Dynamics Of Spin Probes Research Articles

On the Mutual Relationships between Molecular Probe Mobility and Free Volume and Polymer Dynamics in Organic Glass Formers: cis-1,4-poly(isoprene).

We report on the reorientation dynamics of small spin probe 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) in cis-1,4-poly(isoprene) (cis-1,4-PIP10k) from electron spin resonance (ESR) and the free volume of cis-1,4-PIP10k from positron annihilation lifetime spectroscopy (PALS) in relation to the high-frequency relaxations of cis-1,4-PIP10k using light scattering (LS) as well as to the slow and fast processes from broadband dielectric spectroscopy (BDS) and neutron scattering (NS). The hyperfine coupling constant, 2Azz′(T), and the correlation times, τc(T), of cis-1,4-PIP10k/TEMPO system as a function of temperature exhibit several regions of the distinct spin probe TEMPO dynamics over a wide temperature range from 100 K up to 350 K. The characteristic ESR temperatures of changes in the spin probe dynamics in cis-1,4-PIP10k/TEMPO system are closely related to the characteristic PALS ones reflecting changes in the free volume expansion from PALS measurement. Finally, the time scales of the slow and fast dynamics of TEMPO in cis-1,4-PIP10k are compared with all of the six known slow and fast relaxation modes from BDS, LS and NS techniques with the aim to discuss the controlling factors of the spin probe reorientation mobility in polymer, oligomer and small molecular organic glass-formers.

Direct Prediction of EPR Spectra from Lipid Bilayers: Understanding Structure and Dynamics in Biological Membranes.

Of the many biophysical techniques now being brought to bear on studies of membranes, electron paramagnetic resonance (EPR) of nitroxide spin probes was the first to provide information about both mobility and ordering in lipid membranes. Here, we report the first prediction of variable temperature EPR spectra of model lipid bilayers in the presence and absence of cholesterol from the results of large scale fully atomistic molecular dynamics (MD) simulations. Three types of structurally different spin probes were employed in order to study different parts of the bilayer. Our results demonstrate very good agreement with experiment and thus confirm the accuracy of the latest lipid force fields. The atomic resolution of the simulations allows the interpretation of the molecular motions and interactions in terms of their impact on the sensitive EPR line shapes. Direct versus indirect effects of cholesterol on the dynamics of spin probes are analysed. Given the complexity of structural organisation in lipid bilayers, the advantage of using a combined MD‐EPR simulation approach is two‐fold. Firstly, prediction of EPR line shapes directly from MD trajectories of actual phospholipid structures allows unambiguous interpretation of EPR spectra of biological membranes in terms of complex motions. Secondly, such an approach provides an ultimate test bed for the up‐to‐date MD simulation models employed in the studies of biological membranes, an area that currently attracts great attention.

Electron Paramagnetic Resonance Spectroscopic Investigation of the Dynamics of Spin Probe in Room Temperature Ionic Liquid

Rotational motion of the spin probe 4-hydroxyl-2,2,6,6- tetramethyl-1-piperidinyloxy (TEMPOL) in methyl imidazoium based ionic liquid was studied by Electron Paramagnetic Resonance spectroscopy (EPR). The influence of rotational tumbling of the paramagnetic spin probe on the width and shape of EPR transition was studied to calculate rotational correlation time, equilibrium radius, and rotational diffusion coefficient of spin probe in different ionic liquid media; CnmimPF6/NTf2 (n=4, 6, 8). The glass transition temperatures for CnmimNTf2 (n=4, 6) were calculated from the inflection point of τc vs T plots whereas activation energy of the rotation were calculated from the Arrhenius plots of lnD vs 1/ T(K).

Spin probe dynamics in relation to free volume in crystalline organics from ESR and PALS: Cyclohexane

A joint external probe ESR and PALS study on cyclohexane (CHX) using stable free radical 2,2,6,6-tetra-methyl-1-piperidinyloxy (TEMPO) or ortho-positronium (o-Ps), respectively, is reported. The slow to fast transition of TEMPO occurs at the characteristic ESR temperature T50G coinciding with the solid to solid transition temperature, TSS. Moreover, o-Ps lifetime τ3(T50G)=2.0ns is close to an empirical rule τ3(T50G)=2.17±0.15ns found for a series of amorphous glass-formers indicating that main transition of the TEMPO connected with the ‘rigid’ to plastic crystalline state transition in CHX occurs under the similar local free volume condition as in amorphous media.

Spin probe dynamics in relation to free volume in crystalline organics from ESR and PALS: N-tridecane

A joint external probe ESR and PALS study on n-tridecane (n-TRD) using stable free radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) or ortho-positronium (o-Ps), respectively, is presented. In contrast to n-hexadecane (n-HXD) [Physica B 430, 99 (2013)], five crossovers in the spin probe TEMPO mobility coinciding with those in the o-Ps probe annihilation parameters at the characteristic ESR and PALS temperatures in the solid phase were revealed: 1) acceleration within the slow regime at TX1slow in accordance with the changes in both o-Ps lifetime, τ3, and relative o-Ps intensity, I3, at Tb1cr* in the rigid crystalline solid are related to very local end methyl group dynamics, 2) slow to fast regime transition at T50G<<TmDSC is similarly as for n-HXD ascribed to the spin probe TEMPO dynamics in the expanding interlamellar gap due to the molecular probe deformation induced enhanced end-chain mobility close to the rigid to soft crystalline solid transition at Tb1cr being connected also with the starting dominance of electron detrapping over positron mobility, 3) the first change within the fast regime at TX1fast coinciding with reduction of I3 at Tb2cr** is related to strong electron detrapping due to longitudinal disordering and 4) the next one at TX2fast* is related to the soft to plastic crystalline solid phase transition at Tb2cr*=TssDSC=0.95TmDSC. Finally, similarly as for n-HXD the highest T crossover at TX2fast≅TmPALS is related with the collective chain dynamics of the melting transition TmDSC. The underlying structural-dynamic processes behind the observed ESR and PALS crossover coincidencies are discussed in the light of the present thermodynamic DSC and reported dynamic NMR, QENS and IR data.

Spin probe dynamics of n-hexadecane in confined geometry

A combined study of the rotational dynamics of the stable free radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and the phase behavior of n-hexadecane (n-HXD) in the bulk and the confined states in a series of silica gels (SG) by means of ESR and DSC is presented. A slow to fast motion transition of the spin probe TEMPO in the bulk n-HXD occurs at T 50 G,bulk ≪ T m,bulk, i.e., well below the melting temperature due to its trapping and localized mobility in the interlamellar gap of the crystallites [J. Bartos, H. Svajdlenkova, M. Zaleski, M. Edelmann, M. Lukesova, Physica B 430, 99 (2013)]. On the other hand, the dynamics of the TEMPO in the confined systems is strongly slowing down with T 50 G (D pore) > T m (D pore) and slightly increases with the pore size D pore = 60, 100 and 300 A of the SG’s. At the same time, both the corresponding melting temperature, T m (D pore), and melting enthalpy, Δ H m (D pore), decrease with D pore together with the mutual anti-correlation between T 50 G and T m as a function of the inverse of pore diameter, 1/D pore. Moreover, the dynamic heterogeneity of the TEMPO in the confined state below T 50 G (D pore) is closely related to the phase transformation. The strong slowing down of the spin probe motion likely results from its preferential localization at the interface layer of the matrix pore due to specific interaction of TEMPO molecules with the polar silanol groups of the SG matrix. This is supported by special study on a series of the variously filled n-HXD/SG systems, other similar experimental findings as well as by theoretical spectral argument.

Spin probe dynamics in relation to free volume in crystalline organics by means of ESR and PALS: n-Hexadecane

We report on a combined study of the guest dynamics and free volume in n-hexadecane (n-HXD) using two external microscopic probes: stable free radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and ortho-positronium (o-Ps) by means of ESR or PALS, respectively. Dynamic behavior of the molecular TEMPO probe and annihilation properties of the atomistic o-Ps one as a function of temperature are compared. Two coincidencies between the crossover effects at the characteristic ESR and PALS temperatures T50G and Tb1cr below and TX2fast and TmPALS at the melting point TmDSC of n-HXD from macroscopic DSC scan were found. First, the slow to fast regime transition is ascribed to the spin probe TEMPO localization in the expanding interlamellar gap due to the induced enhanced end-chain mobility. Second, the high-temperature crossover within the fast regime is directly connected to the collective chain movements at the melting which takes place under the similar free volume fluctuation condition as revealed recently for a series of amorphous small molecular and polymer glass-formers.

The Spin Probe Dynamics and the Free Volume in a Series of Amorphous Polymer Glass‐Formers

AbstractWe report the results of a combined study of the local structure and the reorientation dynamics in a series of five amorphous polymers of different fragility: cis‐trans‐1,4‐poly(butadiene) (c‐t‐1,4‐PBD), cis‐1,4‐poly(isoprene) (cis‐1,4‐PIP), poly(isobutylene) (PIB), poly(vinyl methylether)(PVME) and poly (propylene glycol) (PPG) by using two different probe methods. The reorientation dynamics of the molecular spin probe 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) from electron spin resonance (ESR) is related to the annihilation behaviour of the atomic ortho‐positronium (o‐Ps) one as obtained by positron annihilation lifetime spectroscopy (PALS). It was found that a slow to fast transition in the spin probe rotation mobility at the operationally defined spectral temperature parameter, T50G, is connected with the mean o‐Ps lifetime, τ3 (T50G) = (2.04 ± 0.26) ns. Consequently, using the free‐volume concept of the o‐Ps annihilation in terms of a quantum‐mechanical model of o‐Ps lifetime this transition can be connected with the occurrence of the mean free volume hole, Vh (T50G) = (102 ± 17) Å3, nearly independent of the chemical composition and the basic structural relaxation parameters of the amorphous polymers investigated. Finally, the free volume hole distribution aspect of the slow to fast transition indicates the presence of a sufficient free volume fluctuation at T50G for both typical fragile PVME and strong PIB polymer and emphasizes the essential role of free volume in the spin probe dynamics.

Adsorption and molecular dynamics of spin probes on hydrophobizated silica gel microparticles: EPR spin-label study

The molecular dynamics parameters and hydrophobicity of the surface of Silasorb silica gel particles covalently modified with alkyl CnH2n + 1 groups differing in length (n = 2, 8, 18) were studied by electron paramagnetic resonance using spin probes of different structures, sizes, and electrical charges. It has been shown that the hydrocarbon shells of the silica gel microparticles in liquid suspensions provide high rotational and conformational mobility and a very high hydrophobicity of the environment for the incorporated molecules. Some regularities in the dependences of the rotational mobility, orientation ordering, and environmental hydrophobicity have on the hydrocarbon shell thickness have been established for spin probes of different structures: a radical containing an aromatic residue which models a fluorescent sensor molecule, lipid spin probes sensitive to the microviscosity and orientation ordering, and positively charged probes sensitive to electrostatic interactions. It has been found that the rotational mobility of the lipid spin probes sharply decreases upon removing the solvent and recovers in the presence of water or water-ethanol vapors. The high values of rotational mobility and environmental hydrophobicity for the spin probes located in the hydrocarbon shell of the Silasorb particles, which are dispersed in suspensions or in the solvent vapors, allow one to conclude that these microparticles can be perspective matrices for molecular optical chemosensors for analytes in solutions or the gas phase.

Protein structural dynamics revealed by site-directed spin labeling and multifrequency EPR

Multifrequency electron paramagnetic resonance (EPR), combined with site-directed spin labeling, is a powerful spectroscopic tool to characterize protein dynamics. The lineshape of an EPR spectrum reflects combined rotational dynamics of the spin probe's local motion within a protein, reorientations of protein domains, and overall protein tumbling. All these motions can be restricted and anisotropic, and separation of these motions is important for thorough characterization of protein dynamics. Multifrequency EPR distinguishes between different motions of a spin-labeled protein, due to the frequency dependence of EPR resolution to fast and slow motion of a spin probe. This gives multifrequency EPR its unique capability to characterize protein dynamics in great detail. In this review, we analyze what makes multifrequency EPR sensitive to different rates of spin probe motion and discuss several examples of its usage to separate spin probe dynamics and overall protein dynamics, to characterize protein backbone dynamics, and to resolve protein conformational states.

Low-temperature electron-spin relaxation in the crystalline and glassy states of solid ethanol

X-band electron paramagnetic resonance (EPR) spectroscopy was used to study the spectral properties of a nitroxide spin probe in ethanol glass and crystalline ethanol, at 5 - 11.5 K. The different anisotropy of molecular packing in the two host matrices was evidenced by different rigid limit values for maximal hyperfine splitting in the signal of the spin probe. The significantly shorter phase memory time, , for the spin probe dissolved in crystalline ethanol, as compared to ethanol glass, was discussed in terms of contribution from spectral diffusion. The effect of low-frequency dynamics was manifested in the temperature dependence of and in the difference between the data measured at different spectral positions. This phenomenon was addressed within the framework of the slow-motional isotropic diffusion model [S. Lee, and S. Z. Tang, Phys. Rev. B 31, 1308 (1985)] predicting the spin probe dynamics within the millisecond range, at very low temperatures. The shorter spin-lattice relaxation time of the spin probe in ethanol glass was interpreted in terms of enhanced energy exchange between the spin system and the lattice in the glass matrix due to boson peak excitations.

Spin probe reorientation and its connections with free volume and relaxation dynamics: Diglycidyl-ether of bis-phenol A

We report the dynamical behavior of a series of four spin probes of different geometries in amorphous van der Waals-type glass-former diglycidyl-ether of bis-phenol A (DGEBA) as measured by electron spin resonance spectroscopy (ESR). To reveal the relative influence of physical factors influencing spin probe dynamics, the reorientation properties were related to the free volume data of DGEBA from positron annihilation lifetime spectroscopy (PALS) and to the relaxation results of DGEBA from broadband dielectric spectroscopy (BDS). The characteristic ESR temperatures (T50G,TX2Azz′) determined from the temperature dependences of the spectral parameter of mobility, 2Azz′, for all spin probes and those (TX1τ and TX2τ) from the rotational correlation time, τc, as a function of temperatures for the smallest spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (Tempo) were found to coincide with the characteristic PALS temperatures Tb1, Tb2, and Tσ3 from phenomenological analyses of the temperature dependencies of the mean o-Ps lifetime, τ3, and the width of the o-Ps lifetime distribution, σ3. Moreover, for the smallest spin probe Tempo, the favorable mutual relationship between the spin probe size VPW and hole size distribution g(Vh) and the closeness of T50G with Tb1 point to the role of free volume in the slow to rapid regime crossover. On the other hand, for the three larger spin probes T50G lies in the vicinity of Tb2∼Tσ3 and is independent of their size, mass, and shape. The origin of these ESR and PALS coincidences can be traced out to the secondary γ-process for the smallest spin probe Tempo and to the primary α-process for all larger spin probes.

A Non-Standard Temperature Dependence of the Order Parameter of the 5CB Liquid Crystal Doped with an Azo-Derivative

The effects of the cis and trans form of the 4-OMe-phenylazobenzene at 1% and 7% mole fraction on order, mesophase stability and dynamics of the 5CB liquid crystal were studied using the ESR spin probe technique. The presence of the trans azo-derivative induced a downshift of the nematic-isotropic transition temperature T NI which increased with the concentration but did not change the dependence of the order parameter on the reduced temperature. The cis isomer caused even larger TNI shifts, determining a marked decrease of the order parameter with a peculiar dependence on the reduced temperature, quite different from the standard Haller-type behavior. The spin probe dynamics appeared instead to be essentially unchanged by the presence of the azo-derivative.

Dynamics of the nitroxide side chain in spin-labeled proteins.

The dynamics of the tether linking methanethiosulfonate (MTSSL) spin probes to alpha-helices has been investigated with the purpose of rationalizing its effects on ESR line shapes. Torsional profiles for the chain bonds have been calculated ab initio, and steric interactions with the alpha-helix and the neighboring residues have been introduced at the excluded-volume level. As a consequence of the restrictions deriving from chain geometry and local constraints, a limited number of allowed conformers has been identified that undergo torsional oscillations and conformational jumps. Torsional fluctuations are described as damped oscillations, while transition rates between conformers are calculated according to the Langer multidimensional extension of the Kramers theory. The time scale and amplitude of the different motions are compared; the major role played by rotations of the outermost bonds of the side chain emerges, along with the effects of substituents in the pyrroline ring on the conformer distribution and dynamics. The extent and symmetry of magnetic tensor averaging produced by the side chain motions are estimated, the implications for the ESR spectra of spin-labeled proteins are discussed, and suggestions for the introduction of realistic features of the spin probe dynamics into the line shape simulation are presented.

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.

Motional heterogeneity of polystyrene- block-polybutadiene: a spin probe study

Polystyrene- block-polybutadiene copolymers (SB) with 0.5 mass fraction of styrene were studied by electron spin resonance (ESR) of nitroxide spin probes. The influence of the block length ( M ̄ w =12,000, M ̄ w =48,000 and M ̄ w =83,000 ) and the solvation power of casting solvents on the motional dynamics of spin probe were measured over a wide temperature range. Two nitroxide radicals as spin probes were selected: 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl benzoate (BzONO) and 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl (Tempol). Irrespective of the spin probe used two ESR spectral components differing in their motional properties above the phase transition of polybutadiene blocks (PB) were observed. The fast component was assigned to spin probes located in polybutadiene-rich domains and the slow component to spin probes in polystyrene-rich domains. The range of two spectral components and the phase transition of the slow ESR component, T 5mT, depend on the block length. The influence of the interphase and accumulation of free volume in the interphase on the Tempol probe motion was investigated by changing copolymer morphology in the films casted from selective and nonselective solvents. The analysis of the motional heterogeneity from the ratio of the fast and slow motional component presents evidence that in the selective solvent for polystyrene (PS) blocks (2-butanone) the most irregular structure with a large interphase is formed. The difference in fast motion of spin probes indicates that the motional dynamic is related to the change of domain structure.

Rotational dynamics ESR investigation of the cholestane spin probe dissolved in a narrow dispersity poly(ethyl acrylate): effect of the presence of trace amounts of free monomer

The dynamics of cholestane spin probes dissolved in poly(ethyl acrylate) (PEA) matrices was fully investigated over a wide temperature range by means of ESR spectroscopy. A PEA sample with narrow molecular weight dispersion (PEA1) and a second one (PEA2) doped with about 2.7 wt.-% free ethyl acrylate were employed. Depending on temperature, different dynamical regimes were identified. The coupling between the spin probe dynamics and structural, β, and γ relaxation processes was recognised. At higher temperatures, Vogel-Fulcher laws well described the temperature dependence of the spin probe rotation, which tracks the backbone dynamics with different coupling degrees. The experiments provide the chance of evaluating the onset of cooperativity in the different PEA matrices. The information is obtained by comparing the temperature dependence of probe rotation with that of rheological measurements. Moreover, the dynamics of cholestane in PEA2 shows an Arrhenius regime in the highest temperature region, which exhibits a complete decoupling from the backbone dynamics. Cholestane spin probe.

Fractionary couplings of spin probe to backbone and side group dynamics of a liquid crystal polymer

The cholestane spin probe dynamics in a nematic polymeric matrix after annealing at a temperature value slightly above the TNI was investigated by ESR spectroscopy in a wide temperature interval. Careful lineshape analysis showed that a two δ-like distribution better represented the experimental spectra. The dependence on temperature of the population in the different sites was evaluated and compared with the analogous one previously found for the same polymer/spin probe system that had been annealed at a temperature well above the TNI.1 The temperature dependence of the spinning correlation time of the spin probe rearranging in slow and fast sites led us to locate them in the backbone and side groups of the polymer. The cooperativity degree in dynamics was quantified. A signature in temperature scale at the onset of cooperativity in backbone dynamics was recognized. Chemical structure of the azobenzene polymethacrylate (PMA).

Free volume structure and spin probe dynamics in solid polymers

Analysis of the structure of nine polymers, in terms of the average size of free volume at T 50G for spin probe Tempo, reveals an empirical connection between this microstructural characteristic of matrices and the change in reorientational regime of the spin probe.

Structure of Drawn Gel-Crystallized UHMWPE as Revealed by Spin-Probe Technique

Abstract The volume distribution and molecular dynamics of spin-probe (nitroxyl radical) in the gel-crystallized UHMWPE of different draw ratios (DR = 12,100) have been investigated. The spin-probes were found to occupy approximately 50% of the polymer volume in the necked samples and 20–30% in the samples drawn up to DR = 100. The mobility of the spin-probe in the samples studied is higher than that in the drawn and even in the undrawn samples of conventional melt-crystallized PE. This fact along with a low value of activation energy of spin-probe rotation (less than 10 kJ/mole) enables a conclusion that the regions of spin-probe localization are those with a significantly reduced density, such as pores, for examples. The analysis of the data obtained and the results of the structural study of the gel-crystallized samples by a SAXS and WAXS technique suggest that these pores are localized in interfibrillar space.