Simulation Of ESR Spectra Research Articles

The orientation order of liquid‐crystalline polymer, measured by ESR and optical dichroism techniques

ABSTRACTMolecular orientation order and rotation mobility of comb‐shaped liquid crystalline polymer forming nematic and smectic mesophases are characterized by spin probe and dichroism technique in the temperature range 100–420 K. The models describing the mechanism of molecular rotations in different temperature ranges were determined by quantitative numerical simulations of ESR spectra. In the vicinity of the glass transition point of the polymer, where the fast amplitude‐restricted molecular reorientations take place, the quasi‐libration model was found to be adequate for the description of ESR spectra, while the often used Brownian rotational diffusion is appropriate at higher temperatures only. Orientation distribution function of probe molecules was characterized quantitatively by order parameters up to sixth rank for a wide temperature range. The orientation order parameters measured via the spin probe technique are shown to correlate with orientation order parameters measured by optical techniques. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 819–825

Spectroscopic study of Mg(II) ion influence on the autoxidation of gallic acid in weakly alkaline aqueous solutions

Gallic acid autoxidation in weakly alkaline aqueous solutions was studied by UV-Vis spectrophotometry and ESR spectroscopy under various conditions. Lowering the pH value from 10 to 8.5 probably changes the mechanism of the autoxidation reaction as evidenced by the different time variations of UV-Vis spectra of solutions. The presence of Mg(II) ions greatly influences the autoxidation reaction at pH 8.5. Although the UV-Vis spectral changes with time follow the similar pattern during the gallic acid autoxidation at pH 10 and at pH 8.5 in the presence of Mg(II) ions, some small differences indicate that Mg(II) ions not only affect the electron density of absorbing species but also influence the overall mechanism of the autoxidation reaction. ESR spectra of free radials formed during the initial stage of gallic acid autoxidation at pH 8.5 in the presence of Mg(II) ions were recorded. Computer simulation of ESR spectra allows partial characterization of these free radicals.

Simulation of ESR spectra of metal nanoparticle aggregates

Aggregates of polydisperse particles characterized by the preset values of the fractal dimensions and prefactor are built with the use of a special algorithm. ESR spectra of such aggregates are calculated in the self-consistent field approximation. It is shown that, even in sufficiently large aggregates with fractal structure, spectra greatly depend on the character of packing of large and small particles in the aggregate.

Modeling the effects of structure and dynamics of the nitroxide side chain on the ESR spectra of spin-labeled proteins.

In the companion paper (J. Phys. Chem. B 2006, 110, jp0629487), a study of the conformational dynamics of methanethiosulfonate spin probes linked at a surface-exposed alpha-helix has been presented. Here, on the basis of this analysis, X-band ESR spectra of these spin labels are simulated within the framework of the Stochastic Liouville equation (SLE) methodology. Slow reorientations of the whole protein are superimposed on fast chain motions, which have been identified with conformational jumps and fluctuations in the minima of the chain torsional potential. Fast chain motions are introduced in the SLE for the protein reorientations through partially averaged magnetic tensors and relaxation times calculated according to the motional narrowing theory. The 72R1 and 72R2 mutants of T4 lysozyme, which bear the spin label at a solvent-exposed helix site, have been taken as test systems. For the side chain of the R2 spin label, only a few noninterconverting conformers are possible, whose mobility is limited to torsional fluctuations, yielding almost identical spectra, typical of slightly mobile nitroxides. In the case of R1, more complex spectra result from the simultaneous presence of constrained and mobile chain conformers, with relative weights that can depend on the local environment. The model provides an explanation for the experimentally observed dependence of the spectral line shapes on temperature, solvent, and pattern of substituents in the pyrroline ring. The relatively simple methodology presented here allows the introduction of realistic features of the spin probe dynamics into the simulation of ESR spectra of spin-labeled proteins; moreover, it provides suggestions for a proper account of such dynamics in more sophisticated approaches.

Thermodynamic Analysis of the Chemical Exchange of β−Phosphorylated Cyclic Nitroxides by Using Two-dimensional (Temperature versus Magnetic Field) Simulation of ESR Spectra: The Impact of Labile Solvent−Solute Interactions on Molecular Dynamics

A complete thermodynamic characterization of the chair-to-chair interconversion in beta-diphosphorylated piperidine-N-oxyl radicals was achieved by means of the analysis of temperature-dependent ESR spectra. A new two-dimensional simulation method was developed with the coordinates temperature and magnetic field, in which the entire set of spectra was simulated simultaneously by adjusting the coefficients in the power expansion, giving the temperature-dependent ESR parameters and the thermodynamic and kinetic parameters determining the site populations and exchange rates, respectively. The new method promotes elimination of the ambiguities inherent in the spectroscopic determination of thermodynamic parameters. Labile solvent-solute interactions can strongly influence the chemical exchange, producing a complex network of symmetric and asymmetric interconversions. The solvent dependence of magnetic relaxation was also analyzed.

Control of formation and molecular orientation of J-aggregates in Langmuir–Blodgett films of mixed merocyanine dyes

We have fabricated Langmuir–Blodgett (LB) films of mixed-merocyanine dyes, DS and its methyl-substituted analog (6-Me-DS), diluted with arachidic acid (AA), expressed as [6-Me-DS] 1− X [DS] X [AA] 2, in order to control the J-aggregation of dye molecules. For most of X -values, the red-shifted J-band peak appeared in the optical absorption spectra with its peak position depending on the mixing ratio of the dyes. In particular, J-band anomalously disappears around X = 0.9 , suggesting the dominance of monomer state. We have also characterized the in-plane preferential orientation of the dye molecules along the dipping direction in the mixed-dye films by the orientation-dependent ESR signals and polarized optical absorption spectra. The optical dichroic ratios were semiquantitatively reproduced by the angular distribution function of the molecular orientation determined by the ESR spectrum simulation. The in-plane orientation of the dye molecules was clearly observed for DS, 6-Me-DS and X = 0.2 films, whereas it was nearly random for X = 0.5 film, suggesting the possibility to control the molecular orientational order of dye molecules by employing mixed-dye system.

A new program based on stochastic Liouville equation for the analysis of superhyperfine interaction in CW-ESR spectroscopy

In the slow-motion region, ESR spectra cannot be expressed as a sum of simple Lorentzian lines. Studies of Freed and co-workers, on nitroxides in liquids gained information on the microscopic models of rotational dynamics, relying much on computer programs for simulation of ESR spectra based on the stochastic Liouville equation (SLE). However, application of Freed’s method to copper system of biological interest has been for a long time precluded by lack of a full program able to simulate ESR spectra containing more than one hyperfine interaction. Direct extension of the Freed’s approach in order to include superhyperfine interaction is not difficult from a theoretical point of view but the resulting algorithm is problematical because it leads to substantial increase in the dimensions of the matrix related to the spin-hamiltonian operator. In this paper preliminary results of a new program, written in C, which includes the superhyperfine interactions are presented. This preliminary version of the program does not take into account a restoring potential, so it can be used only in isotropic diffusion conditions. A comparison with an approximate method previously developed in our laboratory, based on a post-convolution approach, is discussed.

Models of slow rotational mobility of paramagnetic probes in glassy media

Experimental ESR spectra of the 2,2,6,6-tetramethyl-4-oxopiperidinoxyl (TEMPON) radical probe in the glycerol, polystyrene, and polyvinylbutyral matrices measured in the temperature range 77–373 K were quantitatively compared with the ESR spectra calculated using the known theoretical models of rotational mobility. It was shown that simulation of ESR spectra by the nonlinear least-squares method is an efficient procedure for discriminating between theoretical models. The temperature ranges were determined in which it is possible to achieve quantitative agreement between experimental and theoretical spectra as well as the ranges in which theoretical models are insufficient to quantitatively describe the experimental results. It was established that the widely used model of Brownian diffusion in isotropic medium is inadequate to describe the ESR spectra in the case of slow motions of small probe molecules. It was found that specific interactions (formation of weak complexes) between the probe molecules and the molecules of the medium results in strongly anisotropic molecular rotational motions.

ESR studies of photo-generated polarons in undoped poly (p-phenylene vinylene)/PPV

Photo-generated spin species in undopedpoly (p-phenylene vinylene)/PPV and its derivative, CH 3O-PPV have been studied by means of the light-induced ESR (LESR) technique using variable excitation energy. The LESR signal increases with the incident photon-energy and it becomes significant above the threshold energy of 3.1 eV for PPV and 2.7 eV for CH 3O-PPV. Similar increase has been reported for the excitation spectra of photocarriers in MEH-PPV, suggesting that the light-induced spins are charged species. Furthermore, a clear anisotropy of a π-electron on the conjugated chain have been confirmed in the LESR spectra in stretch oriented PPV samples and they were well reproduced by the ESR spectrum simulation using a polaron spin density obtained from Pariser-Parr-Pople (PPP) model. From these results, we conclude that the observed light-induced spins are photo-generated polarons.

Lipid chain dynamics in diacylglycerol-phosphatidylcholine mixtures studied by slow-motional simulations of spin label ESR spectra

Simulation of the ESR spectra from spin-labelled phosphatidylcholine and from spin-labelled diacylglycerol has been used to study the lipid chain dynamics in mixtures of dimyristoyl glycerol with dimyristoyl phosphatidylcholine, as a function of temperature and composition. Successful simulation of the ESR spectra required taking into account slow motional components which was done by using the very anisotropic reorientation model. Chain rotational rates varied with temperature, but not greatly with composition and chain position, which primarily affected chain order. The latter decreased progressively with increasing diacylglycerol content from the lamellar, to inverted hexagonal, to isotropic phase and was consistently lower for the diacylglycerol component than for the phosphatidylcholine component. The order profile of the spin-labelled chains was obtained for both the phosphatidylcholine and diacylglycerol components in a 58:42 mol/mol mixture in the fluid lamellar phase.

Automatic computer simulations of ESR spectra

A versatility of automatic ESR simulation procedures has been developed to obtain high quality of fitting and produce additional information. The following examples are treated: derivation of long-range proton hyperfine coupling constants from unresolved lines; determination of13C hf couplings from the naturally abundant isotope satellites; analyzing chemical exchange phenomena with two-sites model; decomposition of superimposed spectra consisting of poorly resolved components. In order to achieve best agreement between calculated and experimental spectra within a reasonable number of iteration cycles, we combined various approaches, like consecutive independent parameter optimization, least-square approach, optimization on “serpentines” and optimization of compound parameters. The spectra can be computed both for liquid and solid state samples, for non-oriented, partially oriented and single crystal samples. Second order perturbation formulas are used for the spin Hamiltonian includingg- and hyperfine tensors that have isotropic, axial and rhombic symmetry. For chemical exchange the modified Bloch equation for two-site model is used. Various lineshapes, including Lorentzian, Gaussian, mixed, modulation and dispersion distorted forms are applied. Third order parabolic interpolations are used for building up spectra from individual lines. In order to correct sweep non-linearity a third order interpolation converts the spectrum to become equidistant. This procedure can occasionally improve square deviation by an order of magnitudes for well resolved spectra. In the analysis of strongly overlapping superimposed spectra, simultaneous adjustment of two independent experimental spectra can give an exact decomposition. The inclusion of long-range couplings offers highly perfect fitting and allows one to resolve contributions of naturally abundant13C isotopes.

Structure and Dynamics of Perfluorinated Ionomers in Aqueous Solutions and Swollen Membranes Based on Simulations of ESR Spectra from Spin Probes

Structure and Dynamics of Perfluorinated Ionomers in Aqueous Solutions and Swollen Membranes Based on Simulations of ESR Spectra from Spin Probes

Time-domain simulation of ESR spectra of nitroxide spin probes

A time-domain method of simulating the ESR spectrum of a nitroxide spin probe undergoing reorientational motion is described. The method reqiures as input a time trajectory of the stochastic orientation of the nitroxide radical. This trajectory may be generated from any arbitrary prescription for the mode of molecular motion. The potential of the method is demonstrated by the simulation of the ESR spectra of steroid nitroxide spin probes in oriented lipid bilayers over a large range of motional correlation times.

ESR Investigation of the Rotational Mobility of Spin Probes in Ethanol Solvent Adsorbed to .BETA.-Cyclodextrin.

Molecular interactions between spin probe reagents and β-cyclodextrin (β-CyD) in ethanol were investigated on the basis of the rotational correlation time of the spin probes as estimated by the ESR spectrum simulation. The effect of the functional groups of spin probes, possibly hydrogen-bonding interactions, on the hydroxylated surface of the β-CyD molecule were suggested from the thermodynamic data on the rotational correlation times, which could be combined with the rotational mobilities of spin probes confined in a microscopic liquid phase.

Free Radical Production During Photo-Assisted Nadph Reduction of four α-Nitroarenofurans

Generation of radical anions during NADPH reduction of four mutagenic and genotoxic alpha-nitroarenofurans was examined. ESR showed that free radicals were generated during reduction solely in the presence of light. Computer simulations of ESR spectra were in good agreement with the experimental ones.

Evidence of polyphenols nitrosation by computer aided ESR spectroscopy

Polyphenols play an important role as model systems in transition metal derivatives for the preparation of macromolecular systems. Among the metal ions ironnitrosyl coordination chemistry has received much attention in the past because of its important role in inorganic and biological processes. In the case of Fe(I)(NO)2 complexes with polyphenols ligands in solution, difficulties in the interpretation of the ESR spectra arise from complicated patterns due to simultaneous presence of different nitrogen nuclei directly bound to the metal ion or due to the presence of equilibria between species under slow exchange conditions. In order to overcome these difficulties the investigations reported here were carried out using computer simulation of ESR spectra combined with selective isotopic substitution of 14NO with 15NO. Resorcinol displays an unexpected nine lines ESR pattern at g=2.018 which can be explained only by considering more than two nitrogen atoms interacting with the unpaired electron delocalized over the metal complex.

Some recent applications of computer simulations of ESR spectra to the dynamics of spin probes in gels and membranes

Some recent applications of computer simulations of ESR spectra to the dynamics of spin probes in gels and membranes

Characterization of Free Radicals in γ-irradiated Polycrystalline Uridine 5′-monophosphate: A Study Combining ESR, Spin-trapping and HPLC

Free radicals generated in gamma-irradiated polycrystalline uridine 5'-monophosphate (5'-UMP) were studied by ESR, spin-trapping and high-performance liquid chromatography (HPLC). After gamma-irradiation at 0 degree C (70kGy), poly-crystalline 5'-UMP was dissolved in an anaerobic aqueous solution of 2-methyl-2-nitrosopropane as a spin trap at room temperature. Since an ESR spectrum consisting of several components was observed immediately after irradiation, these components were separated with reverse-phase HPLC in the ion-suppression mode and again analysed by ESR spectrometry. Although HPLC ultimately gave four spin-adducts, one component that was originally present disappeared during HPLC. Spin adducts due to two types of C6 radicals were identified. One of these was thought to be formed by electron addition and subsequent protonation at the C6 position, and the other was presumed to be produced by electron addition and subsequent protonation at the O4 position. The spin adducts derived from the C5 and C5' radicals were also identified. The spin adduct that disappeared during HPLC was thought to correspond to the C4'-centred radical. Computer simulation of ESR spectra was carried out to estimate the hyperfine splitting constants.

A very fast program for ESR spectrum simulation

A fast ESR spectrum simulation program suitable for radicals in solution is described. The speed of the convolution algorithm is achieved by the use of integer arithmetic and precalculated lineshapes. FDC-ESR is written in C and runs on an IBM PC and ATARI ST (obtain program via TCM-Online) computers. A math coprocessor is not necessary.

ENDOR Study of Benzothiazolone Azine Cation Radicals by Means of a TM110 Mode Cavity

Abstract The electron nuclear double resonance (ENDOR) spectra of benzothiazolone azine cation radicals were observed by means of an ENDOR spectrometer equipped with a TM110 mode cavity. Highly diluted solutions of the radicals and a high radiofrequency power of 300 W facilitated the ENDOR observation. The obtained ENDOR values are different from the previously published results determined only from the ESR data, especially for the substituted alkyl groups. The assignment of the hyperfine coupling constants was made on the basis of the molecular orbital calculations as well as the re-examination of the ESR spectrum simulation. Good agreement between the calculated and the measured spectra is obtained if one assumes that the hyperfine coupling constants of the two different sets of nitrogen nuclei are equal as has been pointed out earlier.