Water radicals

In the presence of a proton donor, such as water, radical anions accept a proton and the reaction scheme assumes the form... 

Unique radiolytical products (URP) in irradiated food are usually formed by the secondary reactions of water radicals, eh, H, and OH, and to a lesser extent by the direct action of radiation, especially for foods with considerable water content. Due... 

The initial ionization of a water molecule produces an electron and the water radical cation. The water radical cation is a strong acid and rapidly loses a proton to the nearest available water molecule to produce an HO radical and HsO. The electron will lose energy by causing further ionizations and excitations until it solvates (to produce the solvated electron Ca ). In addition to the two radical species HO and e q, a smaller quantity of H-atoms, H2O2, and H2 are also produced. 

Von Sonntag [7] has estimated that the direct effects contribute about 40% to cellular DNA damage, while the effects of water radicals amount to about 60%. A paper by Krisch et al. [15] on the production of strand breaks in DNA initiated by HO radical attack has the direct effects contribution at 50%. 

Ionization of DNA s solvation shell produces water radical cations (H20 ) and fast electrons. The fate of the hole is dictated by two competing reactions hole transfer to DNA and formation of HO via proton transfer. If the ionized water is in direct contact with the DNA (F < 10), hole transfer dominates. If the ionized water is in the next layer out (9 < r < 22), HO formation dominates [67,89,90]. The thermalized excess electrons attach preferentially to bases, regardless of their origin. Thus the yield of one-electron reduced bases per DNA mass increases in lockstep with increasing F, up to an F of 20-25. This means that when F exceeds 9, there will be an imbalance between holes and electrons trapped on DNA, the balance of the holes being trapped as HO . At F = 17, an example where the water and DNA masses are about equal, the solvation shell doubles the number of electron adducts, increasing the DNA-centered holes by a bit over 50% [91-93]. 

Watanabe, R. Usami, N. Takakura, K. Hieda, K. Kobayashi, K. Water radical 5uelds by low energy vacuum ultraviolet photons as measured with Fricke dosimeter. Radiat. Res. 1997,148, 489-490. 

For glutamic acid (18) and glycine (10) the yield of ammonia varies approximately as the cube root of the concentration. This variation agrees with the diffusion of the spur model which derives from the hypothesis that at higher solute concentrations, water radicals are scavenged which would react with each other in more dilute solution. However, for the effect of cathode rays on the aromatic amino acids phenylalanine, tryptophan, and tyrosine and for cystine, this relationship is inverted, and amino acid destruction decreased with an increase in concentration (29). 

In irradiated solutions of proteins, the reacting groups (the amino acid residues) are concentrated into small regions of the solution, and the water radicals are produced in small spurs (approximately 10 A. in radius). The kinetics of their interactions are therefore expected to be different from those of free amino acids which are distributed randomly throughout the solution. A brief treatment of this problem has been presented by Schwartz 32). 

The water radical cation, produced in reaction (1), is a very strong acid and immediately loses a proton to neighboring water molecules thereby forming OH [reaction (3)]. The electron becomes hydrated by water [reaction (4), for the scavenging of presolvated (Laenen et al. 2000) electrons see, e.g., Pimblott and LaVerne (1998) Pastina et al. (1999) Ballarini et al. (2000) for typical reactions of eaq, see Chap. 4], Electronically excited water can decompose into -OH and 11- [reaction (5)]. As a consequence, three kinds of free radicals are formed side by side in the spurs, OH, eaq , and H . To match the charge of the electrons, an equivalent amount of ED are also present. 

Table 2.1. Radiation-chemical yields (G values units 10-7 mol J-1) of water radicals, ions and molecular products at a scavenger capacity of ca. 2 x 10s s-1 under the conditions of sparsely ionizing radiation (60Co-y-rays, high-energy electrons) in the presence of different saturating gases (von Sonntag 1987)...

Chatterjee A, Magee JL (1985) Theoretical investigation of the production of strand breaks in DNA by water radicals. Radiat Protect Dosimetry 13 137-140 Deeble DJ, von Sonntag C (1984) y-Radiolysis of poly(U) in aqueous solution. The role of primary sugar and base radicals in the release of undamaged uracil. Int J Radiat Biol 46 247-260 Deeble DJ, von Sonntag C (1986) Radiolysis of poly(U) in oxygenated solutions. Int J Radiat Biol 49 927-936... 

Verberne JB, Lafleur MVM, Hummel A, Loman H (1987) Radiation chemistry and biological effects. Non-homogeneous kinetics of reactions of water radicals with biologically active DNA. IAEA Panel Proceedings Series 55-68... 

The definition of the sea boundary of the mouth area is related to the term mouth-mixing zone. Water salinity within this zone increases from the salinity inherent in river water (usually 0.2-0.5%o) to the salinity of seawater (usually 10-40%o in different seas). The salt composition of water radically changes within the mixing zone river water of hydrocarbonate class and calcium group transforms into seawater of chloride class and sodium group. 

Water Water radical anion Hydrated electron... 

The initial ionization of a water molecule produces an electron and the water radical cation. The water radical cation is a strong acid and rapidly loses a proton to the... 

Since most studies have been conducted using dilute aqueous solutions, the substrate radicals are mainly formed by attack of water radicals. However, although these must be of great importance in biological conditions, it must be remembered that not all of the key systems are dilute aqueous solutions. This is true of lipids, membrane-bound proteins and tightly packed chromatin, for example. 

Conversion of primary water radicals into secondary radicals... 

WATER — Radical Water of Metals — So called because it is their root and principle. 

Shine and Mach (1965) have reported the spectrum of the pheno-thiazine radical-cation (45), generated by treatment with sulphuric acid of either the parent compound or its neutral radical (i.e. the conjugate base) on treatment with water. Radical-cations of related heterocycles have been studied using similar methods (Shine and Small, 1965 Shine and Davies, 1965). 

The simultaneous presence of pollutants of different nature in wastewater gives rise to a very complex kinetic degradation scheme. The kinetic parameters of each reaction constitute the basic data needed to design an efficient degradation process. A suitable tool to make such kind of investigation is pulse radiolysis (Chapter 2) coupled with a kinetic spectrometry detection system [7]. Aiming to achieve meaningful results, reactions with water radicals are studied separately as outlined below (Chapter 1) ... 

Table I. Rate Constants for Reaction of Primary Water Radicals with Some Amino Acids and Peptides in Fluid Aqueous Solutions"...

The string of beads model has been proposed by Samuel and Magee [12] and has been widely used for the discussion of diffusion-controlled reactions in water. Radicals are supposed to be formed in spherical volumes called spurs . About 40 eV energy is deposited in each spur which are equidistant. The distance between spurs is about 3000 A for a 450 eV electron in water. The initial spur radius is 10—15 A. The picture of an electron track according to this model is given in Fig. 4(a). 

Laboratory experience had convinced chemists earlier that the Chapman mechanism needed a supplement of additional reactions. In 1960, McGrath and Norrish discovered the formation of OH radicals in the reaction of water vapor with 0( D) atoms generated by the photolysis of ozone, and they proposed a chain decomposition of ozone by water radicals. Meinel (1950) had previously demonstrated the existence of OH in the upper... 

Additional chemical losses due to water radical reactions 9.3 9.0... 

Figure 3-7 further shows mixing ratios for nitric acid in the stratosphere. Nitric acid arises from the interaction of N02 with water radicals. The relevant reactions are... 

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