Proton-mediated dynamics of the alkaline conformational transition of yeast iso-1-cytochrome c

The kinetics of the alkaline conformational transition of a Lys 73-->His variant of iso-1-cytochrome c have been investigated using pH jump stopped-flow methods to probe the nature of the ionizable "trigger" group for this conformational change. This mutation moves the pK(a) of the ligand replacing Met 80 from about 10.5 to approximately 6.6 and has unmasked two other ionizable groups, besides the ligand replacing Met 80, that modulate the kinetics of this process. The results are discussed in terms of the impact of ionization equilibria on protein folding mechanisms.     

EPR spectroscopic investigation of the sol-gel transition in modified tetraethylorthosilicate gels

Cu(2+) ions are used to follow the sol-gel transition in amino-modified TEOS mixtures. In contrast to unmodified TEOS gels, fixation of the spin probe (Cu(2+)) to the sol particle surface takes place and changes in mobility are detectable. Furthermore, we find changes in the coordination sphere of the copper ions and a temporary loss of EPR intensity. By using a simple kinetic model it is possible to estimate the rate constants of the condensation and syneresis processes.     

Assessing the Functional and Structural Stability of the Met80Ala Mutant of Cytochrome c in Dimethylsulfoxide

The Met80Ala variant of yeast cytochrome c is known to possess electrocatalytic properties that are absent in the wild type form and that make it a promising candidate for biocatalysis and biosensing. The versatility of an enzyme is enhanced by the stability in mixed aqueous/organic solvents that would allow poorly water-soluble substrates to be targeted. In this work, we have evaluated the effect of dimethylsulfoxide (DMSO) on the functionality of the Met80Ala cytochrome c mutant, by investigating the thermodynamics and kinetics of electron transfer in mixed water/DMSO solutions up to 50% DMSO v/v. In parallel, we have monitored spectroscopically the retention of the main structural features in the same medium, focusing on both the overall protein structure and the heme center. We found that the organic solvent exerts only minor effects on the redox and structural properties of the mutant mostly as a result of the modification of the dielectric constant of the solvent. This would warrant proper functionality of this variant also under these potentially hostile experimental conditions, that differ from the physiological milieu of cytochrome c.     

Phase transition in non-centrosymmetric 2-methyl-5-nitroanilinium dihydrogen phosphate: structural,spectroscopic and optical studies

Structural Chemistry - A reversible phase transition in 2-methyl-5-nitroanilinium dihydrogen phosphate, (H2m5na)H2PO4, has been found and characterized. Lattice parameters of (H2m5na)H2PO4 change...     

Multifrequency high-field EPR study of the tryptophanyl and tyrosyl radical intermediates in wild-type and the W191G mutant of cytochrome c peroxidase.

Multifrequency (95, 190, and 285 GHz) high-field electron paramagnetic resonance (EPR) spectroscopy has been used to characterize radical intermediates in wild-type and Trp191Gly mutant cytochrome c peroxidase (CcP). The high-field EPR spectra of the exchange-coupled oxoferryl--trytophanyl radical pair that constitutes the CcP compound I intermediate [(Fe(IV)=O) Trp*(+)] were analyzed using a spin Hamiltonian that incorporated a general anisotropic spin-spin interaction term. Perturbation expressions of this Hamiltonian were derived, and their limitations under high-field conditions are discussed. Using numerical solutions of the completely anisotropic Hamiltonian, its was possible to simulate accurately the experimental data from 9 to 285 GHz using a single set of spin parameters. The results are also consistent with previous 9 GHz single-crystal studies. The inherent superior resolution of high-field EPR spectroscopy permitted the unequivocal detection of a transient tyrosyl radical that was formed 60 s after the addition of 1 equiv of hydrogen peroxide to the wild-type CcP at 0 degrees C and disappeared after 1 h. High-field EPR was also used to characterize the radical intermediate that was generated by hydrogen peroxide addition to the W191G CcP mutant. The g- values of this radical (g(x)= 2.00660, g(y) = 2.00425, and g(z)= 2.00208), as well as the wild-type transient tyrosyl radical, are essentially identical to those obtained from the high-field EPR spectra of the tyrosyl radical generated by gamma-irradiation of crystals of tyrosine hydrochloride (g(x)= 2.00658, g(y) = 2.00404, and g(z) = 2.00208). The low g(x)-value indicated that all three of the tyrosyl radicals were in electropositive environments. The broadening of the g(x) portion of the HF-EPR spectrum further indicated that the electrostatic environment was distributed. On the basis of these observations, possible sites for the tyrosyl radical(s) are discussed.     

Electron spectroscopic studies of the adsorption and decomposition of methanol on transition metals. A review

Results of investigations of the adsorption and decomposition of methanol on the surface of transition metals such as Fe, Ni, Cu, Pd, Ag, Mo, W and Pt byuv and x-ray photoelectron spectroscopy, electron energy loss spectroscopy, Auger electron spectroscopy and thermal desorption spectroscopy have been reviewed. The first step in the decomposition of CH₃OH on these metal surfaces is the formation of the methoxy species, OCH₃ radical. In the case of Fe, Mo and W, complete decomposition of CH₃OH occurs leaving CO(β), H₂ and CH₄ on the surface. Dissociation proceeds upto CO(α) and H₂ on the surface of Ni, Pd and Pt whereas on Ag and Cu, selective oxidation of CH₃OH to H₂CO is preferred. The difference in the reactivity of metals towards CH₃OH is rationalised from the heats of adsorption of O₂, CO and H₂ on these metals. Contribution No. 253 from the Solid State and Structural Chemistry Unit.     

Detection of metaphosphate intermediates in reaction solutions of phosphate esters: Chromatographic and spectroscopic studies

This study was undertaken to gain a better understanding of uncatalysed phosphoryl transfer reactions under neutral pH conditions in aqueous media. A combination of chromatographic (Ultra high performance liquid chromatography, UHPLC) and spectroscopic (Quadrupole time-of-flight-mass spectrometry- qTOF-MS;) UV-vis; and FTIR techniques were utilized to probe the mechanism of reaction under physiological conditions (aqueous medium; pH 7.0; 25°C). Mass spectroscopic data from these studies reveal that the phosphate monoesters, phenyl phosphate (PP) and 4-nitrophenyl phosphate (NPP) undergo collision-induced dissociation dynamics through the formation of the metaphosphate intermediate, [PO₃]^? – m/z 79, a process that was found to be independent of the pKa of the leaving group. Under conditions of the present experiment, the metaphosphate intermediate does not undergo protonation to either metaphosphoric acid, HPO₃ or the dibasic phosphonic acid, H₃PO₃.     

Configuration-selective spectroscopic studies of Er3+ centers in ErSc2N@C80 and Er2ScN@C80 fullerenes

Low temperature photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy of high purity ErSc(2)N@C(80) and Er(2)ScN@C(80) fullerenes reveal at least two metastable configurations of the Er(3+) ion within the cage, consistent with previous observations from x-ray diffraction. Using PLE measurements at a number of different emission wavelengths we have characterized the ground state, (4)I(152), and the first excited state, (4)I(132), of the various Er(3+) configurations and their crystal-field splitting. We present detailed energy level diagrams for the ground and excited states of the two dominant configurations of ErSc(2)N@C(80) and Er(2)ScN@C(80).     

EPR and optical studies of erbium-doped beta-PbF2 single-crystals and nanocrystals in transparent glass-ceramics

beta-PbF(2) single-crystals and nanocrystals in transparent glass-ceramics doped with ErF(3) have been synthesized and studied with two complementary techniques: electron paramagnetic resonance (EPR) and optical spectroscopy (absorption, selective excitation, fluorescence). A comparative study shows that, in both single-crystals and glass-ceramics, Er(3+) ions occupy the same types of sites, leading to similar optical properties. An EPR investigation demonstrates that, in these materials, part of the Er(3+) ions occupy cubic symmetry sites. For these ions, we determine the crystal field splitting of the ground state (4)I(15/2) and the symmetry of its sublevels. We also provide evidence for the presence of another type of Er(3+) ions, not detectable by EPR but evidenced by optical spectroscopy. We clearly show that this Er(3+), which gives rise to up-conversion luminescence, corresponds to clusters associating Er(3+) and F(-) ions. In the single-crystals, the proportion of these two types of erbium ions is estimated. It strongly depends on the doping rate of the beta-PbF(2) crystals.     

EPR and optical absorption studies of VO(II)-doped zinc Tris thiourea sulfate.

Electron paramagnetic resonance (EPR) of vanadyl ion as an impurity in polycrystalline zinc tris thiourea sulfate (ZTS), a prominent nonlinear optical host, has been studied at X-band frequencies at room temperature. Spin-Hamiltonian parameters and molecular orbital coefficients have been calculated. The optical absorption spectra have also been studied to deduce the crystal field parameters.     

EPR, 1H and 2H NMR, and reactivity studies of the iron-oxygen intermediates in bioinspired catalyst systems

Complexes [(BPMEN)Fe(II)(CH(3)CN)(2)](ClO(4))(2) (1, BPMEN = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane) and [(TPA)Fe(II)(CH(3)CN)(2)](ClO(4))(2) (2, TPA = tris(2-pyridylmethyl)amine) are among the best nonheme iron-based catalysts for bioinspired oxidation of hydrocarbons. Using EPR and (1)H and (2)H NMR spectroscopy, the iron-oxygen intermediates formed in the catalyst systems 1,2/H(2)O(2); 1,2/H(2)O(2)/CH(3)COOH; 1,2/CH(3)CO(3)H; 1,2/m-CPBA; 1,2/PhIO; 1,2/(t)BuOOH; and 1,2/(t)BuOOH/CH(3)COOH have been studied (m-CPBA is m-chloroperbenzoic acid). The following intermediates have been observed: [(L)Fe(III)(OOR)(S)](2+), [(L)Fe(IV)═O(S)](2+) (L = BPMEN or TPA, R = H or (t)Bu, S = CH(3)CN or H(2)O), and the iron-oxygen species 1c (L = BPMEN) and 2c (L = TPA). It has been shown that 1c and 2c directly react with cyclohexene to yield cyclohexene oxide, whereas [(L)Fe(IV)═O(S)](2+) react with cyclohexene to yield mainly products of allylic oxidation. [(L)Fe(III)(OOR)(S)](2+) are inert in this reaction. The analysis of EPR and reactivity data shows that only those catalyst systems which display EPR spectra of 1c and 2c are able to selectively epoxidize cyclohexene, thus bearing strong evidence in favor of the key role of 1c and 2c in selective epoxidation. 1c and 2c were tentatively assigned to the oxoiron(V) intermediates.     

Electrochemical and FTIR spectroscopic studies of tyrosine oxidation at polycrystalline platinum surfaces in alkaline solutions

The electrochemical oxidation of tyrosine has been studied on polycrystalline platinum in sodium hydroxide using cyclic voltammetry and in situ Fourier transform infrared spectroscopy (FTIRS). According to infrared data, the first interaction of tyrosine through the carboxylate group on platinum is proposed. The results showed an onset for tyrosine oxidation at ca. 0.5 V with the formation of soluble formate ion as product. The sharp increase in the current vs potential slope from ca. 1.1 V obeys the detection of cyanate ions together with p-(methyl)-phenoxide ions. No adsorbed carbon monoxide as well as carbon dioxide was detected by this methodology in the whole studied potential range. These results contrast with those obtained in acid solution where two parallel reactions, one yielding the γ-lactone of hydroxy benzofurane and the other producing p-benzoquinone occurs for the oxidation of tyrosine on platinum. Dedicated to Prof. Dr. Teresa Iwasita on the occasion of her 65th birthday in recognition of her numerous contributions to interfacial electrochemistry.     

EPR and optical absorption studies of VO2+ doped L-arginine phosphate monohydrate single crystals--part I.

The electron paramagnetic resonance (EPR) studies on VO2+ doped L-arginine phosphate monohydrate (LAP) single crystals at room temperature at X-band frequencies reveal the presence of two magnetically inequivalent VO2+ sites occupying interstitial positions in the lattice with fixed orientations and show very high angular dependence. The principal values of the g and A tensors indicate that the electrostatic field around the VO2+ ion is rhombic. The optical absorption spectra at room temperature show four absorption bands at 16155, 14775, 10928 and 10526 cm(-1), characteristic of rhombic symmetry. From EPR and optical absorption data, the molecular orbital bonding coefficients (beta2, epsilon2, P and k) and the crystal field parameters have been evaluated.     

EPR and DFT studies of the structure of phosphinyl radicals complexed by a pentacarbonyl transition metal

Paramagnetic complexes M(CO)5P(C6H5)2, with M = Cr, Mo, W, have been trapped in irradiated crystals of M(CO)5P(C6H5)3 (M = Cr, Mo, W) and M(CO)5PH(C6H5)2 (M = Cr, W) and studied by EPR. The radiolytic scission of a P-C or a P-H bond, responsible for the formation of M(CO)5P(C6H5)2, is consistent with both the number of EPR sites and the crystal structures. The g and 31P hyperfine tensors measured for M(CO)5P(C6H5)2 present some of the characteristics expected for the diphenylphosphinyl radical. However, compared to Ph2P*, the 31P isotropic coupling is larger, the dipolar coupling is smaller, and for Mo and W compounds, the g-anisotropy is more pronounced. These properties are well predicted by DFT calculations. In the optimized structures of M(CO)5P(C6H5)2 (M = Cr, Mo, W), the unpaired electron is mainly confined in a phosphorus p-orbital, which conjugates with the metal d(xz) orbital. The trapped species can be described as a transition metal-coordinated phosphinyl radical.     

Crystal growth and vibrational spectroscopic studies of the semiorganic non-linear optical crystal--bisthiourea zinc chloride

The semiorganic non-linear optical crystal bisthiourea zinc chloride (BTZC) was grown by mixed solvent slow evaporation technique. The solubilities under various solvents in different proportions have been studied. Vibrational spectra were recorded to determine the symmetries of molecular vibrations. The observed Raman and infrared bands were also assigned and discussed. The optical transmission spectral study was also carried out. The property of second harmonic generation of BTZC was also verified.     

Crystal growth and vibrational spectroscopic studies of the semiorganic non-linear optical crystal--bisthiourea magnesium sulphate

The semiorganic non-linear optical crystal bisthiourea magnesium sulphate (BTMS) was grown by slow evaporation technique using water as solvent. Vibrational spectra were recorded to determine the symmetries of molecular vibrations. The observed Raman and infrared bands were also assigned and discussed. The optical transmission spectral study was carried out to test the transmitting ability of the crystal in the visible range. The second harmonic generation test of BTMS revealed the non-linear nature of the crystal. The TGA/DTA curve was also recorded for the experimental crystal.     

Theoretical studies on the optical spectra and EPR parameters for trigonal Yb3+ center in CsCdBr3 crystal

In this paper, the crystal-field energy levels, the EPR g factors g//, g(perpendicular) of Yb3+ and hyperfine structure constants A//, A(perpendicular) of 171Yb3+ and 173Yb3+ isotopes in CsCdBr3 crystal are calculated from the crystal-field theory. The calculated results (seven energy levels and six EPR parameters) are in reasonable agreement with the observed values. In the calculation, we find that Yb3+ ion does not occupy the exact Cd2+ site, but is shifted from the center of bromine octahedron by a distance (Delta)Z approximately 0.184 angstroms along C3 axis. The results are discussed.     

Single crystal EPR and optical absorption studies of Cu2+ ion in L-arginine sulphophosphate monohydrate--a nonlinear optical crystal

The EPR spectra of Cu2+ ion in L-arginine sulphophosphate monohydrate (LASP), a nonlinear optical material (NLO) have been studied at room temperature. Two magnetically inequivalent interstitial Cu2+ sites in the lattice were identified. The principal values of g- and A-tensors indicate that the symmetry of the electrostatic field around the Cu2+ ion is rhombic. Bonding parameters have been evaluated and the ground state wave function of Cu2+ ion is constructed. The ground state is found to be an admixure of x2 - y2 and 3z2 - r2 orbitals. By correlating EPR and optical absorption spectra the crystal field parameters have been evaluated.     

Coordination chemistry of the alkaline earth metal ions with Zwitterionic forms of the Schift bases. X-Ray studies and other spectroscopic properties

The non-ionized forms of tetradentate Schiff bases NN′-ethylenebis(salicylideneimine), H₂L and NN′-propane-1,3-diylbis(salicylideneimine), H₂L′ react with hydrated alkaline earth halide and nitrate to give complexes of the type: M(H₂L)Cl₂·nH₂O [M = Mg(II), Ca(II), Sr(II); n = 0–4], M(H₂L)₂Cl₂ [M = Ca(II), Sr(II), M(H₂L)_nBr₂ [M = Ca(II), Sr(II); n = 2, 3 and Mg₂(H₂L)₃Br₄], M(H₂L)_nI₂ [M = Mg(II), Ca(II), Sr(II), Ba(II); n = 2, 3)], M(H₂L)_n(NO₃)₂ and M(H₂L′)_n(NO₁)₂[M = Mg(II), Ca(II);n = 1, 2)]. Because of distinct spectral similarities with structurally known Ca(H₂L′)(NO₃)₂ compound, the Schiff bases are coordinated through the negatively charged phenolic oxygen atoms and not the nitrogen atoms of the azomethine groups, which carry the protons transferred from phenolic groups on complexation. Halide and nitrate are coordinated to the central metal ion except in 2:1 nitrato complexes where the presence of both ionic and coordinated nitrate groups are evident and also in 3:1 halide complexes where the presence of non-coordinated halide cannot be excluded. X-Ray powder photographs showed no marked similarities between Ca(H₂L′)(NO₃)₂ and Mg(H₂L′)(NO₃)₂ while there are some isomorphic features between the same types of halide complexes. Infrared spectra and other structural information revealed the polymeric nature of the complexes. Therefore the coordination numbers exhibited by the alkaline earth metal cations would be 4, 6 or 8 in these series of Schiff base complexes.