A Pulse EPR Study of Longitudinal Relaxation of the Stable Radical in γ-Irradiated L-Alanine

Rakvin, Boris; Maltar-Strmečki, Nadica; Cevc, P.; Arčon, Denis

doi:10.1006/jmre.2001.2373

Cited by 15 publications

(14 citation statements)

References 25 publications

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“…For example, for irradiated alanine, the values of T 1e obtained by CW-SR are significantly longer than the "slow" component that was obtained previously from an inversion recovery study of irradiated L-alanine by fitting data to the sum of exponentials. 56 For the glycylglycine radical, an 8-level model was used to analyze the contributions of relaxation processes to the ELDOR curves (Figure 2A). In the inversion recovery or saturation recovery experiments, transition 3 f 4 is both excited and observed.…”

Section: Methodsmentioning

confidence: 99%

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

Harbridge

Eaton

2003

J. Phys. Chem. A

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Electron spin-lattice relaxation times (T 1e ) for the major radicals in γ-irradiated polycrystalline samples of glycylglycine, L-alanine, 2,4,6-tri-tert-butyl-phenol, and 4-methyl-2,6-di-tert-butyl-phenol were measured as a function of temperature using pulsed EPR. CW-saturation recovery (CW-SR) were obtained at X-band (9.1 GHz) and S-band (3.0 GHz) between about 10 and 295 K. Inversion recovery, echo-detected saturation recovery (ED-SR), and pulsed electron-electron double resonance (ELDOR) curves were obtained at X-band between 77 and about 295 K. For 2,4,6-tri-tert-butyl-phenoxy radical, which has a single-line EPR spectrum, the recovery times obtained by the three methods were in good agreement and were assigned as T 1e . For the three radicals with resolved hyperfine splitting, spectral diffusion caused the recovery times observed by inversion recovery or ED-SR to be significantly shorter than T 1e obtained by CW-SR or ELDOR. Spectral diffusion processes were observed directly by pulsed ELDOR experiments, and time constants for cross relaxation and nuclear relaxation were obtained by modeling the ELDOR curves. For irradiated L-alanine and for the 4-methyl-2,6-tert-butyl-phenoxy radical at some temperatures, the effects of rapid cross relaxation on CW-SR curves could not be fully mitigated even by long saturating pulses, and T 1e could only be determined by ELDOR. For the radicals in γ-irradiated L-alanine, 2,4,6-tri-tert-butyl-phenol, and 4-methyl-2,6-di-tert-butyl-phenol, methyl group rotation makes significant contributions to T 1e at temperatures where the rate of rotation of a methyl group is comparable to the microwave frequency. Activation energies for methyl rotation were determined by modeling the temperature dependence of T 1e at X-band and S-band. In temperature ranges where methyl rotation did not dominate, T 1e was dominated by Raman, direct, or local mode processes.

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Section: Methodsmentioning

confidence: 99%

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

Harbridge

Eaton

2003

J. Phys. Chem. A

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“…4). In this calculation the experimental setup parameters (Hmw, com) and the chosen interval of relaxation times T~ (containing values experimentatly measured by various other techniques) were employed [7][8][9]. In order to check the reliability of the catculated T2, one can approximate 1/2rcT 2 to the known ENDOR linewidth for this center.…”

Section: Transfer Saturation Eprmentioning

confidence: 99%

“…Thus, it is convenient to relate the linewidth broadening con- The temperature dependence of the in-phase spectrum was described by modified Bloch equations and the correlation time of proton motion, z-p, was deduced earlier [11]. It should be noted here that the same effect of methyl dynamics was studied with several different methods: ENDOR [6,12], pulse EPR [7,8,10], longitudinally detected EPR [15], and double modulation EPR [t6]. Al1 these experiments yield different values for activation energies and preexponentiat factors of rp.…”

Section: Transfer Saturation Eprmentioning

confidence: 99%

“…This effect has been ascribed to an extreme behavior of spin-lattice relaxation due to the motional dynamics of the CH 3 group of stable paramagnetic centers [5] rather than to the Curie dependence of the EPR susceptibility. However, in the recent more detaited investigation of the rotational dynamics and the rotational correlation time, ~p, of CH 3 group [6] as well as the longitudinal relaxation rate [7] the extreme behavior of relaxation was found at lower and at higher temperatures but not in the vicinity of 190 K. Thus, the mechanism for enhanced relaxation at 190 K is still not completely clear.…”

Section: Introductionmentioning

confidence: 97%

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Relaxation mechanism in γ-ray-irradiated L-alanine studied by transfer saturation EPR and pulse EPR

et al. 2002

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Stabie paramagnetic centers m y-ray-irradiated L-alanine dosimeters exhibit a maximum in relaxation rate in the vicinity of 190 K. The mechanism of this relaxation rate has been investigated on the first stable atanine radical center, SAR1, by employing continuous-wave transfer saturation electron paramagnetic resonance and pulse electron paramagnetic resonance techniques. The detected in-phase and out-of-phase spectra as well as phase memory times, TM, indicate that besides the wellknown rp of the CH 3 group of SAR1 an additional correlation time, r~ (AE/k = 2689• K. and r~0 = 0.15+0.03 ps), is involved in the transverse relaxation process and effects the SARI center. For the SARI center this mechanism o¡ from the hindered motion of undamaged CH 3 and NH] groups in the lattice. The motion of these groups additionally effects the spectrum of the SAR1 center through averaging out of the anisotropic splitting.

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“…The first stable l -alanine radical, • CH(CH 3 )COOH, henceforth denoted SAR1, has been studied for many years and remains the most frequently studied paramagnetic amino acid center in the solid state. − It has been examined by various EPR techniques (continuous wave and pulsed EPR, electron−nuclear double resonance, ENDOR, ENDOR induced EPR, and ELDOR). Despite the huge number of investigations, the molecular environment of the radical, in particular the unusually large displacement of the central carbon atom, C α , which contains 75% of the spin density, as well as the dynamic processes associated with the radical motion, are still not fully understood.…”

Section: Introductionmentioning

confidence: 99%

ENDOR Study on the Dynamic Properties of the First Stable Paramagnetic Center in γ-Irradiated l-Alanine Crystals

et al. 2010

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Dynamic properties of the first stable l-alanine radical, SAR1, induced by gamma-irradiation of l-alanine crystals, have been investigated by the electron nuclear double resonance technique (ENDOR). The study focuses on the dynamic properties of the alpha-proton hyperfine splitting in the temperature range from 180 to 320 K. In this region the motion of the NH(3)(+) and CH(3) groups exhibits slow and fast motional dynamics in comparison to the nuclear and electron Larmor frequencies, respectively. Evidence for different conformations of the SAR1 center is presented on the basis of thermodynamic properties of the alpha-hyperfine splitting. The activation processes causing the broadening of the ENDOR lines are studied. At room temperature the motional dynamics of the SAR1 center are modulated by the dynamics of the charged, neighboring NH(3)(+) group.

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A Pulse EPR Study of Longitudinal Relaxation of the Stable Radical in γ-Irradiated L-Alanine

Cited by 15 publications

References 25 publications

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

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

Relaxation mechanism in γ-ray-irradiated L-alanine studied by transfer saturation EPR and pulse EPR

ENDOR Study on the Dynamic Properties of the First Stable Paramagnetic Center in γ-Irradiated l-Alanine Crystals

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