Radiation chemistry of solids

The present chapter is not meant to be an arcanum accessible only to the trained radiation chemist, but is intended to give comprehensive information to readers generally interested in carbohydrate and free-radical chemistry. Following a general description of the free-radical reactions thus far discovered for this class of compounds, there is a Section dealing with individual compounds, as well as a special Section on the radiation chemistry of solid carbohydrates, including aspects of preparative interest. 

Pharmaceutical and surgical supplies are often in the solid state when irradiated. Certainly, their containers are solid. It is therefore in order to consider some of the radiation chemistry of solids. 

Radiation Chemistry of Solids Irradiated with Heavy Ions... 

However, the chemical changes observed in low molecular weight compounds can be quite misleading as models for polymers. Difficulties include the high concentration of end groups, e.g. COOH in N-acetyl amino acids, which can dominate the radiation chemistry of the models. Low molecular weight compounds are usually crystalline in the solid state and reactions such as crosslinking may be inhibited or severely retarded. 

The previous section outlined the typical e loss and e gain products observed in the nucleic acid bases in the solid state. These studies can be applied to the study of the radiation chemistry of DNA. The relevance of the study of model systems is shown by considering the following remarkable observations. Years ago, Ehrenberg et al. showed the EPR spectra of the 5,6-dihydrothymine-5-yl radical observed in thymine, thymidine, and DNA. The spectra are nearly identical [46]. The reduction product observed in cytosine monohydrate is the N3 protonated anion. In solution, this reduction product gives rise to a 1.4-mT EPR doublet. The same feature is present in irradiated DNA at 77 K. Likewise, the result of e loss in guanine bases is characterized by a broad EPR singlet. The same feature is also evident in the EPR spectrum of DNA irradiated and observed at 77 K. 

Work along these lines could give a better understanding of the role played by ions in the radiation chemistry of polymers. The fact that ions can induce chemical reactions in irradiated organic solids is clearly demonstrated by the polymerization of certain crystalline monomers which are known to polymerize only when treated with ionic catalysts (3). 

Huttermann J (1970) Electron-spin-resonance spectroscopy of radiation-induced free radicals in irradiated single crystals of thymine monohydrate. Int J Radiat Biol 17 249-259 Huttermann J (1982) Solid-state radiation chemistry of DNA and its constituents. Ultramicroscopy 10 25-40... 

In general, however, the effect of phase is much less marked than for ionic species and results for different phases will not be considered separately in this section. Since, in fact, more experiments have been carried out on the radiation chemistry of liquids than of gases or solids, most of the results discussed in this section refer to the liquid state. 

Analytical techniques for the identification of the structure and chemistry of solid surfaces are based on detection of particles photons, electrons, and ions. The techniques are classified according to the particles or radiation used to excite the sample and detected particles (emission) to obtain information about the sample. Fig. 4.5 schematically shows the interaction of the excitation sources with the solid surfaces of materials various forms of energy are emitted from the solid surfaces and detected by energy analyzers. 

The ionization of a molecule and the rupture of a chemical bond by ionizing radiation necessarily result in the pairwise formation of radical species. The pairwise correlation of radical species will be more or less retained in solid polymers where the radical migration is restricted. This heterogeneity of spatial distribution of radical species affects the radiation chemistry of polymers. Another source of spatial heterogeneity is the heterogeneous deposition of radiation energy [6, 7]. Low LET radiations such as y-rays produce an ensemble of isolated spurs. Each spur is composed of a few ion-pairs and/or radical... 

Schuchmann [1], and the book by von Sonntag, The Chemical Basis of Radiation Biology both contain an enormous amount of useful information [2], Bernhard s review article Solid-State Radiation Chemistry of DNA The Bases , covers the early work in the same area as presented here [3], The review article entitled Radical Ions and Their Reactions in DNA Constituents EPR/ENDOR Studies of Radiation Damage in the Solid-State was an attempt to update Bernhard s 1981 review [4],... 

Concurrently with the view that reactions of electrons and positive ions play an important part in the radiation chemistry of liquids, it was being demonstrated, notably by Hamill(56) and his co-workers over the period 1962-1966, that charge trapping, migration and reaction were also important In the radiation chemistry of many solid systems, especially at low temperatures. As a result of these studies, >-irradiation of low temperature solids has become perhaps the most versatile method of studying the spectroscopic properties of radical anions and cations. 

In real-world applications, the importance of interfaces is hard to overestimate and three chapters are devoted to the effects of radiation at aqueous-solid boundaries. Jonsson focuses on applications within the nuclear industry where basic studies on radiation effects at water-metal interfaces have enabled a proposal for safe storage of spent nuclear fuel. Also with implications for the nuclear industry, Musat et al. document alterations in the radiation chemistry of liquid water confined on the nanoscale. Such nanoconfmed solutions are prevalent in the media proposed and indeed in use for waste storage. In another application, radiation chemistry has successfully been used to produce nanoscale objects such as metallic clusters and nanoparticles, an area summarized by Remita and Remita. 

G. J. Dienes, G. H. Vineyard, Radiation Effects in Solids, Interscience, London, 1957 A. J. Swallow, Radiation Chemistry of Organic Compounds, Pergamon, Oxford, 1960 A. Charlesby, Atomic Radiation and Polymers, Pergamon, Oxford, 1960... 

The effect of ionizing radiation on phenols has been stndied mainly in aqneons soln-tions nnder oxidizing conditions, where the phenols are reacted with hydroxyl radicals or with transient one-electron oxidants to yield, indirectly or directly, phenoxyl radicals. The reactions leading to formation of phenoxyl radicals, as well as the properties and reactions of phenoxyl radicals in aqneons solntions, are discnssed in the chapter on transient phenoxyl radicals. In this chapter, other aspects of the radiation chemistry of phenols are summarized. These include studies with phenols in organic solvents and in the solid state, reactions leading to reduction of substituted phenols in various media and radiation treatment of phenols for detoxification purposes. 

IV. RADIATION CHEMISTRY OF NEAT PHENOLS (AND IN SOLID MATRICES)... 

Since phenols are solid under ambient conditions, few studies were concerned with the radiation chemistry of phenols in the gas phase. An early study demonstrated the acetylation of phenols when irradiated in a specific gaseous mixture. Gas-phase y-irradiation of a mixture of CH3F and CO was found to form the acetyl cation, CHsCO , and to lead to acetylation of substrates . Gaseous phenol, cresols and xylenols present in such a mixture were acetylated mainly at the OH group to form 80-97% aryl acetate. The remaining products, hydroxyacetophenones, were mainly the ortho and para derivatives. 

The radiation chemistry of poly(tetrafluoroethylene-co-perfluoropro-pylene) copolymers has been studied using Co-60 y-radiation at temperatures of 300 and 363 K. New structure formation in the copolymers was analysed by solid-state NMR. ... 

Paramagnetic species, generated in the vapor phase in a crossed-beam experiment by irradiation with 1 Mev. He ions, have been trapped at 77 °K. and detected by electron spin resonance (ESR). This paper describes the results obtained from irradiated methyl-, ethyl-, and tert-butyl alcohol, acetone, and ethylene. Trapped electrons together with the radicals CH2OH, CHsCHOH, and (CHa)2C(OH)CH2- and (CH i).tC are formed in methyl-, ethyl-, and tert-butyl alcohol respectively. Ethyl radicals are formed from ethylene. Acetone gives rise to CHjCOCH, and CHS radicals and appears to form trapped electrons in the deposit. The results are compared with the radiation chemistry of these systems in the solid and vapor phase. 

The experiments with 5,6-dihydrothymine are a preliminary attempt to compare rates of formation of transients by addition and by abstraction. Dihydrothymine is the saturated analog of thymine. The transient species which it forms by reaction with OH have an absorption maximum at 400 n.m. at pH 7 and at 320 n.m. or lower at pH 12.4. The rate of formation of these transients appears to be slightly less than one-third of the rate of formation of transients by addition to thymine. At pH 7, OH adds to the 5,6 double bond of thymine. The aqueous solution radiation chemistry of dihydrothymine has not been investigated, but electron paramagnetic resonance (EPR) studies in the solid phase show that H atoms abstract... 

The Radiation Chemistry of Anhydrous Solid State Glycine, Alanine, and Glycine Salts... 

A major chemical effect of y-rays on simple peptides such as the N-acylamino acids under oxygen-free conditions, both in the solid state and in concentrated aqueous solution, leads to formation of labile amidelike compounds which are readily degraded on mild hydrolysis to yield ammonia as a characteristic product. Several classes of nitrogen-deficient products are formed concomitantly with the ammonia. Earlier communications have discussed certain limited aspects of the radiolytic lability of simple peptides in the solid state and in concentrated solutions (9, 10, 18). The radiation chemistry of these systems is more complex than that involved in the radiolysis of simple peptides in dilute oxygen-free aqueous solution under which conditions main-chain degradation is of minor importance (10). In this paper we report detailed experimental evidence... 

Radiation-induced curing of plastic coatings has been discussed,8 and radiation-induced polymerization in the solid state reported.87 The radiation chemistry of epoxy-containing electron resists88 and polycondensation induced by ionizing radiation in the urea-formaldehyde system88 have been described. Radiation-induced copolymerization of the following pairs of monomers has been achieved ethylene-hexafluoropropylene,80 tetrafluoroethylene-propylene,81 tetrafluoroethylene-hexafluoropropylene,82 hexafluoroacetone-a-olefins,83 MMA-di- and tri-methacrylates,84 styrene-acrylonitrile,85 buta-1,3-diene-acrylonitrile,88 and acenaphthylene-vinylcarbazole.87... 

Protein radiation chemistry has been studied for more than 30 years and a wealth of data has been accumulated. In the solid phase, only modifications of the polypeptidic backbone were shown. They concern the surface. More precisely it seems that weak points are turns and loops. Nothing is known concerrung modifications of residues. The knowledge about radiation chemistry of membrane proteins is also extremely poor. Efforts in this field would be relevant for biology. Let us mention that one of the most important free radical producer systems of living cells is partly buried in a membrane (NADPH oxidase) (see for instance 238). 

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