Pulse radiolysis radiation-generated radicals

Radiation methods, pulse radiolysis (22, 23), and 7 irradiation techniques (24) prove to be elegant and versatile methods not only for generation of unstable radicals, but also for the study of their physical properties and reactivity. 

Carotenoid radicals — Many of the important oxidations are free-radical reactions, so a consideration of the generation and properties of carotenoid radicals and of carbon-centered radicals derived from carotenoids by addition of other species is relevant. The carotenoid radicals are very short-lived species. Some information has been obtained about them by the application of radiation techniques, particularly pulse radiolysis. Carotenoid radicals can be generated in different ways. "... 

A very interesting technique for radical generation is flash photolysis, which employs a very intense pulse of radiation (visible or u.v.) of very short duration. This produces a very high immediate concentration of radicals, which may be detected—and whose fate may be followed—by spectroscopy through one or more subsequent pulses of lower intensity radiation of suitable wavelength. This is, of course, primarily a technique for the study of radicals rather than for their use in preparative procedures. Radicals may also be generated, in suitable cases, by irradiation of neutral molecules with X-rays or with y-rays radiolysis. 

Radiation-induced reactions include photo-induced reactions or those initiated by radicals generated by an electron beam—the pulse radiolysis method. Reactions initiated by a light signal clearly can be of conventional time range or rapid. The latter... 

Although, in principle, ESR spectroscopy is the most powerful method for detecting radical intermediates, it has not been widely used in conjunction with steady-state or pulse radiolysis, mainly because of technical difficulties. Important early work was done by Fessenden, Schuler and their co-workers (see for example Eiben and Fessenden, 1968 and Fessenden and Schuler, 1971) using steady-state radiation from a Van der Graaf accelerator. In this way, liquid-phase ESR spectra were generated for a range of radicals never previously observed by ESR methods. 

Pulse Radiolysis A technique related to flash photolysis pulse radiolysis uses very short (nanosecond) intense pulses of ionizing radiation to generate transient high concentrations of reactive species. See Salmon, G. A. and Sykes, A.G., Pulse radiolysis, Methods Enzymol. 227, 522-534, 1993 Maleknia, S.D., Kieselar, J.G., and Downard, K.M., Hydroxyl radical probe of the surface of lysozyme by synchrotron radiolysis and mass spectrometry. Rapid Commun. Mass Spectrom. 16, 53-61, 2002 Nakuna, B.N., Sun, G., and Anderson, V.E., Hydroxyl radical oxidation of cytochrome c by aerobic radiolysis, Free Radic. Biol. Med. 37, 1203-1213, 2004 BataiUe, C., Baldacchino, G., Cosson, R.P. et al., Effect of pressure on pulse radiolysis reduction of proteins, Biochim. Biophys. Acta 1724, 432-439, 2005. 

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. 

The use of radiation chemistry to study proteins in water can be accomplished through slow or fast techniques gamma radiolysis and pulse radiolysis, respectively. The difference between these two applications of radiation chemistry is that, in the former case, a continual irradiation of the water produces a steady-state flux of radicals and usually involves a gamma-ray generator such as a °Co source to produce the radicals. In the latter case, an electron accelerator is used to deliver short bursts of electrons to water in the nanosecond (10 s) to picosecond (10 s) time scale. 

Research on ascorbate and dehydroascorbate can be performed by conventional biochemical methods, but study of the relatively short-lived ascorbate free radical requires methods such as flow techniques with rapid mixing or pulse radiolysis coupled with polarography, ESR, or spectrophotometry. Ascorbate free radicals have been generated preferentially by oxidation of ascorbic acid [enzymatic (14-17), chemical (18-20), radiation chemical (21-26), and photochemical (27)] because ascorbic acid is easily available in a high purity grade but dehydro-ascorbic acid is not. 

The largest contribution of radiation chemical techniques to general free-radical chemistry has been made in aqueous solution because they provide a very convenient way of generating an enormous variety of highly reactive species which cannot readily be generated by thermal or photochemical methods. In particular, the technique of pulse radiolysis has provided a wealth of kinetic and mechanistic information in inorganic, organic, and biochemistry [4,5]. 

Pulse radiolysis is a unique technique to generate specific radicals and to study the kinetic parameters for the primary radical attack, the stability of the secondary formed radicals, and the redox potentials of these secondary formed radicals [110]. In pulse radiolysis a short pulse of ionizing radiation will generate excited states, ions, and radicals in a sample solution [111]. In aqueous solutions a short electron pulse will result within 1 nanosecond (10-9 sec) in the formation of the following primary radicals (Eq. 11) [112] ... 

In another radiation technique, pulse radiolysis, high-energy electrons formed in a Van de Graaff generator lead to cation radical reactions in solution with very short periods of irradiation. Solvent ions or radicals are formed which may next remove an electron from the solute. For example, TMPD and some triarylaminc cation radicals have been made in carbon tetrachloride solution [(66) and (67)] by... 

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