J. Agric. Food Chem. 2001, 49, 5273−5278
5273
EPR Studies on γ-Irradiated Barley Seeds: Identification of Trapped Electrons Lata Israni Shukla,*,† V. Natarajan,‡ T. P. A. Devasagayam,§ M. D. Sastry,‡ and P. C. Kesavan# School of Life Sciences, Jawaharlal Nehru University, New Delhi 110 067, India; Radiochemistry Division and Cell Biology Division, Bhabha Atomic Research Centre, Mumbai 400 085, India; and M. S. Swaminanthan Research Foundation, Chennai, India
Electron paramagnetic resonance (EPR) studies were conducted on barley seeds exposed to normal (H2O) and deuterated (D2O) moisture, irradiated with 750 Gy at 77 K. Reported here, for the first time, are the trapped electrons formed on γ-irradiation of seeds at 77 K. Electrons are stabilized/ solvated with an increase in the moisture content (H2O/D2O) of seeds. The recombination of the trapped electron with radical cation gave intense thermoluminescence emission at 110 K. With the increase in temperature and the destruction of singlet, unmasking of an underlying heterogeneous population of free radicals was observed. These free radicals emanate mainly from the endosperm (∼95% by wt of the seed), whereas irradiated embryos show a broad multiplet of comparatively low amplitude. Radiolysis of carbohydrate, proteins (∼95% of endosperm), and lipids could possibly be responsible for the heterogeneous population of free radicals. Peroxyl radicals were also observed on annealing. Keywords: Radiation effects; electron paramagnetic resonance; thermoluminescence; barley seeds; trapped electrons; organic peroxyl radical; endosperm; embryo INTRODUCTION
Radiation damage in seeds and other agricultural products is of intense current interest, both from a fundamental radiobiological point of view and also with respect to radiation preservation of seeds (1-5). In this context, a first logical step is the identification of radiation-induced free radicals as “transients”/“reaction intermediates” and their thermal stability leading to final stable species. Electron paramagnetic resonance (EPR) spectroscopy is an ideal technique for elucidation of the radiation-induced paramagnetic defect centers. Some of these free radical species could be the precursors of the radiation-induced damage in the biological systems. It may be noted that the investigations reported herein are primarily directed toward understanding the reactive species amenable to EPR investigations. In this context the diamagnetic species created during the radiolytic processes were considered to be ineffective for radiobiological changes. In this paper, we present the result of our investigations on the EPR of γ-irradiated barley seeds, which have several essential features to investigate mechanistic aspects in basic radiobiology (6, 7). Barley seeds are very suitable for the study of both the physicochemical and biochemical stages of development of the radiobiological damage (8). The seeds of low moisture content (
