Alanine Radicals. 2. The Composite Polycrystalline Alanine EPR Spectrum Studied by ENDOR, Thermal Annealing, and Spectrum Simulations

Abstract

Radiation-induced free radical formation in the amino acid l-R-alanine has been studied using powder and single-crystal X-, K-, and Q-band electron paramagnetic resonance (EPR) spectroscopy, X-band powder electron-nuclear double resonance (ENDOR), thermal annealing, and EPR spectrum simulations. The spectra obtained after room temperature irradiations are composite, consisting of resonances from mainly three radicals denoted R1, R2, and R3. R1 is the well-known, stable room-temperature species formed by deamination from a protonated alanine anion radical. On the basis of simulations of EPR spectra obtained at X-, K-, and Q-bands, the room-temperature EPR spectrum seems to consist of about 55% of R1. Upon thermal annealing, the R1 resonance disappears faster than those of the other two components. The R2 species is presumably formed in the oxidative chain of radiation-induced events by net H-abstraction from the central alanine carbon atom. Q-band EPR was used to determine the g-tensor of R2. This species contributes about 35% to the resonance recorded at room temperature. Upon thermal annealing this radical decays slower than R1, resulting in the predominance of R2 in spectra obtained after prolonged warming at 480 K. Powder ENDOR was used to verify that the dominating species remaining after thermal annealing at this temperature indeed is R2 and not a successor species of either of the room-temperature radicals. The R3 species was previously assigned to an N-deprotonated version of R2 being additionally protonated at the carboxyl group. Detailed spectral data for this resonance are missing but a set of parameters based on available data and otherwise estimated using literature values for similar products was constructed. Simulations indicated that 5-10% of the roomtemperature resonance could be ascribed to R3. R3 is more heat-resistant than the R1 and R2 radicals, and after prolonged annealing at 480K it was estimated that the resulting resonance consisted of about 51% R2 and 43% R3. The remaining part (about 6%) of the resonance was due to R1. These numbers must, however, be considered as tentative because of the lack of precise spectral data for R3.

Bond dissociation of the dipeptide dialanine and its derivative alanine anhydride induced by low energy electrons

“…In addition, the amino acid alanine has attracted attention due to its radiation dosimetric properties and has been formally accepted as a secondary standard for high-dose and transfer dosimetry. [10][11][12][13] When two molecules of alanine join covalently through the formation of a peptide bond, L-alanine-L-alanine (shortly dialanine) is formed. Some of the dialanine derivatives have recently been developed as water-soluble photosensitizers with the potential for application in photodynamic therapy and treatment of malignant tissues.…”

Electron impact ionization of alanine: Appearance energies of the ions

“…The formation of radicals in single crystals of l-alanine exposed to energetic radiation has been subject of investigation during the past four decades [1]. Alanine has attracted attention due to its radiation dosimetric properties and has been formally accepted as a secondary standard for high-dose and transfer dosimetry [2][3][4][5]. The interaction of ionizing radiation with matter produces secondary reactive species along the track of the high-energy projectile.…”

Applications of EPR and ENDOR Spectrum Simulations in Radiation Research

“…The simulated powder spectra at saturation conditions deviated more from those obtained experimentally, possibly due to the influence of two other radicals known to be present in irradiated L-alanine. The unsaturated powder spectrum could in fact be accurately simulated as a superposition of three components [157]. The saturation properties of the two minor components are not known, however.…”