Dilute solutions, 7-radiolysis

Rao and Symons49 studied the formation of radicals in y-radiolysis of dilute solutions of dimethyl sulfoxide in fluorotrichloromethane. By ESR studies they found the radical cation (CH3)2SOt whose ESR spectrum show considerable g anisotropy and small methyl proton hyperfine coupling. 

While aqueous solutions mean only dilute solutions, mixtures can be any proportion of HjO and DMSO. Cooper and coworkers found in the pulse radiolysis of H2O/DMSO mixtures two easily resolvable absorption bands at wavelengths >400nm. One band corresponds to the oxidizing species with a maximum at 600 nm and a relatively long half-life (1 to 4/is). The second band with higher wavelength (720-1500 nm) and shorter half-life is attributed to the solvated electrons. 

Pulse radiolysis is used also for preparation of excited states of dienes and polyenes. This is done by irradiation of the diene/polyene in toluene solution. The radiolysis of toluene yield high concentration of molecules in the triplet excited state of the solute. Wilbrandt and coworkers61 pulse-radiolysed 1 mM solution of al I -lrans-1,3,5-heptatriene in toluene solution and observed the absorption spectra of the triplet state of the heptatriene with a maximum at 315 nm. The same group62 produced and measured the absorption spectra of several isomeric retinals in their lowest excited triplet state by pulse irradiation of their dilute solution in Ar-saturated benzene containing 10 2 M naphthalene. Nakabayashi and coworkers63 prepared the lowest triplet states of 1,3-cyclohexadiene,... 

Ionizing radiation can act in two distinct ways on organic substances. In the absence of water, in condensed systems or in concentrated solution, the predominant effects occur directly on the organic molecule and produce electronic excitations or ionizations which may lead to chemical modification. In dilute solution (1% or less) the major effects are the result of reactions between the solute and reactive species produced by the radiolysis of water. These indirect effects are the subject of this article. 

The primary effect of radiation on aqueous solution is the decomposition of the water, followed by reactions of the transients from water with the solutes present. Direct effect of radiation on the solute is practically negligible up to concentrations of about 1 M, and because we are usually dealing with reasonably dilute solutions the discussion will be restricted to these indirect effects. It is, therefore, important to know what radicals are produced in irradiated water, how they react with the organic solutes, and how they can be manipulated for the production of certain organic radicals. These subjects have been studied very thoroughly and are sufficiently well understood to enable us to use the radiolysis of aqueous solutions for studies of diverse chemical problems. 

However, this mechanism does not explain the chain reaction. Tabata and coworkers measured the optical spectrum of the dimer cation radical, by pulse radiolysis of benzonitrile solution of the dimer immediately after the pulse. They found only a peak at 770 nm without other peaks, except for a possible small shoulder at 740 nm (which is within the limit of experimental error). Addition of cation scavengers leads to elimination of this spectrum, while oxygen does not remove it, suggesting that the spectrum is due to a cation. This 770-nm peak of the cation of the cyclodimer of VC reminds one of the 770-nm peak found 1.6 jus after the pulse in the case of 1 M VC solution. It should be noticed that while in this second paper the authors also mentioned this shift from 790 nm to 770 nm, the data in their figure show a peak at 790 nm both immediately and 1.6 jus after the pulse. Consequently, Tabata and coworkers suggested that the observed spectrum in pulse radiolysis of aerated solution of VC in benzonitrile is a composite of the spectrum of VC cation together with that of the cation of the cyclodimer of VC. The contribution of each intermediate to the observed spectrum depends on the concentration of VC and how long after the pulse the spectrum was taken. In a dilute solution, the dimer cation will be produced as time proceeds, but it is absent immediately after the pulse. In concentrated solutions, both cations coexist even immediately after a pulse. 

A separate development of the TRMC techniques was their application to the study of dipolar and excitonic species formed on flash-photolysis of dilute solutions and, more recently, to charge transport and charge separation in thin (aligned) solid films. In the present review we restrict ourselves to results that we have obtained on pulse-irradiated materials, for which the method has become known as the pulse-radiolysis time-resolved microwave conductivity or PR-TBAIC technique. 

This review covers the interaction of radicals generated from low LET radiation in water with protein components, proteins and, finally, the metallocenters themselves. It commences with a discussion of the experimental techniques that have been the most amenable to these studies, those of gamma and pulse radiolysis. It then addresses, very generally, the radiolysis of the building blocks of proteins, amino acids, and follows with the radiolysis of the proteins themselves. In both cases, the discussion is limited to radiolysis of dilute solute in water, where the initial radiation is absorbed totally by water and does not directly interact with the amino acids/proteins. 

As has been discussed here, the reactions that occur upon radiolysis of dilute solutions of proteins in water are very much dependent upon the amino acid composition of the protein, protein folding (i.e. residues that are surface exposed), the presence of metal catalytic centers and the particular radicals that are generated in solution. As has been illustrated in the last two examples, radiation chemistry can serve as a probe of the redox processes that occur upon catalysis and, under very specific systems, can also serve to generate the substrate for enzyme catalysis. 

The mechanistic details of amine oxidation has also been extensively studied by use of radiation chemical methods [84-90]. In the radiolysis of dilute solutions, interaction of the ionizing radiation occurs predominantly with the solvent molecules resulting in the formation of reactive intermediates derived from the solvent [91]. 

Tladiation chemists have been aware for about 15 years that the presence of dilute solutes in liquid hydrocarbons can change the course of radiation chemical reactions by other than the normally expected secondary radical reactions. For example, Manion and Burton (40) in early work on the radiolysis of benzene-cyclohexane solutions, drew attention to the possibility of energy transfer from solvent to solute. Furthermore, it is known that in hydrocarbon solvents certain solutes are capable of capturing electrons, thus interfering with the normal ion-recombination process (14, 20, 65, 72). Though ionic products can be observed readily in hydrocarbon glasses [e.g., (19, 21)] demonstration of effects which can be specifically ascribed to electron capture in the liquid state has been elusive until recently. Reaction of positive ions prior to neutralization can play an important role as demonstrated recently by studies on... 

Until 1970, pulse radiolysis studies were limited to those species with lifetime >10" sec and observations on the hydrated electron were therefore carried out using only dilute solutions of scavenger. More recently, it has been possible to develop pulses of lO" sec and consequently it has been possible to study much earlier events in the radiolysis and also to study the disappearance of the hydrated electron in concentrated scavenger solutions. Rate coefficients have been found to depend on the concentration of scavenger [66—68]. Thus in the competition of hydrogen ion and acetone for the hydrated electron, viz. 

It is interesting that the greater stability of the ammoniated electron makes its observation easier in continuous radiolysis than that of the hydrated electron. The ESR spectrum of dilute solutions shows a single, narrow line. The g factor is 2.0012 which is close to the free electron value and indicates only a weak interaction between electron and solvent [84]. 

Radiolysis of the aqueous nitrate system is discussed in terms of (a) indirect effect in dilute solution, and (b) concurrent indirect and direct effects in concentrated solution. Analysis of energy fractionation breaks down (b), gives G(N02 )no3- = 4.0, and demonstrates stoichiometry for direct effect according to... 

In these reactions the unidentified species is symbolized by its formal oxidation number—i.e., N03 by N(VI), N02 by N(IV), etc.) The clue to the behavior of OH radicals came from the observation that concentration vs. dose curves for N02" formation were initially nonlinear until concentrations of N02" reached about 2 yM and were thereafter independent of N02". This behavior was observed over a wide range of nitrate concentrations from 10"3 to 6.0M, but the nonlinearity was eliminated by adding small quantities of N02 before irradiation. These facts clearly indicate that Reaction 4 is the fate of the OH radicals, and this was confirmed by demonstrating competition kinetics (5) between H2 and NOo" for the OH species. The main features of the radiolysis in dilute solution can thus be accounted for by Reactions 1 and 4, followed by a hydrolytic dismutation process... 

The pulse radiolysis work of Pikaev et al. (15) further verifies this scheme. They measured N02" and H202 formation as a function of intensity at very high intensities. For dilute solutions both G(N02 ) and G(H202) increased considerably at higher intensities, owing to the incidence of Reaction 6... 

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