PRODUCT YIELDS FROM IRRADIATED GLYCYLGLYCINE IN OXYGEN-FREE SOLUTIONS- MONTE-CARLO SIMULATIONS AND COMPARISON WITH EXPERIMENTS

The radiation chemistry of photon-irradiated aqueous solutions of biol ogical molecules may be considered under four distinct time regimes: p hysical transport (less than or equal to 10(-15) s); prechemical conve rsion of H2O+, H2O, and subexcitation electrons into free radicals an d molecular products (10(-15) s to 10(-12) s); chemical reactions with in individual electron tracks (10(-12) s to 10(-6) s); and chemical re actions within overlapping tracks (>10(-6) s). We have previously repo rted of the use of the Monte Carlo radiation transport/chemistry codes OPEC and RADLYS to model the radiolysis of glycylglycine in oxygen-fr ee solution to a time of 1 mu s. These simulations successfully predic ted the yields of free ammonia, an end product created solely in the r eaction of the hydrated electron with the solute within individual tra cks. Other measurable products are only partially created during intra track reactions, and thus one must additionally consider the late, int ertrack chemistry of this system. In this paper, we extend our simulat ions of glycylglycine radiolysis to model for the first time the event s which occur during this late chemistry stage. The model considers th e product rates of the reactants in bulk solution by using previously available microsecond intratrack yields given by single-track OREC/RAD LYS simulations and an x-ray dose rate of 2.80 Gy min(-1) as used in a companion experimental program. These rates are then applied in a ser ies of coupled, differential rate equations that describe the solution chemistry of glycylglycine radiolysis. Product yields are reported as a function of time over a total irradiation period of 10(4) s. Excell ent overall agreement is seen between the theoretical predictions and measurements of five radiolysis end products: free ammonia, acetylglyc ine, diaminosuccinic acid, aspartic acid, and succinic acid. The model also gives the explicit contributions of intratrack and intertrack re actions to the various end products. For example, the model predicts t hat similar to 56% and 93% of succinic acid and aspartic acid, respect ively, are produced during intertrack reactions at a solute concentrat ion of 0.05 M; these contributions drop to 0.07% and 11%, respectively , at 1.2 M.