Lauraine A. Dalton

Molecular and applied modulation effects in electron electron double resonance. IV. Stationary ELDOR of very slowly tumbling spin labels

The investigation of very slowly tumbling spin labels by stationary electron electron double resonance (ELDOR) is discussed. When a Zeeman modulation frequency of 270 Hz was employed, spectra which were independent of modulation frequency but not modulation amplitude resulted. Under such conditions, the ELDOR technique permits characterization of rotational processes with correlation times from 10−7 to 10−3 sec even though normal electron spin resonance (ESR) spectra are insensitive to motion in these regions, appearing to be superimposable with the ESR powder pattern of a static random collection of molecules. Quantitative analysis of stationary ELDOR spectra is accomplished employing a density matrix treatment that explicitly includes the interaction of the spins with the applied electromagnetic radiation and Zeeman modulation fields. The effect of molecular motion inducing random modulation of the anisotropic hyperfine and electron Zeeman interactions can be calculated employing either an orthogonal ei...

Saturation transfer spectroscopy: signals sensitive to very slow molecular reorientation

Abstract A density matrix-transition rate matrix formalism is employed to compute the eight unique electron paramagnetic resonance signals at the first two harmonics of the applied Zeeman modulation. Calculations are carried out for conditions appropriate for investigating spin-labeled biomolecules with partially saturating microwave fields and Zeeman modulation amplitudes comparable to resonance linewidths. Spectra are computed for rotational correlation times ranging from 10 −7 to 10 −3 s and for modulation frequencies of 50 and 100 kHz. These simulations indicate that of the eight signals the in-phase dispersion signal at the first harmonic of the modulation and the out-of-phase dispersion signal at the first harmonic afford the best sensitivity to molecular motion and the largest signal amplitudes. It is suggested that study of these signals is the method of choice for monitoring slowly tumbling spin labels when signal-to-noise considerations are critical. The conclusions derived from computer simulations are borne out by experimental measurements performed on 10 −3 M solutions of the steroid spin label 17β-hydroxy-4′,4′-dimethylspiro-[5α-androstrance-3,2′-oxazolidin]-3′-oxyl in sec-butylbenzene.

Fast computer calculation of ESR and nonlinear spin response spectra from the fast motion to the rigid lattice limits

Abstract Fast computer simulation of the electron spin resonance and adiabatic rapid passage spectra of spin labels characterized by rotational correlation times ranging from the fast motion to the rigid lattice limits is demonstrated. Calculations are based upon a modification of the stochastic Liouville equation for the density matrix which explicitly includes interaction of the spins with applied radiation and modulation fields. Several mathematical simplifications of previous calculations are demonstrated, permitting computation with core and CPU requirements compatible with small computers.

Molecular and applied modulation effects in electron electron double resonance. V. Passage effects in high resolution frequency and field swept ELDOR

The observation of electron electron double resonance (ELDOR) linewidths for nitroxide radicals which are significantly less than the hyperfine envelope widths is reported. For a 1.5×10−3M solution of 2,2,6,6‐tetramethyl‐4‐piperidinol‐1‐oxyl (TANOL) in sec‐butylbenzene (SBB) between the temperatures of −40 °C and −70 °C, the ELDOR linewidths correspond to single spin packet widths determined by the effective electron spin–spin relaxation rates. An analysis of the precise dependence of frequency swept absorption and dispersion ELDOR signals upon microwave radiation field intensities and upon the details of the applied Zeeman modulation and accompanying phase‐sensitive detection is also presented. Quantitative prediction of molecular and applied modulation effects is accomplished employing a modified density matrix treatment. Best fit parameters for TANOL in SBB at −63 °C include we(0) = 1.75×105 Hz, Te1e(0) = 1.3×10−6 sec, Te2e(0) = 2.7×10−7 sec, wn(14N) = 2.6×105 Hz, wn(all 1H) = 0 Hz, τ2 = 2×10−10 sec, ?...

Molecular and applied modulation effects in electron—electron double resonance. III. Bloch equation analysis for inhomogeneous broadening

Abstract The dependence of electron—electron double resonance (ELDOR) reduction factors, obtained employing field or frequency swept modes of operation and dispersion or absorption modes detection, upon experimental condition such as Zeeman modulation frequency, Zeeman modulation phase, and pump microwave power and upon molecular conditions such as homogeneous and inhomogeneous broadening of the hyperfine lines are experimentally and theoretically investigated. Experimental studies were carried out on solutions of 2,2,6,6-tetramethyl-4-piperidenol-1-oxyl and perdeuterio-2,2,6,6-tetramethyl-4-piperidone-oxyl in sec-butylbenzene at −63 °C. For 1 × 10−3M tetramethylpiperidinol spin label, the methyl and methylene proton nuclear relaxation rates are significantly longer than nitrogen (14N) nuclear relaxation rates that the electron spin resonance (ESR) hyperfine lines demonstrate the characteristics of inhomogeneous broadening. Frequency swept dispersion and absorption ELDOR linewidths are observed to correspond to the linewidth (approximately one megahertz between points of maximum slope) of an ESR spin packet or superhyperfine line as compared to the ESR envelope or hyperfine linewidth (approximately four megahertz for the mI(14N) = +1 (low field) line when a peak to peak modulation field of 0.24 gauss is employed). Moreover, field dispersion ELDOR reduction factors are observed to depend strongly upon modulation frequency (in contrast to the behavior for homogeneous lines reported earlier), with reduction factors decreasing with modulation frequency. Theoretical calculations based upon a modified (interaction of the spins with Zeeman modulation and multiple radiation fields is introduced) decreasing equation Bloch treatment quantitatively reproduce the experimental results when interaction of the unpaired electron with nearby protons as well as with 14N is taken into account.

Theory of modulation effects in electron electron double resonance

Abstract The nonlinear response of spin systems to intense radiation fields is quantitatively treated by a modification of the stochastic Liouville equation for the spin density matrix. In particular, applied modulation terms are included in this equation. The resulting formalism provides a general method for calculating nonlinear spin response for dilute systems of radicals in a high magnetic field. In this communication, frequency and field swept absorption and dispersion electron-electron double resonance spectra are calculated and compared with experimental spectra recorded under conditions of sinusoidal magnetic field modulation and phase-sensitive detection. Good reproduction of the detailed lineshapes of experimental spectra is observed in all cases. The dependence of ELDOR reduction factors upon modulation frequency is discussed. A theoretical analysis such as employed in the present communication is shown to be essential if ELDOR reduction factors are to be related to relaxation times and hence to molecular dynamics, and if the design of ELDOR experiments is to be optimized.

Very slowly tumbling spin labels: Saturation recovery

Abstract The recovery of electron magnetization following pulsed saturation is determined of spectral diffusion and spin—lattice relaxation processes. For dilute solutions of slowly tumbling spin labels (nitroxide radicals), spectral diffusion arises as the result of rotational diffusion modulating anisotropic electron Zeeman and electron—nuclear hyperfine interactions. The effects of such spectral diffusion upon pulsed microwave experiments (pulsed electron—electron double resonance, saturation recovery employing a Bloch scheme, saturation recovery, and spin echo) are treated quantitavely by employing a form of the stochastic Liouville equation for the spin density matrix appropriately modified to include Markoffian diffusion operators modulating oriental variables and hence anisotropic magnetic interactions. Particular attention is paid to the effects of pulse conditions (pump field intensity, pulse length) and to the details of the isotropic rotational diffusion process (motional model such as brownian, free, or jump diffusion; correlation time).

Endor induced electron paramagnetic resonance: Application to the resolution of overlapping spectra

Abstract A technique for resolving overlapping EPR spectra is demonstrated in a study of X- and γ-irradiated crystals of 1,1-cyclobutanedicarboxylic acid. Discrimination of overlapping spectra is accomplished by recording the electron paramagnetic resonance desaturation induced by a radiofrequency field which excites nuclear spin transitions of only one of the several paramagnetic species present. This spectrum, referred to as ENDOR induced electron paramagnetic resonance (EI-EPR), is the difference between the ordinary partially saturated EPR spectrum and the EPR spectrum recorded in the presence of a resonant radiofrequency field. An instrumental arrangement for the fast and convenient measurement of EI-EPR at several Zeeman modulation frequencies is described. The technology is employed to resolve the complicated EPR spectrum which arises from three distinct radical species existing in radiation damaged 1,1-cyclobutanedicarboxylic acid. The EI-EPR, electron nuclear double resonance, and electron electron double resonance spectra of the alicyclic and aliphatic radical are recorded as a function of temperature and crystal orientation. EI-EPR is demonstrated to permit resolution of EPR spectra due to different radical species, different molecular conformations of a given species, and radicals in different (magnetically inequivalent) sites in the crystal unit cell. All alicyclic radicals exhibit temperature dependent beta hyperfine interactions although alpha and gamma interactions show little variation with temperature. EI-EPR signal intensities are found to vary with nuclear transition probability, relaxation parameters which characterize the various transitions, and with the applied de magnetic field (i.e., the nuclear Zeeman interaction).

Biological Applications of Time Domain ESR

Studies of biological materials by pulsed EPR techniques are generally regarded as exotic. Up to the present time, all pulsed EPR spectrometers have been homemade and represent an investment of

Dynamics of strained ring systems as studied by ESR and saturation transfer sensitive spectroscopic techniques

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