Narrow Electron Paramagnetic Resonance Line Research Articles

Chemical Gardens Mimic Electron Paramagnetic Resonance Spectra and Morphology of Biogenic Mn Oxides.

Manganese (Mn) oxides are ubiquitous in nature and occur as both biological and abiotic minerals, but empirically distinguishing between the two remains a problem. Recently, electron paramagnetic resonance (EPR) spectroscopy has been proposed for this purpose. It has been reported that biogenic Mn oxides display a characteristic narrow linewidth in contrast to their pure abiotic counterparts, which is explained in part by the large number of cation vacancies that form within the layers of biogenic Mn oxides. It was, therefore, proposed that natural samples that display a narrow EPR linewidth, ΔHpp < 580G, could be assigned to a biogenic origin. However, in poorly crystalline or amorphous solids, both dipolar broadening and exchange narrowing simultaneously determine the linewidth. Considering that the spectral linewidth is governed by several mechanisms, this approach might be questioned. In this study, we report synthetic chemical garden Mn oxide biomorphs that exhibit both morphologically life-like structures and narrow EPR linewidths, suggesting that a narrow EPR line may be unsuitable as reliable evidence in assessment of biogenicity.

Microstructure and Conduction Electron Quantum Properties of Small Diamond Cubic α-Sn Nanocrystals Embedded in Cubic Boron Nitride Crystals.

The morphology, structure, composition, and conduction electron properties of quasi-spherical tin nanocrystals (NCs) of 2.5 nm average diameter, with unstrained, bulk-like α-Sn diamond cubic structure, observed in dark cubic boron nitride (cBN) crystallites, were determined by correlated analytical high-resolution scanning transmission electron microscopy and multifrequency electron spin resonance (ESR) investigations. The narrow Lorentzian ESR line with g = 2.0028 is attributed to the conduction ESR of the α-Sn NCs, consistent with the temperature- and frequency-independent small g-shift and intensity reduction under high temperature (950 °C) vacuum annealing when the α-Sn NCs are thermally dissolved in the host cBN crystallites. The ESR linewidth and line intensity vs temperature dependences recorded in the 20 to 295 K range are quantitatively described considering the presence of discrete, quantum confinement-induced conduction electron energy levels with ΔQC/k B = 125 K separation, close to the theoretical value for conductive α-Sn NCs of 2.5 nm in diameter. The observed properties are tentatively explained with the predicted nanosize induced band-gap opening and change of band ordering from bulk α-Sn to small unstrained α-Sn NCs, resulting in a topological phase transition that also explains the predominantly s-like character of the conduction band electron orbitals.

Weak Antiferromagnet Iron Borate FeBO3. Classical Object for Magnetism and the State of the Art

The simple lattice and magnetic structure, the high Neel temperature, the narrow antiferromagnetic resonance line of FeBO3, and the narrow electron paramagnetic resonance line of its isostructural diamagnetic analogs MBO3:Fe3+ (M = Ga, In, Sc, Lu) make iron borate unique for investigations and applications. Iron borate is a model crystal for numerous experimental and theoretical studies, including spin crossovers and metallization at megabar pressures and many-electron effects in optics and X-ray spectroscopy. The recent works dealing with the investigation of the properties of FeBO3 are reviewed.

Oxygenic photosynthesis: EPR study of photosynthetic electron transport and oxygen-exchange, an overview.

In this review, we consider the applications of electron paramagnetic resonance (EPR) methods to the study of the relationships between the electron transport and oxygen-exchange processes in photosynthetic systems of oxygenic type. One of the purposes of this article is to encourage scientists to use the advantageous EPR oximetry approaches to study oxygen-related electron transport processes in photosynthetic systems. The structural organization of the photosynthetic electron transfer chain and the EPR approaches to the measurements of molecular oxygen (O2) with O2-sensitive species (nitroxide spin labels and solid paramagnetic particles) are briefly reviewed. In solution, the collision of O2 with spin probes causes the broadening of their EPR spectra and the reduction of their spin-lattice relaxation times. Based on these effects, tools for measuring O2 concentration and O2 diffusion in biological systems have been developed. These methods, named "spin-label oximetry," include not only nitroxide spin labels, but also other stable-free radicals with narrow EPR lines, as well as particulate probes with EPR spectra sensitive to molecular oxygen (lithium phthalocyanine, coals, and India ink). Applications of EPR approaches for measuring O2 evolution and consumption are illustrated using examples of photosynthetic systems of oxygenic type, chloroplasts in situ (green leaves), and cyanobacteria.

BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21.1 T.

Dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (NMR) has developed into an invaluable tool for the investigation of a wide range of materials. However, the sensitivity gain achieved with many polarizing agents suffers from an unfavorable field and magic angle spinning (MAS) frequency dependence. We present a series of new hybrid biradicals, soluble in organic solvents, that consist of an isotropic narrow electron paramagnetic resonance line radical, α,γ-bisdiphenylene-β-phenylallyl (BDPA), tethered to a broad line nitroxide. By tuning the distance between the two electrons and the substituents at the nitroxide moiety, correlations between the electron-electron interactions and the electron spin relaxation times on one hand and the DNP enhancement factors on the other hand are established. The best radical in this series has a short methylene linker and bears bulky phenyl spirocyclohexyl ligands. In a 1.3 mm prototype DNP probe, it yields enhancements of up to 185 at 18.8 T (800 MHz 1H resonance frequency) and 40 kHz MAS. We show that this radical gives enhancement factors of over 60 in 3.2 mm sapphire rotors at both 18.8 and 21.1 T (900 MHz 1H resonance frequency), the highest magnetic field available today for DNP. The effect of the rotor size and of the microwave irradiation inside the MAS rotor is discussed. Finally, we demonstrate the potential of this new series of polarizing agents by recording high field 27Al and 29Si DNP surface enhanced NMR spectra of amorphous aluminosilicates and 17O NMR on silica nanoparticles.

Defects induced in cerium dioxide single crystals by electron irradiation

Micro-Raman spectroscopy, X-band electron paramagnetic resonance (EPR) spectroscopy, and UV-visible optical absorption spectroscopy were used to study the damage production in cerium dioxide (CeO2) single crystals by electron irradiation for three energies (1.0, 1.4, and 2.5 MeV). The Raman-active T2g peak was left unchanged after 2.5-MeV electron irradiation at a high fluence. This shows that no structural modifications occurred for the cubic fluorite structure. UV-visible optical absorption spectra exhibited a characteristic sub band-gap tail for 1.4-MeV and 2.5-MeV energies, but not for 1.0 MeV. Narrow EPR lines were recorded near liquid-helium temperature after 2.5-MeV electron irradiation; whereas no such signal was found for the virgin un-irradiated crystal or after 1.0-MeV irradiation for the same fluence. The angular variation of these lines in the {111} plane revealed a weak g-factor anisotropy assigned to Ce3+ ions (with the 4f1 configuration) in a high-symmetry local environment. It is concluded that Ce3+ ions may be produced by a reduction resulting from the displacement damage process. However, no evidence of F+ or F0 center or hole center formation due to irradiation was found from the present EPR and optical absorption spectra.

Spin Resonance Clock Transition of the Endohedral Fullerene ^{15}N@C_{60}.

The endohedral fullerene ^{15}N@C_{60} has narrow electron paramagnetic resonance lines which have been proposed as the basis for a condensed-matter portable atomic clock. We measure the low-frequency spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. We infer a linewidth at the clock field of 100kHz. Using experimental data, we are able to place a bound on the clock's projected frequency stability. We discuss ways to improve the frequency stability to be competitive with existing miniature clocks.

Triarylmethyl Radicals: EPR Study of 13C Hyperfine Coupling Constants.

Triarylmethyl (TAM) radicals are widely used in Electron Paramagnetic Resonance (EPR) spectroscopy as spin labels and in EPR imaging as spin probes for in vivo oxymetry. One of the key advantages of TAMs is extremely narrow EPR line, especially in case of deuterated analogues (~5 μT). Another advantage is their slow spin relaxation even at physiological temperatures allowing, in particular, application of pulsed dipolar EPR methods for distance measurements in biomolecules. In this paper a large series of TAM radicals and their deuterated analogues is synthesized, and corresponding spectroscopic parameters including 13C hyperfine constants are obtained for the first time. The negligible dependence of 13C hyperfine constants on solvent, as well as on structure and number of substituents at para-C atoms of aromatic rings, has been found. In addition, we have demonstrated that 13C signals at natural abundance can be employed for successful room-temperature distance measurements using Pulsed Electron Double Resonance (PELDOR or DEER).

Multi-Frequency Pulsed Overhauser DNP at 1.2 Tesla

Abstract Dynamic nuclear polarization (DNP) is a methodology to increase the sensitivity of nuclear magnetic resonance (NMR) spectroscopy. It relies on the transfer of the electron spin polarization from a radical to coupled nuclear spins, driven by microwave excitation resonant with the electron spin transitions. In this work we explore the potential of pulsed multi-frequency microwave excitation in liquids. Here, the relevant DNP mechanism is the Overhauser effect. The experiments were performed with TEMPOL radicals in aqueous solution at room temperature using a Q-band frequency (1.2 T) electron paramagnetic resonance (EPR) spectrometer combined with a Minispec NMR spectrometer. A fast arbitrary waveform generator (AWG) enabled the generation of multi-frequency pulses used to either sequentially or simultaneously excite all three 14N-hyperfine lines of the nitroxide radical. The multi-frequency excitation resulted in a doubling of the observed DNP enhancements compared to single-frequency microwave excitation. Q-band free induction decay (FID) signals of TEMPOL were measured as a function of the excitation pulse length allowing the efficiency of the electron spin manipulation by the microwave pulses to be extracted. Based on this knowledge we could quantitatively model our pulsed DNP enhancements at 1.2 T by numerical solution of the Bloch equations, including electron spin relaxation and experimental parameters. Our results are in good agreement with theoretical predictions. Whereas for a narrow and homogeneous single EPR line continuous wave excitation leads to more efficient DNP enhancements compared to pulsed excitation for the same amount of averaged microwave power. The situation is different for radicals with several hyperfine lines or in the presence of inhomogeneous line broadening. In such cases pulsed single/multi-frequency excitation can lead to larger DNP enhancements.

Electron-spin resonance of transparent conductive oxide β-Ga2O3

β-Ga2O3 crystals are one of the transparent conductive oxides (TCOs). The n-type conductivity of β-Ga2O3 is attributed to an oxygen deficit. In order to examine a mechanism of the conductivity, electron-spin resonance (ESR) measurements were carried out for β-Ga2O3 doped with various impurity ions (Mn, Cr, Si, Zr, Hf). A single narrow ESR line with an anisotropic g value (~1.96) was observed for Si, Zr, or Hf doped β-Ga2O3 and ascribed to “motional narrowing” of electrons trapped around oxygen vacancies, namely, conduction electrons. Extinction of the narrow ESR line for Mn or Cr doped β-Ga2O3 may be due to a decrease of the conduction electron density and/or a strong spin–spin interaction between the conduction electron and the paramagnetic ions. The fine structure of the ESR spectrum for Mn or Cr doped β-Ga2O3 gives evidence that Mn2+ and Cr3+ ions substitute for octahedral Ga3+ ions in the lattice without charge compensators.

Synthesis of Trityl Radical-Conjugated Disulfide Biradicals for Measurement of Thiol Concentration

Measurement of thiol concentrations is of great importance for characterizing their critical role in normal metabolism and disease. Low-frequency electron paramagnetic resonance (EPR) spectroscopy and imaging, coupled with the use of exogenous paramagnetic probes, have been indispensable techniques for the in vivo measurement of various physiological parameters owing to the specificity, noninvasiveness and good depth of magnetic field penetration in animal tissues. However, in vivo detection of thiol levels by EPR spectroscopy and imaging is limited due to the need for improved probes. We report the first synthesis of trityl radical-conjugated disulfide biradicals (TSSN and TSST) as paramagnetic thiol probes. The use of trityl radicals in the construction of these biradicals greatly facilitates thiol measurement by EPR spectroscopy since trityls have extraordinary stability in living tissues with a single narrow EPR line that enables high sensitivity and resolution for in vivo EPR spectroscopy and imaging. Both biradicals exhibit broad characteristic EPR spectra at room temperature because of their intramolecular spin-spin interaction. Reaction of these biradicals with thiol compounds such as glutathione (GSH) and cysteine results in the formation of trityl monoradicals which exhibit high spectral sensitivity to oxygen. The moderately slow reaction between the biradicals and GSH (k(2) ∼ 0.3 M(-1) s(-1) for TSSN and 0.2 M(-1) s(-1) for TSST) allows for in vivo measurement of GSH concentration without altering the redox environment in biological systems. The GSH concentration in rat liver was determined to be 3.49 ± 0.14 mM by TSSN and 3.67 ± 0.24 mM by TSST, consistent with the value (3.71 ± 0.09 mM) determined by the Ellman's reagent. Thus, these trityl-based thiol probes exhibit unique properties enabling measurement of thiols in biological systems and should be of great value for monitoring redox metabolism.

Aluminum doped silicon carbide thin films prepared by hot-wire CVD: Investigation of defects with electron spin resonance

Al-doped p-type μc-SiC:H is prepared in a wide range of HWCVD preparation parameters like Al-doping ratio, deposition pressure, substrate and filament temperatures. We investigate the structural and electrical properties, and focus on identification of paramagnetic defect states by electron spin resonance (ESR). Nominally undoped μc-SiC:H is of a high n-type conductivity (σ D = 10 − 6 –10 − 1 S/cm) and shows a narrow central ESR line (g ≈ 2.003, peak-to-peak linewidth ∆H pp ≈ 4 G) with two pairs of satellites and a spin density N S = 10 19 cm − 3 . Al-doping results in the compensation of dark conductivity to as low as σ D = 10 − 11 S/cm and at higher doping concentrations to effective p-type material. Increase of Al-doping results in reduction of crystallinity ( I C IR ), ESR line shifts to g ≈ 2.01 and becomes as broad as ∆H pp ≈ 30 G, not unlike to the resonance of singly occupied paramagnetic valence band tail states in a-Si:H. ESR spectrum of highly crystalline Al-doped μc-SiC:H however has a g-value very close to undoped μc-SiC:H. Electron spin density in compensated material decreases to 5 × 10 17 cm − 3 before it increases again for the highly doped material.

Molecular Details of Bax Activation, Oligomerization, and Membrane Insertion

Bax and Bid are pro-apoptotic members of the Bcl-2 protein family. Upon cleavage by caspase-8, Bid activates Bax. Activated Bax inserts into the mitochondrial outer membrane forming oligomers which lead to membrane poration, release of cytochrome c, and apoptosis. The detailed mechanism of Bax activation and the topology and composition of the oligomers are still under debate. Here molecular details of Bax activation and oligomerization were obtained by application of several biophysical techniques, including atomic force microscopy, cryoelectron microscopy, and particularly electron paramagnetic resonance (EPR) spectroscopy performed on spin-labeled Bax. Incubation with detergents, reconstitution, and Bid-triggered insertion into liposomes were found to be effective in inducing Bax oligomerization. Bid was shown to activate Bax independently of the stoichiometric ratio, suggesting that Bid has a catalytic function and that the interaction with Bax is transient. The formation of a stable dimerization interface involving two Bcl-2 homology 3 (BH3) domains was found to be the nucleation event for Bax homo-oligomerization. Based on intermolecular distance determined by EPR, a model of six adjacent Bax molecules in the oligomer is presented where the hydrophobic hairpins (helices alpha5 and alpha6) are equally spaced in the membrane and the two BH3 domains are in close vicinity in the dimer interface, separated by >5 nm from the next BH3 pairs.

Spin dynamics and charge transfer in C60 · 2ferrocene studied by electron spin resonance

Electron spin resonance (ESR) has been applied to study spin dynamics and charge transfer interactions in C60(Fe(C5H5)2)2. A low concentration of C60− spins is derived by ESR, suggesting the presence of weak charge transfer between C60 and ferrocene. The C60− ESR spectra display an appreciable anisotropy at T<40K reflecting molecular rotation freezing, while their low temperature variation (T<10K) implies development of antiferromagnetic spin–spin interactions. At high temperatures, the narrow ESR line of C120O− impurity spins inhibits observation of a C60− ESR.

Ageing effect in nanocrystalline TiCx/C studied by EPR

Abstract TiC/C nanocrystalline material: titanium carbide TiC dispersed in a carbon matrix has been prepared by a nonhydrolytic sol–gel process. Temperature dependence of the electron paramagnetic resonance (EPR) spectra of this material has been studied in the 3.5–120 K range. Two very different EPR lines have been recorded in fresh sample at temperatures below 120 K arising from the Ti(III) complex (broad and asymmetric line) and conduction electrons (very narrow line). In the same aged sample (1 year old) the magnetic anisotropy of Ti(III) line has increased while a narrow line attributed to conduction electrons has vanished. The existence of the paramagnetic centers connected with trivalent titanium ions could the result of disordering processes. The increase of anisotropy in Ti(III) line could be connected with the oxidation processes. The temperature dependence of the integrated intensity of the broad line revealed the presence of titanium antiferromagnetic dimers. The disappearance of a narrow EPR line suggests that the oxidation process (ageing effect) could influence also the electrical properties of titanium carbide.

Paramagnetic defects and exchange coupled spins in pristine ultrananocrystalline diamonds

Electron paramagnetic resonance (EPR) and magnetic susceptibility measurements were done on nanodiamond samples fabricated by the detonation method and purified by acids. Comprehensive acid treatment leads to the reduction of EPR signals of magnetic impurities and revealing two weak and narrow EPR lines with g 1 = 4.26, ▵ H pp = 2.9 mT and g 2 = 4.00, ▵ H pp = 1.4 mT at T = 4 K, separated by the distance of 10.4 mT. The origin of this new doublet EPR signal observed in the well purified sample is discussed. The magnetic susceptibility behavior and doublet EPR signal ( g ∼ 4) suggest the weak antiferromagnetically exchange coupling of S = 1/2 paramagnetic defects as well as the presence of isolated dimers with S = 1.

Oligothiophene Bipyridine Alternate Copolymers and Their Ruthenium Metalated Analogues: In Situ ESR and UV−Vis Investigations of Metal−Chain Interactions

In situ electron spin resonance (ESR) and UV-vis spectro-electrochemical studies have been performed on two copolymers consisting of alternating subunits of regioregular head to tail (HT) coupled 3-octylthiophene tetramer and 2,2'-bipyridine subunits (P4) or 3-octylthiophene hexamer subunits of the same regioregularity and 2,2'-bipyridine subunits (P6). Both P4 and P6 have been investigated in their metal-free form as well as in the ruthenium(II) metalated form (P4-Ru and P6-Ru). P4 and P6 in the p-doped state exhibit a clear ESR signal characteristic of the presence of polarons in the oligothienylene subunits. In the case of P4, no recombination of polarons into bipolarons is observed, whereas the recombination process takes place in P6. The formation of bipolarons is well-rationalized in terms of the conjugation length, and it seems clear that the higher length of the oligothiophene subunit in P6( )()stabilizes bipolarons(.)() The same effect, is induced by the coordination of -Ru(bpy)(2)(2+) to the bipyridine unit in the metalated form of both polymers, which results in an increase of the conjugation length. Important information is gained from the analysis of the ESR spectra of both nonmetalated and metalated in the oxidized (p-doped) and reduced (n-doped) forms. In the p-doped state both nonmetalated and metalated polymers reveal the presence of a narrow ESR line characteristic of the mobile spin carriers in the polymer matrix. The oxidation of the metal center occurs at higher potentials and leads to an irreversible destruction of the system. To the contrary, in the reduced (n-doped) state the ESR lines of the nonmetalated and metalated polymers markedly differ. A significant line broadening with simultaneous change of the g-value is caused by spin-orbit coupling phenomenon induced by the presence of the coordinating metal. Finally, the observation of a clear polaronic band in the UV-vis spectrum of p-doped P4 and its strong dependence on the applied potential can be clearly correlated with the potential induced changes in the ESR spin density. The same applies to P4-Ru, where the changes in the polaronic and bipolaronic bands can also be correlated with the ESR spin density changes.

Electron paramagnetic resonance and other allied studies of sol–gel derived nanocrystallinechromium oxides in a silica glass matrix

Monolithic silica gels doped with various concentrations (1, 5, 10 mol%) of chromium oxide areprepared by a sol–gel process and are subjected to calcination at different temperatures up to1200 °C. This has principally led to the formation ofCr2O3 nanocrystalsand also of CrO3 nanocrystals at the higher calcination temperatures. The sizes of theCr2O3 nanocrystals have been determined by x-ray diffraction (XRD) and transmission electronmicroscopy (TEM) studies and have been found to lie in the range 5–30 nm depending upon thedopant concentration as well as the calcination temperature. The formation of nonmagneticCrO3 crystals in 10 mol% chromium oxide doped silica glass calcined at1200 °C as a result ofoxidation of Cr5+ ions to Cr6+ ions has been confirmed from XRD studies. Optical, infrared and X-band electron paramagneticresonance (EPR) spectra of these nanocrystals have also been studied. EPR behaviours such as theg-factor andlinewidth of Cr2O3 nanocrystals investigated at room and liquid nitrogen temperatures havebeen found to be quite distinct from those of antiferromagnetic (AF) bulkCr2O3 crystals and may be manifestations of superparamagnetism/ferromagnetism in thesenanocrystals. A narrow anisotropic EPR line () has been detected for chromium oxide doped silica gel calcined at800 °C and has beenassigned to isolated Cr5+ ions in a tetragonally distorted tetrahedral environment. Studies of the thermaldependence of zero-field-cooled (ZFC) and field-cooled (FC) magnetization(σ) 10 mol% chromium oxide doped silica glasses calcined at800 °C by aSQUID magnetometer in the 5–300 K temperature range have revealed the occurrence of the blockingtemperature TB at about 250 K, which is a characteristic feature of superparamagnetism inCr2O3 nanocrystals. The superparamagnetic effects are almost quenched in the samples calcined at 1000and 1200 °C because of the presence of larger nanocrystals in these samples. Magnetichysteresis effects in these calcined samples at 300 K as well as at 5 K in the ± 6 T magnetic field range have also been investigated. The existence of ferromagnetic order atlow temperature (5 K) as well as at room temperature (300 K) (for samples calcined athigher temperatures) has been confirmed.

Metallic Phase in Lightly DopedLa2−xSrxCuO4Observed by Electron Paramagnetic Resonance

In the low doping range of $x$ from 0.01 to 0.06 in ${\mathrm{L}\mathrm{a}}_{2\ensuremath{-}x}{\mathrm{S}\mathrm{r}}_{x}{\mathrm{C}\mathrm{u}\mathrm{O}}_{4}$, a narrow electron paramagnetic resonance (EPR) line has been investigated. This line is distinct from the known broad line and both lines are due to probing ${\mathrm{M}\mathrm{n}}^{2+}$ ions. The narrow line is ascribed to metallic regions in the material, and its intensity increases exponentially upon cooling below $\ensuremath{\sim}150\text{ }\text{ }\mathrm{K}$. The activation energy deduced $\ensuremath{\Delta}=460(50)\text{ }\text{ }\mathrm{K}$ is nearly the same as that found in the doped superconducting regime by Raman and neutron scattering. The intensity of the narrow EPR line follows the same temperature dependence as the in-plane resistivity anisotropy in lightly doped ${\mathrm{L}\mathrm{a}}_{2\ensuremath{-}x}{\mathrm{S}\mathrm{r}}_{x}{\mathrm{C}\mathrm{u}\mathrm{O}}_{4}$ single crystals.

Electrochemical Preparation and EPR Studies of Lithium Phthalocyanine. 3. Measurements of Oxygen Concentration in Tissues and Biochemical Reactions

A systematic investigation of the preparation, characterization, and use of lithium phthalocyanine (LiPc) microcrystalline powder as a probe for electron paramagnetic resonance (EPR) oximetry applications has recently been initiated (Ilangovan et al. J. Phys. Chem. B 2000, 104, 4047−4059). Electrochemical preparation of LiPc under potentiostatic conditions (+0.4 V) yielded microcrystalline paramagnetic particles that showed extremely narrow (<10 mG) and oxygen-dependent EPR line width measured at 9.82 GHz (Ilangovan et al. J. Phys. Chem. B 2000, 104, 9404−9410). In this report, we demonstrate the application of this material to measure oxygen concentration in a biochemical (xanthine/xanthine oxidase, X/XO), cellular (human polymorphonuclear neutrophils, PMNs), and in vivo (mice) biological systems. The microcrystalline particles showed high oxygen-sensitivity and stability in aqueous and physiological environments. The particles implanted in the gastrocnemius muscle of living mice were found to be stable ...