Evaluation of the radioprotective ability of cystamine for 150 keV ‐ 500 MeV proton irradiation: a Monte Carlo track chemistry simulation study

Cystamine, an organic diamino-disulfide, is among the best of the known radiation-protective compounds, although the underlying molecular mechanisms by which it operates remain poorly understood. This study aims to use the aqueous ferrous sulfate (Fricke) dosimeter to evaluate the protective properties of this compound when present during irradiation by fast incident protons in the energy range of 150 keV - 500 MeV, that is, for "linear energy transfer" (LET) values ranging from ⁓72.3 to 0.23 keV/μm. The presence of cystamine in irradiated Fricke solutions prevents the oxidation of Fe²⁺ ions by the oxidizing species produced in the radiolysis of acidic water, resulting in reduced Fe³⁺ ion yields. A Monte Carlo computer code is used to simulate the radiation-induced chemistry of the studied Fricke-cystamine solutions under aerated conditions while covering a wide range of cystamine concentrations from 5 × 10⁻⁷ to 1 mol/L. Results indicate that the protective activity of cystamine is due to its radical-capturing ability, a clear signature of the strong antioxidant profile of this compound. In addition, our simulations show that at low and intermediate concentrations of cystamine, its protective efficiency decreases with increasing LET, which is consistent with previous work. This finding stems from differences in the geometry of the track structures that change from low-LET isolated spherical "spurs" to high-LET dense continuous cylindrical tracks as LET increases. This study concludes that Monte Carlo simulations represent a powerful method for understanding, at the molecular level, indirect radiation damage to complex molecules such as cystamine. (author)