co(III) nitrile hydratase model.

To determine how a substitutionally inert metal can play a catalytic role in the metalloenzyme nitrile hydratase (NHase), a reactive five-coordinate Co(III) thiolate complex ([Co(III)(S(2)(Me2)N(3)(Pr,Pr))](PF(6)) (1)) that resembles the active site of cobalt containing nitrile hydratase (Co NHase) was prepared. This was screened for reactivity, by using low-temperature electronic absorption spectroscopy, toward a number of biologically relevant "substrates". It was determined 1 will react with azide, thiocyanate, and ammonia, but is unreactive toward nitriles, NO, and butyrate. Substrate-bound 1 has similar spectroscopic and structural properties as [Co(III)(ADIT(2))](PF(6)) (2). Complex 2 is a six-coordinate Co(III) complex containing cis-thiolates and imine nitrogens, and has properties similar to the cobalt center of Co NHase. Substrate binding to 1 is reversible and temperature-dependent, allowing for the determination of the thermodynamic parameters of azide and thiocyanate binding and the rates of ligand dissociation. Azide and thiocyanate bind trans to a thiolate, and with similar entropies and enthalpies (thiocyanate: DeltaH = -7.5 +/- 1.1 kcal/mol, DeltaS = -17.2 +/- 3.2 eu; azide: DeltaH = -6.5 +/- 1.0 kcal/mol, DeltaS = -12.6 +/- 2.4 eu). The rates of azide and thiocyanate displacement from the metal center are also comparable to one another (k(d) = (7.22 +/- 0.04) x 10(-)(1) s(-)(1) for thiocyanate and k(d) = (2.14 +/- 0.50) x 10(-)(2) s(-)(1) for azide), and are considerably faster than one would expect for a low-spin d(6) six-coordinate Co(III) complex. These rates are comparable to those of an analogous Fe(III) complex, demonstrating that Co(III) and Fe(III) react at comparable rates when in this ligand environment. This study therefore indicates that ligand displacement from a low-spin Co(III) center in a ligand environment that resembles NHase is not prohibitively slow so as to disallow catalytic action in nonredox active cobalt metalloenzymes.

EPR investigation of compound I in Proteus mirabilis and bovine liver catalases: formation of porphyrin and tyrosyl radical intermediates.

Compound I of Proteus mirabilis and bovine liver catalases (PMC and BLC, respectively) were studied combining EPR spectroscopy and the rapid-mix freeze-quench techniques. Both enzymes, when treated with peroxyacetic acid, form a catalytic intermediate which consists of an oxoferryl porphyrin pi-cation radical. In PMC this intermediate is semistable, and an unexpected reversible equilibrium under pH influence takes place between two forms of compound I with different coupling between the oxoferryl and the porphyrin pi-cation radical. At acid pH, one form has a ferromagnetic character as in Micrococcus luteus compound I. At neutral pH, another form with a much smaller coupling, reminiscent of the horse radish peroxidase compound I, is detected. The approximate midpoint, estimated for these changes in the range 5.3 < pH < 6.0, approaches the pKa value of an histidyl residue. The residues possibly involved in the transformation are discussed in terms of the known structure of PMC compound I. The EPR spectrum of BLC compound I (pH 5.6), obtained in the millisecond time scale (40 ms), also showed a mixture of two forms which, most probably, correspond to two different magnetic exchange interactions, as in the case of PMC. Taken together, the low-temperature electronic absorption and the EPR spectra of BLC compound I formed in the 0.04-15 s range show that the porphyrin pi-cation radical disappears and, instead, a tyrosyl radical is formed. ENDOR experiments confirm our previously estimated hyperfine couplings to the C2,6 and C3,5 ring protons and the beta-methylene protons of the purported tyrosyl radical. Candidates for such a tyrosyl radical are discussed in connection with the possible electron transfer pathways between the heme active site and the NADPH cofactor.

Electronic states of semiconductor clusters: Homogeneous and inhomogeneous broadening of the optical spectrum

The homogeneous (single-cluster) and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35–50 Å diameter CdSe clusters are separated using transient photophysical hole burning. The clusters have the cubic bulk crystal structure, but their electronic states are strongly quantum confined. The inhomogeneous broadening of these features arises because the spectrum depends upon cluster size and shape, and the samples contain similar, but not identical, clusters. The homogeneous spectrum, which consists of a peak 140 cm−1 (17 meV) wide, with a phonon sideband and continuum absorption to higher energy, is compared to a simple molecular orbital model. Electron–vibration coupling, which is enhanced in small clusters, contributes to the substantial broadening of the homogeneous spectrum. The inhomogeneous width of the lowest allowed optical transition was found to be 940 cm−1, or seven times the homogeneous width, in the most monodisperse sample.

Spectroscopic studies of ascorbate oxidase. Electronic structure of the blue copper sites.

Low-temperature electronic absorption and room temperature circular dichroism (CD) and magnetic circular dichroism (MCD) spectra are reported for ascorbate oxidase. Bands attributable to d-d electronic transitions in the type 1 (blue) coppers (2B2 ground state) have been observed at 5800, 10000, and 12000 cm-1 (1725, 1000, and 835 nm). The three bands are assigned to the transitions 2B2 leads to 2E, 2B2 leads to 2B1, and 2B2 leads to 2A1, respectively, in the slightly flattened tetrahedral blue copper centers. The ligand field theoretical treatment predicts a type 1 copper reduction potential of 417 mV. The MCD spectrum of ascorbate oxidase contains one major band at 14000 cm-1 (710 nm) and a very weak feature at 13 200 cm-1 (550 nm). The intensities of the CD and MCD bands are consistent with two or three type 1 coppers in each enzyme molecule, and the near-infrared spectroscopic results suggest that the electronic structures of the blue coppers are closely similar.

Optical hole burning and matrix effects in a phthalocyanine complex of ruthenium(II)

A doubling of the origin is observed in the low-temperature electronic absorption spectrum of the complex carbonyl(pyridine)-phthalocyaninatoruthenium(II) in glassy matrices. The effect is studied by optical hole burning and results suggest a non-degenerate singlet ground state of near C 2v symmetry to be invoked.

Electronic structure of various ferricenium systems as inferred from Raman, infrared, low-temperature electronic absorption, and electron paramagnetic resonance measurements

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTElectronic structure of various ferricenium systems as inferred from Raman, infrared, low-temperature electronic absorption, and electron paramagnetic resonance measurementsD. Michael Duggan and David N. HendricksonCite this: Inorg. Chem. 1975, 14, 5, 955–970Publication Date (Print):May 1, 1975Publication History Published online1 May 2002Published inissue 1 May 1975https://doi.org/10.1021/ic50147a001RIGHTS & PERMISSIONSArticle Views936Altmetric-Citations148LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Low Temperature Electronic Absorption Spectra of Oxidized and Reduced Spinach Ferredoxins. Evidence for Nonequivalent Iron(III) Sites

The electronic absorption spectra of oxidized and reduced spinach ferredoxins have been measured between 1200 and 600 nm at low temperature in D(2)O/ethylene glycol glasses. Relatively weak absorption bands are observed at 720, 820, and 920 nm in oxidized ferredoxin, and at 652, 820, and 920 nm in reduced ferredoxin. The spectral results show that the two Fe(III) centers in oxidized ferredoxin are not equivalent, and that the 820- and 920-nm bands are associated with the nonreducible site. Assignment of the reducible site as tetrahedral Fe(III) is indicated. The 720-nm (13.9 kcm(-1)) band in oxidized ferredoxin is attributed to an intensity-enhanced (6)A(1) --> (4)T(1)d-d transition, whereas the 652-nm (15.3 kcm(-1)) feature of reduced ferredoxin could be due either to (5)E --> (3)T(1) in tetrahedral Fe(II)S(4) or an Fe(II) --> Fe(III) intervalence excitation.