Echo-detected Spectra Research Articles

Electron Spin Relaxation of Tb3+ and Tm3+ Ions.

Electron spin relaxation times T1 and Tm of Tb3+ and Tm3+ in 1:1 water:ethanol and of Tb3+ doped (2%) in crystalline La2(oxalate)3 decahydrate were measured between about 4.2 and 10 K. Both cations are non-Kramers ions and have J = 6 ground states. Echo-detected spectra are compared with CW spectra and with field-stepped direct-detected EPR spectra. Due to the strong temperature dependence of T1, measurements were not made above 10 K. Between about 4.2 and 6 K T1 is strongly concentration dependent between 1 and ~50 mM. T1 values at 4.2 K are in the μs range which is orders of magnitude faster than for 3d transition metals. Phase memory times, Tm, are less than 500 ns, which is short relative to values observed for 3d transition metals and organic radicals at 4 K. Tm is longer in the oxalate lattice which is attributed to the lower proton concentration in oxalate than in the organic solvent, which decreases nuclear spin diffusion. The rigidity of the crystalline lattice also may contribute to longer Tm.

Triarylmethyl Radical: EPR Signal to Noise at Frequencies between 250 MHz and 1.5 GHz and Dependence of Relaxation on Radical and Salt Concentration and on Frequency.

In vivo oximetry by pulsed electron paramagnetic resonance is based on measurements of changes in electron spin relaxation rates of probe molecules, such as the triarylmethyl radicals. A series of experiments was performed at frequencies between 250 MHz and 1.5 GHz to assist in the selection of an optimum frequency for oximetry. Electron spin relaxation rates for the triarylmethyl radical OX063 as a function of radical concentration, salt concentration, and resonance frequency were measured by electron spin echo 2-pulse decay and 3-pulse inversion recovery in the frequency range of 250 MHz-1.5 GHz. At constant OX063 concentration, 1/T1 decreases with increasing frequency because the tumbling dependent processes that dominate relaxation at 250 MHz are less effective at higher frequency. 1/T2 also decreases with increasing frequency because 1/T1 is a significant contribution to 1/T2 for trityl radicals in fluid solution. 1/T2-1/T1, the incomplete motional averaging contribution to 1/T2, increases with increasing frequency. At constant frequency, relaxation rates increase with increasing radical concentration due to contributions from collisions that are more effective for 1/T2 than 1/T1. The collisional contribution to relaxation increases as the concentration of counter-ions in solution increases, which is attributed to interactions of cations with the negatively charged radicals that decrease repulsion between trityl radicals. The Signal-to-Noise ratio (S/N) of field-swept echo-detected spectra of OX063 were measured in the frequency range of 400 MHz-1 GHz. S/N values, normalized by √Q, increase as frequency increases. Adding salt to the radical solution decreased S/N because salt lowers the resonator Q. Changing the temperature from 19 to 37 °C caused little change in S/N at 700 MHz. Both slower relaxation rates and higher S/N at higher frequencies are advantageous for oximetry. The potential disadvantage of higher frequencies is the decreased depth of penetration into tissue.

X-band rapid-scan EPR of samples with long electron spin relaxation times: a comparison of continuous wave, pulse and rapid-scan EPR

X-band room temperature spectra obtained by rapid-scan, continuous wave, field-swept echo-detected and Fourier transform electron paramagnetic resonance (FTEPR) were compared for three samples with long electron spin relaxation times: amorphous hydrogenated silicon (T1 = 11 μs, T2 = 3.3 μs), 0.2% N@C60 solid (T1 = 120–160 μs, T2 = 2.8 μs) and neutral single substitutional nitrogen centres (NS0) in diamonds (T1 = 2300 μs, T2 = 230 μs). For each technique, experimental parameters were selected to give less than 2% broadening of the lineshape. For the same data acquisition times, the signal-to-noise for the rapid-scan spectra was one-to-two orders of magnitude better than for continuous wave or field-swept echo-detected spectra. For amorphous hydrogenated silicon, T2* (∼ 10 ns) is too short to perform FTEPR. For 0.2% N@C60, the signal-to-noise ratio for rapid scan is about five times better than for FTEPR. For NS0 the signal-to-noise ratio is similar for rapid scan and FTEPR.

Backbone Dynamics of Alamethicin Bound to Lipid Membranes: Spin-Echo Electron Paramagnetic Resonance of TOAC-Spin Labels

Alamethicin F50/5 is a hydrophobic peptide that is devoid of charged residues and that induces voltage-dependent ion channels in lipid membranes. The peptide backbone is likely to be involved in the ion conduction pathway. Electron spin-echo spectroscopy of alamethicin F50/5 analogs in which a selected Aib residue (at position n=1, 8, or 16) is replaced by the TOAC amino-acid spin label was used to study torsional dynamics of the peptide backbone in association with phosphatidylcholine bilayer membranes. Rapid librational motions of limited angular amplitude were observed at each of the three TOAC sites by recording echo-detected spectra as a function of echo delay time, 2τ. Simulation of the time-resolved spectra, combined with conventional EPR measurements of the librational amplitude, shows that torsional fluctuations of the peptide backbone take place on the subnanosecond to nanosecond timescale, with little temperature dependence. Associated fluctuations in polar fields from the peptide could facilitate ion permeation.

Multifrequency Pulsed EPR Studies of Biologically Relevant Manganese(II) Complexes.

Electron paramagnetic resonance studies at multiple frequencies (MF EPR) can provide detailed electronic structure descriptions of unpaired electrons in organic radicals, inorganic complexes, and metalloenzymes. Analysis of these properties aids in the assignment of the chemical environment surrounding the paramagnet and provides mechanistic insight into the chemical reactions in which these systems take part. Herein, we present results from pulsed EPR studies performed at three different frequencies (9, 31, and 130 GHz) on [Mn(II)(H(2)O)(6)](2+), Mn(II) adducts with the nucleotides ATP and GMP, and the Mn(II)-bound form of the hammerhead ribozyme (MnHH). Through line shape analysis and interpretation of the zero-field splitting values derived from successful simulations of the corresponding continuous-wave and field-swept echo-detected spectra, these data are used to exemplify the ability of the MF EPR approach in distinguishing the nature of the first ligand sphere. A survey of recent results from pulsed EPR, as well as pulsed electron-nuclear double resonance and electron spin echo envelope modulation spectroscopic studies applied to Mn(II)-dependent systems, is also presented.

Echo-Detected Electron Paramagnetic Resonance Spectra of Spin-Labeled Lipids in Membrane Model Systems

The dynamics of spin-labeled lipid chains in the low-temperature phases of dipalmitoyl phosphatidylcholine (DPPC) membranes, with and without equimolar cholesterol, have been investigated by pulsed electron paramagnetic resonance (EPR) spectroscopy. Echo-detected spectra from the two-pulse, primary spin-echo (pulse sequence: π/2-τ-π-τ-echo) are used to detect rapid angular motions, on the time scale of the phase memory time (T 2 M ) that is in the nanosecond regime. Echo-detected spectra from the three-pulse, stimulated spin-echo (pulse sequence: π/2-τ-π/2-T-π/2-τ-echo) are used to detect slow angular motions, on the time scale of the spin-lattice relaxation time (T 1 ) that is in the microsecond regime. Spectra recorded at very low temperature (77 K) are used to correct the two-pulse echo spectra for instantaneous diffusion that arises from dipolar spin-spin interactions between different spin labels. Echo-detected inversion recovery spectra are used to correct the three-pulse echo spectra for intrinsic spin-lattice relaxation and large-scale spectral diffusion induced by nitrogen nuclear spin flips. The dependence of the echo-detected spectral line shapes on the two time delays, r and T, can be simulated adequately by using a simple two-state model to represent the small-amplitude librational motions in the low-temperature membrane phases. The fast librational motion has isotropic character, no singly defined direction of the librational axis, and is segmental in nature, depending on chain position and also on the presence of cholesterol. The slow librational motion is of a more global, cooperative nature, being independent of chain position and cholesterol content.

Selection of Species in Divalent Rhodium Hexacyanide Using Pulsed EPR

The superposition of paramagnetic species in CW EPR spectra usually prevents the measurement of spectral parameters. Separation of superimposed species can be performed in field-swept pulsed EPR by choosing a convenient pulse sequence in such a way that species with different relaxation times can be distinctly affected. Computer subtraction between spectra taken under different conditions may lead to better species identification. ForT2-based selection, the difference between FID-detected and long τ echo-detected spectra is used. This technique is successfully applied to the [Rh(CN)6]4−paramagnetic species created by radiation damage in [Rh(CN)6]3−in a KCl host lattice, where spectra corresponding to different positions of a charge-compensating positive-ion vacancy are superimposed.

Anisotropic motion of semiquinones in photosynthetic reaction centers of Rhodobacter sphaeroides R26 and in frozen isopropanol solution as measured by pulsed high-field EPR at 95 GHz

Pulsed high-field/high-frequency EPR (3.4 T magnetic field, 95 GHz microwave frequency) is performed on the primary electron acceptor ubisemiquinone-10 anion radical (UQ-10 −.) in Zn-substituted photosynthetic reaction centers (RCs) of Rhodobacter sphaeroides R26. In addition, in vitro measurements of UQ-10 −. in a frozen solution of deuterated isopropanol are presented. The method is two-dimensional EPR, with the swept magnetic field as the first and the pulse separation time as the second variable of the echo-detected spectra. The high Zeeman field leads to a spectral resolution of the anisotropic g-tensor. This allows a sensitive detection of the orientation-dependent echo-decay functions. The T 2 relaxation anisotropy at high field is dominated by librations. Comparison of in vivo and in vitro measurements shows different anisotropies of the echo-decay functions. The results are interpreted by calculations considering angular motions around the principal axes of the g-tensor. Hydrogen bonds between UQ-10 −. and its protic environment are considered.

Impact of Electron–Electron Spin–Spin Coupling on Electron-Spin Turning Angle in a Spin-Labeled Copper(II) Complex

The continuous wave EPR spectrum of copper bis(hexafluoroacetylacetonate) complexed with 4-((2,2,6,6-tetramethyl-4-piperidinyl-1-oxy)iminomethyl)pyridine in 2:1 toluene:chloroform glass exhibits an electron–electron AB spin-coupled pattern with |J| = 70 × 10−4cm−1and interspin distance = 12 Å. In two-pulse field-swept echo-detected spectra, the relative intensities of the inner and outer lines of the AB pattern for this complex are dependent upon incident microwave power because of the dependence of turning angle on effective spin S. This observation confirms the impact of the electron–electron spin–spin interaction on effective S: the inner lines have effectiveS>[formula]and the outer lines have effectiveS>[formula].

Molecular Motion of the Cholestane Spin Label in a Multibilayer in the Gel Phase Studied Using Echo-Detected EPR

Echo-detected EPR spectra of the cholestane spin label in a macroscopically oriented lipid multibilayer have been obtained at temperatures between 50 and 210 K. The drastic temperature dependence which the spectra reveal was attributed to relaxation induced by the orientational oscillations of the spin-label molecule. The scheme of computer simulations of echo-detected spectra for the model of uniaxial orientational oscillations (librations) is described in detail. The simulated spectra show a remarkable dependence on the orientation of the motional axis in the molecular frame. The simulation and experimental results agree when the projection of this axis in the plane of the nitroxide fragment makes an angle of 70° ± 10° with the direction of the NO bond. This was attributed to librational motion of the label about the long axis of the cholestane moiety.