Photo-oxidation of diglycine in confined media Relaxation of longitudinal magnetization in spin correlated radical pairs.

Time-resolved electron paramagnetic resonance spectra (X-band) of correlated radical pairs created in AOT reverse micelles are presented and simulated using the microreactor model. They are discussed in terms of the two-site model with a particular emphasis on longitudinal relaxation mechanisms. The geminate radical pair is created by photo-oxidation of dyglicine by the excited triplet states of an anthraquinone salt. The strong chemically induced electron spin polarization observed is due to three mechanisms: TM, RPM, and SCRPM. Relative contributions from these mechanisms depend on the water pool volume and the time of observation. There are three types of longitudinal relaxation in radical pairs. The first is relaxation of the RPM induced longitudinal magnetization in spin correlated radical pairs. The second is the longitudinal relaxation in radical pairs which are not correlated (with a zero value of the double quantum coherence). In such pairs, the generation of longitudinal magnetization due to RPM is impossible, and the spin-selective recombination of the pairs is ineffective. Under all experimental conditions, the first type of relaxation is slower than the second type. For both, the physical mechanism leading to relaxation is modulation of the Heisenberg electron spin exchange interaction. This is an internal relaxation process. The third relaxation type occurs in radical pairs due to ordinary longitudinal relaxation in non-interacting radicals. Normally, relaxation of the third type is the slowest of the three. This explains time and micelle size dependence of the relative contribution of RPM into TREPR spectra. It seems reasonable to suggest that the creation of the initial spin state populations is partially adiabatic.

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 biological molecules may be considered under four distinct time regimes: physical transport (< or = 10(-15) s); prechemical conversion of H2O+, H2O*, and subexcitation electrons into free radicals and molecular products (10(-15) s to 10(-12) s); chemical reactions within individual electron tracks (10(-12) s to 10(-6) s); and chemical reactions within overlapping tracks (>10(-6) s). We have previously reported of the use of the Monte Carlo radiation transport/chemistry codes OREC and RADLYS to model the radiolysis of glycylglycine in oxygen-free solution to a time of 1 micros. These simulations successfully predicted the yields of free ammonia, an end product created solely in the reaction of the hydrated electron with the solute within individual tracks. Other measurable products are only partially created during intratrack reactions, and thus one must additionally consider the late, intertrack chemistry of this system. In this paper, we extend our simulations of glycylglycine radiolysis to model for the first time the events which occur during this late chemistry stage. The model considers the product rates of the reactants in bulk solution by using previously available microsecond intratrack yields given by single-track OREC/RADLYS 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 series 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. Excellent overall agreement is seen between the theoretical predictions and measurements of five radiolysis end products: free ammonia, acetylglycine, diaminosuccinic acid, aspartic acid, and succinic acid. The model also gives the explicit contributions of intratrack and intertrack reactions to the various end products. For example, the model predicts that approximately 56% and 93% of succinic acid and aspartic acid, respectively, are produced during intertrack reactions at a solute concentration of 0.05 M; these contributions drop to 0.07% and 11%, respectively, at 1.2 M.

Concentration- and pH-dependent ammonia yields in the reaction of hydrated electrons with glycylglycine.

Competition between recombination of the radicals formed in the gamma radiolysis of water (eaq-, OH, H) and the reaction of eaq- with glycylglycine is investigated in glycylglycine solutions of various concentrations and pH. First, the concentration dependence of the ammonia yields is measured in the glycylglycine solutions of between 0.5 mM and 1.0 M. The G values of ammonia appear to reach a plateau of 2.47 +/- 0.05 molecules/100 eV at the low concentrations (1-5 mM) and increase to 3.81 +/- 0.07 molecules/100 eV at the high concentration (1 M). Secondly, the pH dependence of the ammonia yields is measured at low (1 and 5 mM) and high (0.1 and 0.2 M) concentrations of glyclyglycine. For concentrated glyclyglycine solutions, the ammonia yields increase with increasing pH, as expected based on comparison of the probabilities of the competitive reactions. However, the G values of ammonia reach a maximum at pH 9.0 and then decrease as the pH increases for dilute solutions. The lower G values at higher pH for dilute glycylglycine solutions may result from increased interspur recombination since the rate constant for the reaction of eaq- with glycylglycine is decreased and, as a consequence, the scavenging capacity of glycylglycine for eaq- decreases approximately 4.7-fold from low to high pH.

Measurement of products from X-irradiated glycylglycine in oxygen-free aqueous solutions.

The product yields in X-irradiated aqueous solutions of glycylglycine (0.05 M and 1.0 M) were measured under deoxygenated conditions. Comparison was made between the results obtained from X- and 60Co gamma-irradiated glycylglycine solutions reported by Garrison, Sokol, and Bennett-Corniea (Radiat. Res. 53, 376-384, 1973). The mechanisms proposed by Garrison et al. were tested by evaluating the stoichiometric relationships. The two intermediate radicals, deamination and H-abstraction radicals, were produced in the initial interactions of glycylglycine with reactive species (e-aq, OH, H) formed in H2O. Although the difference was fairly large at 0.05 M, the production of deamination radicals agreed well with the consumption of the radicals at 1.0 M. The production and the consumption of H-abstraction radicals were within the estimated experimental error in dilute solutions. Among all the products only the G value of aspartic acid decreased with increasing concentration of glycylglycine. This could be attributed to the fact that more acetylglycine is formed at the expense of aspartic acid at 1.0 M than at 0.05 M glycylglycine solutions. Competitive reactions involved with deamination radicals under conditions of homogeneously distributed reactants are discussed to elucidate the radiation chemistry of glycylglycine.

Radiation damage to a biomolecule: new physical model successfully traces molecular events.

For the first time, a complete computer simulation of physical and chemical reactions at the molecular level has been used to calculate the yield of a chemical species resulting from irradiation of a biological molecule in aqueous solution. Specifically, when a solution of glycylglycine is irradiated anaerobically, an ammonia molecule is released by the action of a hydrated electron, which is produced by irradiation of water. In the computations, Monte Carlo techniques are used to simulate the statistical progression of molecular events as they are assumed to occur. These include the initial physical ionization and excitation of water molecules along a particle track in the liquid; the subsequent formation of free radicals and other species: and the random diffusion and chemical reactions of the species with each other, the solvent, and solute molecules. We have calculated and measured the yield of ammonia from irradiation of glycylglycine with 250 kVp x-rays as a function of glycylglycine concentration between 0.01 and 1.2 M. Excellent agreement is obtained between predicted and measured results. The literal simulation of events, combined with specific experimental measurements, offers a powerful new tool for studying mechanisms of radiation action and damage at the molecular level.

Monte Carlo calculations of free ammonia production in deoxygenated solutions of glycylglycine irradiated by X rays and 60Co gamma rays.

Detailed-history Monte Carlo computer codes were used to simulate the formation, diffusion, and chemical reaction of free-radical species within deoxygenated aqueous solutions of glycylglycine irradiated by 250-kVp X rays and by 60Co gamma rays. In one reaction, hydrated electrons react with the glycylglycine solute to produce unbound, or free, ammonia. This reaction is complete by 10(-6) s within individual electron tracks for glycylglycine concentrations greater than or equal to 0.025 M. For solute concentrations from 0.025 to 1.2 M, calculated G values of free ammonia are in excellent agreement with measured values. In addition, the computer model predicts a statistically significant difference between the G value of free ammonia produced under X irradiation and that produced under 60Co gamma irradiation.

Measurement of free ammonia produced by X irradiation of glycylglycine in aqueous solution.

This research was initiated to test the validity of predictions based on Monte Carlo calculations of the effect of ionizing radiation on a simple dipeptide. The mechanism for the formation of ammonia, proposed by Garrison, Sokol, and Bennett-Corniea (Radiat. Res. 53, 376-384, 1973), was reevaluated by measuring the yields under deoxygenated and oxygenated conditions. Although free ammonia was formed under both conditions, the yields were different, depending on the concentrations of solute and molecular oxygen. The reaction probabilities of the specific interactions of free radicals formed in pure water with solute and oxygen are discussed to account for the observed difference. Our results obtained after low-dose-rate X irradiation are compared with those obtained by Garrison et al. after high-dose-rate 60Co gamma irradiation.

Radiation protection of glycine and glycylglycine.

The protective action of a series of organic sulfur compounds and amino acids has been investigated by using glycine and glycylglycine as substrates. A linear relationship is shown to exist between the reciprocal of the G-m value for the substrate in the presence of the protecting agent, and the concentration of this protecting agent. The "protecting activity" of the agents, expressed as the ratio of the rate constants for the reaction of radicals with the protecting agent and the substrate, is determined.