Electron Spin-Lattice Relaxation Processes of Radicals in Irradiated Crystalline Organic Compounds

Magic angle spinning dynamic nuclear polarization solid-state NMR spectroscopy of γ-irradiated molecular organic solids

In the past 15 years, magic angle spinning (MAS) dynamic nuclear polarization (DNP) has emerged as a method to increase the sensitivity of high-resolution solid-state NMR spectroscopy experiments. Recently, γ-irradiation has been used to generate significant concentrations of homogeneously distributed free radicals in a variety of solids, including quartz, glucose, and cellulose. Both γ-irradiated quartz and glucose previously showed significant MAS DNP enhancements. Here, γ-irradiation is applied to twelve small organic molecules to test the applicability of γ-irradiation as a general method of creating stable free radicals for MAS DNP experiments on organic solids and pharmaceuticals. Radical concentrations in the range of 0.25 ​mM-10 ​mM were observed in irradiated glucose, histidine, malic acid, and malonic acid, and significant ¹H DNP enhancements of 32, 130, 19, and 11 were obtained, respectively, as measured by ¹H→¹³C CPMAS experiments. However, concentrations of free radicals below 0.05 ​mM were generally observed in organic molecules containing aromatic rings, preventing sizeable DNP enhancements. DNP sensitivity gains for several of the irradiated compounds exceed that which can be obtained with the relayed DNP approach that uses exogeneous polarizing agent solutions and impregnation procedures. In several cases, significant ¹H DNP enhancements were realized at room temperature. This study demonstrates that in many cases γ-irradiation is a viable alternative to addition of stable exogenous radicals for DNP experiments on organic solids.

Electron Spin Relaxation of Tb3+ and Tm3+ Ions

Electron spin relaxation times T₁ and T_m 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, T_m, are less than 500 ns, which is short relative to values observed for 3d transition metals and organic radicals at 4 K. T_m 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 T_m.

Structural insights for vanadium catecholates and iron‑sulfur clusters obtained from multiple data analysis methods applied to electron spin relaxation data

Electron paramagnetic resonance (EPR) inversion recovery curves for vanadium catecholates and iron‑sulfur clusters were analyzed with three models: the sum of two exponentials, a stretched exponential, and a model-free distribution of exponentials (UPEN). For all data sets studied fits with a stretched exponential were statistically indistinguishable from the sum of two exponentials, and were significantly better than for single exponentials. UPEN provides insights into the structures of the distributions. For a vanadium(IV) tris catecholate the distribution of relaxation rates calculated with UPEN shows the contribution from spectral diffusion at low temperatures. The energy of the local mode for this complex, found from the temperature dependence of the spin lattice relaxation, is consistent with values expected for a metal-ligand vibration. For the [2Fe-2S]+ cluster in pyruvate formate lyase activating enzyme (PFL-AE) the small stretched exponential β values (0.3) at low temperature and the distributions calculated with UPEN reflect the contribution from a second rapidly relaxing species that could be difficult to detect by continuous wave EPR. The distributions in 1/T1 for the [4Fe-4S]+ clusters in Mycofactocin maturase were about a factor of four wider than for the three other systems studied. The very broad distribution of relaxation rates may be due to protein mobility and distributions in electronic energies and local environments for the clusters. UPEN provides insight into several situations that can result in low values of stretch parameter β including contributions from spectral diffusion, overlapping signals from distinguishable clusters, or very wide distributions.

An x-band continuous wave saturation recovery electron paramagnetic resonance spectrometer based on an arbitrary waveform generator

An X-band (ca. 9-10 GHz) continuous wave saturation recovery spectrometer to measure electron spin-lattice relaxation (T1) was designed around an arbitrary waveform generator (AWG). The AWG is the microwave source and is used for timing of microwave pulses, generation of control signals, and digitizer triggering. Use of the AWG substantially simplifies the hardware in the bridge relative to that in conventional spectrometers and decreases the footprint. The bridge includes selectable paths with different power amplifications to permit experiments requiring hundreds of milliwatts to fractions of nanowatts for the pump and observe periods. The signal is detected with either a single or quadrature-output double balanced mixer. The system can operate with reflection or crossed-loop resonators. The source noise from the AWG was decreased by addition of a Wenzel high-stability clock. The source is sufficiently stable that automatic frequency control is not needed. The spectrometer was tested with samples that contained 1 × 1015 to 8 × 1017 spins and have T1 between a few hundred ns and hundreds of μs. Excellent signal-to-noise ratio was obtained with acquisition times of 2-90 s. Signal-to-noise performance is similar to that of a conventional saturation recovery spectrometer with a solid-state source. The stability and data reproducibility are better than with conventional sources. With replacement of frequency-sensitive components, this spectrometer can be used to perform saturation recovery measurements at any frequency within the range of the AWG.

Dynamic Nuclear Polarization of β-Cyclodextrin Macromolecules

¹H dynamic nuclear polarization and nuclear spin-lattice relaxation rates have been studied in amorphous complexes of β-cyclodextrins doped with different concentrations of the TEMPO radical. Nuclear polarization increased up to 10% in the optimal case, with a behavior of the buildup rate (1/T_POL) and of the nuclear spin-lattice relaxation rate (1/T_1n) consistent with a thermal mixing regime. The temperature dependence of 1/T_1n and its increase with the radical concentration indicate a relaxation process arising from the modulation of the electron-nucleus coupling by the glassy dynamics. The high-temperature relaxation is driven by molecular motions, and 1/T_1n was studied at room temperature in liquid solutions for dilution levels close to the ones typically used for in vivo studies.

Rapid scan electron paramagnetic resonance at 1.0 GHz of defect centers in γ-irradiated organic solids

The radicals in six ⁶⁰Co γ-irradiated solids: malonic acid, glycylglycine, 2,6 di-t-butyl 4-methyl phenol, L-alanine, dimethyl malonic acid, and 2-amino isobutyric acid, were studied by rapid scan electron paramagnetic resonance at L-band (1.04 GHz) using a customized Bruker Elexsys spectrometer and a locally-designed dielectric resonator. Sinusoidal scans with widths up to 18.2 mT were generated with the recently described coil driver and Litz wire coils. Power saturation curves showed that the rapid scan signals saturated at higher powers than did conventional continuous wave signals. The rapid scan data were deconvolved and background subtracted to obtain absorption spectra. For the same data acquisition time the signal-to-noise for the absorption spectra obtained in rapid scans were 23 to 37 times higher than for first-derivative spectra obtained by conventional continuous wave electron paramagnetic resonance.

Multifrequency Pulsed EPR and the Characterization of Molecular Dynamics

In fluid solution, motion-dependent processes dominate electron spin-lattice relaxation for nitroxides and semiquinones at frequencies between 250 MHz and 34 GHz. For triarylmethyl radicals, motion-dependent processes dominate spin-lattice relaxation at frequencies below about 3 GHz. The frequency dependence of relaxation provides invaluable information about dynamic processes occurring with characteristic times on the order of the electron Zeeman frequency. Relaxation mechanisms, methods of measuring spin-lattice relaxation, and motional processes for nitroxide, semiquinone, and triarylmethyl radicals are discussed.

Electron spin-lattice relaxation mechanisms of rapidly-tumbling nitroxide radicals

Electron spin relaxation times at 295 K were measured at frequencies between 250 MHz and 34 GHz for perdeuterated 2,2,6,6-tetramethyl-4-piperidone-1-oxyl (PDT) in five solvents with viscosities that result in tumbling correlation times, τR, between 4 and 50 ps and for three (14)N/(15)N pairs of nitroxides in water with τR between 9 and 19 ps. To test the impact of structure on relaxation three additional nitroxides with τR between 10 and 26 ps were studied. In this fast tumbling regime T2(-1)~T1(-1) at frequencies up to about 9 GHz. At 34 GHz T2(-1)>T1(-1) due to increased contributions to T2(-1) from incomplete motional averaging of g-anisotropy, and T2(-1)-T1(-1) is proportional to τR. The contribution to T1(-1) from spin rotation is independent of frequency and decreases as τR increases. Spin rotation dominates T1(-1) at 34 GHz for all τR studied, and at all frequencies studied for τR=4 ps. The contribution to T1(-1) from modulation of nitrogen hyperfine anisotropy increases as frequency decreases and as τR increases; it dominates at low frequencies for τR>~15 ps. The contribution from modulation of g anisotropy is significant only at 34 GHz. Inclusion of a thermally-activated process was required to account for the observation that for most of the radicals, T1(-1) was smaller at 250 MHz than at 1-2 GHz. The significant (15)N/(14)N isotope effect, the small H/D isotope effect, and the viscosity dependence of the magnitude of the contribution from the thermally-activated process suggest that it arises from intramolecular motions of the nitroxide ring that modulate the isotropic A values.

Investigation of the nitrogen hyperfine coupling of the second stable radical in γ-irradiated l-alanine crystals by 2D-HYSCORE spectroscopy

The second stable radical, NH(3)(+)C()(CH(3))COO(-), R2, in the γ-irradiated single crystal of l-alanine and its fully (15)N-enriched analogue were studied by an advanced pulsed EPR technique, 2D-HYSCORE (two-dimensional hyperfine sublevel correlation) spectroscopy at 200K. The nitrogen hyperfine coupling tensor of the R2 radical was determined from the HYSCORE data and provides new experimental data for improved characterization of the R2 radical in the crystal lattice. The results obtained complement the experimental proton data available for the R2 radical and could lead to increased accuracy and reliability of EPR spectrum simulations.

Electron spin relaxation governed by Raman processes both for Cu²⁺ ions and carbonate radicals in KHCO₃ crystals: EPR and electron spin echo studies

EPR studies of Cu²⁺ and two free radicals formed by γ-radiation were performed for KHCO₃ single crystal at room temperature. From the rotational EPR results we concluded that Cu²⁺ is chelated by two carbonate molecules in a square planar configuration with spin-Hamiltonian parameters g(||)=2.2349 and A(||)=18.2 mT. Free radicals were identified as neutral HOCO· with unpaired electron localized on the carbon atom and a radical anion CO₃·⁻ with unpaired electron localized on two oxygen atoms. The hyperfine splitting of the EPR lines by an interaction with a single hydrogen atom of HOCO· was observed with isotropic coupling constants a₀=0.31 mT. Two differently oriented radical sites were identified in the crystal unit cell. Electron spin-lattice relaxation measured by electron spin echo methods shows that both Cu²⁺ and free radicals relax via two-phonon Raman processes with almost the same relaxation rate. The temperature dependence of the relaxation rate 1/T₁ is well described with the effective Debye temperature Θ(D)=175 K obtained from a fit to the Debye-type phonon spectrum. We calculated a more realistic Debye temperature value from available elastic constant values of the crystal as Θ(D)=246 K. This Θ(D)-value and the Debye phonon spectrum approximation give a much worse fit to the experimental results. Possible contributions from a local mode or an optical mode are considered and it is suggested that the real phonon spectrum should be used for the relaxation data interpretation. It is unusual that free radicals in KHCO₃ relax similarly to the well localized Cu²⁺ ions, which suggests a small destruction of the host crystal lattice by the ionizing irradiation allowing well coupling between radical and lattice dynamics.

Suppression of Raman electron spin relaxation of radicals in crystals. Comparison of Cu2+ and free radical relaxation in triglycine sulfate and Tutton salt single crystals

Electron spin-lattice relaxation was measured by the electron spin echo method in a broad temperature range above 4.2 K for Cu(2+) ions and free radicals produced by ionizing radiation in triglycine sulfate (TGS) and Tutton salt (NH4)(2)Zn(SO4)2 ⋅ 6H2O crystals. Localization of the paramagnetic centres in the crystal unit cells was determined from continuous wave electron paramagnetic resonance spectra. Various spin relaxation processes and mechanisms are outlined. Cu(2+) ions relax fast via two-phonon Raman processes in both crystals involving the whole phonon spectrum of the host lattice. This relaxation is slightly slower for TGS where Cu(2+) ions are in the interstitial position. The ordinary Raman processes do not contribute to the radical relaxation which relaxes via the local phonon mode. The local mode lies within the acoustic phonon band for radicals in TGS but within the optical phonon range in (NH4)(2)Zn(SO4)2 ⋅ 6H2O. In the latter the cross-relaxation was considered. A lack of phonons around the radical molecules suggested a local crystal amorphisation produced by x- or γ-rays.

Electron spin relaxation rates for semiquinones between 25 and 295K in glass-forming solvents

Electron spin lattice relaxation rates for five semiquinones (2,5-di-t-butyl-1,4-benzosemiquinone, 2,5-di-t-amyl-1,4-benzosemiquinone, 2,5-di-phenyl-1,4-benzosemiquinone, 2,6-di-t-butyl-1,4-benzosemiquinone, tetrahydroxy-1,4-benzosemiquione) were studied by long-pulse saturation recovery EPR in 1:4 glycerol:ethanol, 1:1 glycerol:ethanol, and triethanolamine between 25 and 295K. Although the dominant process changes with temperature, relaxation rates vary smoothly with temperature, even near the glass transition temperatures, and could be modeled as the sum of contributions that have the temperature dependence that is predicted for the direct, Raman, local mode and tumbling-dependent processes. At 85K, which is in a temperature range where the Raman process dominates, relaxation rates along the g(xx) (g approximately 2.006) and g(yy) (g approximately 2.005) axes are about 2.7-1.5 times faster than along the g(zz) axis (g=2.0023). In highly viscous triethanolamine, contributions from tumbling-dependent processes are negligible. At temperatures above 100K relaxation rates in triethanolamine are unchanged between X-band (9.5GHz) and Q-band (34GHz), so the process that dominates in this temperature interval was assigned as a local mode rather than a thermally activated process. Because the largest proton hyperfine couplings are only 2.2G, spin rotation makes a larger contribution than tumbling-dependent modulation of hyperfine anisotropy. Since g anisotropy is small, tumbling-dependent modulation of g anisotropy makes a smaller contribution than spin rotation at X-band. Although there was negligible impact of methyl rotation on T(1), rotation of t-butyl or t-amyl methyl groups enhances spin echo dephasing between 85 and 150K.

Dynamic Nuclear Polarization of [1-13C]pyruvic acid at 4.6 tesla

Dynamic Nuclear Polarization (DNP) of the (13)C nucleus has been investigated for [1-(13)C]pyruvic acid, doped with the trityl radical OX063Me, at 4.64 T and 1.15K. The dependence of the polarization on microwave frequency, radical concentration and electron saturation was studied. For optimized conditions, a (13)C polarization equal to 64+/-5% was obtained, an increase by more than a factor of two compared with earlier results at 3.35 T of the same system. It was furthermore observed that the addition of gadolinium, which resulted in a twofold polarization increase at 3.35 T, only resulted in a minor improvement at 4.64 T. The dependence of the electron saturation on microwave frequency and microwave power was quantified by first moment measurements which were obtained by nucleus-electron double resonance (NEDOR) experiments. Complete electron saturation was observed for a microwave frequency close to the centre frequency of the ESR line, and by using maximum power of the microwave source. The DNP build-up time at 4.64 T (approximately 3000 s) was prolonged by approximately a factor three over the build-up time at 3.35 T (approximately 1200 s). However, after approximately 20 min of microwave irradiation the polarization at 4.64 T exceeded the polarization at 3.35 T.

Evidence for disorder in L-alanine lattice detected by Pulsed-EPR spectroscopy at cryogenic temperatures

The unusual behavior of lattice dynamics of L-alanine has been assigned to intermolecular dynamics and localization of vibrational energy. Recent heat capacity and Pulsed-EPR measurements support presence of thermally activated dynamic orientational disorder in the L-alanine lattice below 20 K. In the present study, the additional evidence for possible thermally activated disordered behavior of L-alanine lattice have been obtained by investigating dependences of longitudinal relaxation time of first stable L-alanine radical, SAR1, on sample cooling rates for the same low temperature interval. The obtained relaxation time by Pulsed-EPR shows clear dependence on cooling rates and this behavior can be explained within two types of suggested spin-lattice relaxation mechanisms for the paramagnetic centers in the hydrogen-bonded organic crystal.

Formates and dithionates: sensitive EPR-dosimeter materials for radiation therapy

Polycrystalline formates and dithionates are promising materials for EPR dosimetry, as large yields of radiation induced stable radicals are formed with a linear dose response. Rapid spin relaxation rates were detected in many of the substances, indicating that a high microwave power can be applied during EPR acquisition in order to improve sensitivity. Different techniques used to further improve the sensitivity, such as the replacement of 7Li with 6Li or exchange of protons with deuterons in the corresponding crystalline matrices and metal ion doping are discussed. It is concluded that formates and dithionates may be up to 10 times as sensitive as L-alpha-alanine.

Electron spin relaxation times for the alanine radical in two dosimeters

The spin-lattice relaxation times for the alanine radical in dosimeters were measured by long-pulse saturation recovery and by inversion recovery. The strong dependence of the recovery time constants on experimental conditions show that spectral diffusion processes contribute to enhanced relaxation rates, analogous to what was observed previously for 60Co gamma-irradiated polycrystalline l-alanine. The spectral diffusion processes make it possible to run quantitative CW spectra of the alanine radical at substantially higher microwave powers than could be used in the absence of spectral diffusion processes.

Determination of T1-spin-lattice relaxation time in a two-level system by continuous wave multiquantum electron paramagnetic resonance spectroscopy in a presence of tetrachromatic microwave irradiation

Applicability of continuous wave multiquantum EPR methods to study relaxation times at X-band is examined. Multiquantum transitions excited in a two-level system by tetrachromatic irradiation are used for these studies. The Bloch equation model is applied to simulate lineshapes of the three quantum transitions as a function of frequency difference between exciting fields. The dependence of multiquantum transition signals on relaxation times and microwave amplitude is shown. On this basis a method of deducing relaxation times from these signals is formulated. The case of a homogeneously and inhomogeneously broadened resonance line is considered. Two experimental methods are used to verify the proposed hypothesis: the X-band continuous wave multiquantum EPR with four frequencies microwave field and saturation recovery EPR. The values of T(1) obtained from CW MQ EPR and SR EPR are compared.

Heat capacity and electron spin echo evidence for low frequency vibrational modes and lattice disorder in L-alanine at cryogenic temperatures

With the view of understanding the low frequency (40-50 cm(-1)) motional processes in L-alanine around 4 K, we have carried out heat capacity (CP) and electron spin echo (ESE) measurements on L-alanine and L-alanine-d7. The obtained CP data show the so-called boson peak (seen as a maximum in CP/T3 versus T plots) in the low temperature region (1.8-20 K). The phase memory time, T(M), and spin lattice relaxation time, T1, of the spin probe, the so-called first stable alanine radical (SAR1), *CHCH3COOH, have been measured between 4 and 105 K. The obtained relaxation rate 1/T1 shows an anomalous increase which coincides with the emergence of a boson peak in the low temperature region (4-20 K). Together, the ESE and the CP data confirm the existence of a thermally activated dynamic orientational disorder in the lattices of both compounds below 20 K. The results help explain the discrepancy between the CP data from powders and single crystals of alanine, as well as the proanomalous relaxation mechanisms for SAR1 in these lattices, and they also provide a mechanism for the spin-lattice relaxation process for SAR1 at cryogenic temperatures.

Comparison of electron spin relaxation times measured by Carr-Purcell-Meiboom-Gill and two-pulse spin-echo sequences

Electron spin relaxation times obtained by two-pulse spin-echo and Carr-Purcell-Meiboom-Gill (CPMG) experiments were compared for samples with: (i) low concentrations of nuclear spins, (ii) higher concentrations of nuclear spins and low concentrations of unpaired electrons, (iii) higher concentrations of nuclear spins and of electron spins, and (iv) dynamic averaging of inequivalent hyperfine couplings on the EPR timescale. In each case, the CPMG time constant decreased as the time between the refocusing pulses increased. For the samples with low concentrations of nuclear spins (the E' center in irradiated amorphous SiO2) the limiting value of the CPMG time constant at short interpulse spacings was similar to the Tm obtained by two-pulse spin echo at small turning angle. For the other samples, the time constants obtained by CPMG at short interpulse spacings were systematically longer than Tm obtained by two-pulse spin echo. For most of the samples, the CPMG time constant decreased with increasing electron spin concentration, which is consistent with the expectation that the CPMG sequence does not refocus dephasing due to electron-electron dipolar interaction between resonant spins. Dynamic processes that average inequivalent hyperfine couplings contributed less to the CPMG time constant than to the spin-echo decay time constant. The impact of nuclear echo envelope modulation on CPMG time constants also was examined. For a Nycomed trityl radical in glassy D2O:glycerol-d8 solution, the CPMG time constant was up to 20 times longer when the time between pulses was approximately equal to integer multiples of the reciprocal of the deuterium Larmor frequency than when the time between pulses was an odd multiple of half the reciprocal of the deuterium Larmor frequency.