In Quest of the Alanine R3 Radical: Multivariate EPR Spectral Analyses of X-Irradiated Alanine in the Solid State

Abstract

The amino acid l-α-alanine is the most commonly used material for solid-state electron paramagnetic resonance (EPR) dosimetry, due to the formation of highly stable radicals upon irradiation, with yields proportional to the radiation dose. Two major alanine radical components designated R1 and R2 have previously been uniquely characterized from EPR and electron-nuclear double resonance (ENDOR) studies as well as from quantum chemical calculations. There is also convincing experimental evidence of a third minor radical component R3, and a tentative radical structure has been suggested, even though no well-defined spectral signature has been observed experimentally. In the present study, temperature dependent EPR spectra of X-ray irradiated polycrystalline alanine were analyzed using five multivariate methods in further attempts to understand the composite nature of the alanine dosimeter EPR spectrum. Principal component analysis (PCA), maximum likelihood common factor analysis (MLCFA), independent component analysis (ICA), self-modeling mixture analysis (SMA), and multivariate curve resolution (MCR) were used to extract pure radical spectra and their fractional contributions from the experimental EPR spectra. All methods yielded spectral estimates resembling the established R1 spectrum. Furthermore, SMA and MCR consistently predicted both the established R2 spectrum and the shape of the R3 spectrum. The predicted shape of the R3 spectrum corresponded well with the proposed tentative spectrum derived from spectrum simulations. Thus, results from two independent multivariate data analysis techniques strongly support the previous evidence that three radicals are indeed present in irradiated alanine samples.

Application of Amino Acids for High-Dosage Measurements with Electron Paramagnetic Resonance Spectroscopy

“…Many studies on the composition of the alanine EPR spectrum are available in the literature [ 32 , 33 , 34 , 35 ]. It has been shown that the EPR spectrum of irradiated alanine consists of at least three different radical species [ 36 , 37 , 38 ].…”

ENDOR-Induced EPR of Disordered Systems: Application to X-Irradiated Alanine

“…9 Its spectrum was only recently isolated in a statistical decomposition study of the EPR spectrum. 11 In more fortunate cases, for example, when the EPR components of minority species are narrow and easier to distinguish, better sensitivity could be expected.…”

“…It has to be noted, however, that the aforementioned radical was difficult to characterize even in single crystals due to the weakness of its relative contribution (<10%) and the simultaneous presence of several conformations . Its spectrum was only recently isolated in a statistical decomposition study of the EPR spectrum . In more fortunate cases, for example, when the EPR components of minority species are narrow and easier to distinguish, better sensitivity could be expected.…”

“…These radicals were labeled as R1−R3, and their chemical structures can be found in Figure 1B−D, respectively. Decomposition of the dosimetric powder spectrum was accomplished by exploiting the different decay rates of the three species at elevated temperatures, 10,11 and simulations of the complete spectrum became possible only after the g̃tensors of the stable radicals had been determined. 12 The 14 N HF interactions are smaller than the majority of the 1 H interactions, and they are often neglected in simulations of the powder EPR spectrum at RT, when reproduction of the main features of the spectrum is the main aim.…”

“…These radicals were labeled as R1–R3, and their chemical structures can be found in Figure B–D, respectively. Decomposition of the dosimetric powder spectrum was accomplished by exploiting the different decay rates of the three species at elevated temperatures, , and simulations of the complete spectrum became possible only after the g ̃ tensors of the stable radicals had been determined …”