Quantitative ESR-studies of decay-produced radicals in 5-iododeoxyuridine labeled with 125I, 131I or tritium: role of the Auger effect

In view of the enhanced biological damage caused by the "Auger nuclide" iodine-125, we have carried out quantitative ESR-studies of the radical formation in polycrystalline 5-iododoeoxyuridine (IUdR) resulting from the following internal or external radiation sources: (1) Decay of 3H, 125I or 131I in labeled IUdR; (2) Lanthanum K-photons corresponding to the K-edge of iodine; (3) 60Co gamma-rays. The results clearly indicate that inner shell ionization with its accompanying Auger effect as caused by the lanthanum K X-ray produces about 30% more free radicals per unit dose absorbed than 60Co gamma-rays, when considering the long-lived secondary radicals. Similarly, the concentration of free radicals is by about 30% higher in 125I- than in 131I-labeled IUdR at comparable doses. In the case of 3H-labeled IUdR the dose curve is almost identical with that observed for 125I-labeled IUdR. The results are discussed in terms of a localized radiation damage from low energy electrons.

Decay-induced incorporation of tritium into nucleosides in aqueous solutions

In view of environmental and biological implications, the decay-induced incorporation of tritium from T2 into nucleosides in aqueous solutions has been studied by chromatographic methods. It could be shown that at T2 concentrations of about 1 to 2 mCi/ml, labeling of thymidine or deoxyuridine mainly occurs via the 3HeT+ decay ion rather than via beta--radiolysis. In a saturated solution of thymidine, the number of thymidine molecules labeled per tritium decay (L-value) is only 8 X 10(-4), concomitantly 0.6 HTO molecules are also formed, together with CH3T (L = 0.2) and an unidentified organic product (L = 10(-3). Compared to identical concentrations of HT and HTO in aqueous deoxyuridine solutions, the labeling efficiency of T2 is an order of magnitude higher.

Reactions of Nucleogenic Chlorine Atoms with Simple Aromatic Compounds

Reactions of recoil chlorine resulting from the 37Cl(n, γ)38Cl-process have been studied in liquid aromatic compounds of the type C6H5X [X = F, Cl, I, CH3, CF3, NO2], using both binary CCl4/C6H5X-systems and 2 mole % Cl2-containing pure aromatic substrates.

No pronounced orientation effects for the o-, m-, p-distribution of the monochloro isomers are observed at high ArH concentration. The functional group has little or no influence on the reactivity of the aromatic compound towards hydrogen substitution. The lack of selectivity- and reactivity effects indicates the absence of electrophilic substitution processes. It is suggested that recoil chlorination largely proceeds via hot, neutral chlorine atoms. Comparison with results obtained from the γ-radiolysis of the corresponding systems show that deviations from purely non-discriminating reactions can be ascribed to caged thermal radical reactions.

The yield for heavy atom- and group displacement at high ArX concentration increases in the following sequence of the displaced group X:CH3 < CF3 < F < I < NO2 < CI. The mass of the displaced atom or group and cross sectional effects, rather than bond energies are shown to be yield-determining factors the lack of bond energy effects and the almost complete retention of the original isomeric form after chlorine replacement in o- and m-dichlorobenzene rules out an addition-type reaction followed by a decomposition of the excited intermediate. Both hot replacement reactions and caged thermal radical reactions are assumed to be responsible for the formation of C6H5 38Cl from C6H5X.

Recoil Tritium Reactions in Simple Aliphatic Compounds

Some reactions of recoil tritium produced via the 3He (n, p) T- and the 6Li (n, α) T-process, respectively, have been studied in gaseous and liquid aliphatic compounds of the type RX (X = NH2, OH, SH, halogen).

Except for a remarkably high HT-yield in gaseous and liquid labile Η-containing systems the product forming mechanisms seem to follow the general pattern of the chemistry of energetic tritium atoms.

Liquid phase competition experiments in equimolar CH3NH2/C2H5NH2(I2)- and CH3NH2/n-C3H7NH2(I2)-systems show a stronger structure effect on hydrogen replacement than expected on the basis of the "impact model". The ratios of the relative yields per C-Η bond are 1.00 : 0.67 : 0.50 for methyl-, ethyl- and n-propylamine, respectively.

The effect of bond energies on excitation-decomposition following hot hydrogen substitution has been studied using scavenged gaseous, equimolar CH3X/C2He-systems. The CH2TX-yields are shown to decrease in the order

CH2TF > CH2TOH > CH2TCl > CH2TNH2 > CH2TSH

thus indicating that the energy of the weakest bond in the molecule plays a major yield-determining role. Pressure- and phase-effects confirm that a considerable fraction of the hot substitution product can decompose at 1 atmosphere before collisional de-excitation occurs. Primary steric and chemical factors seem to be small compared to the secondary decomposition effects, particulary for compounds with weaker bonds.