Scavenger molecules

Fig. 23. Recombination probability of radical pairs formed in the presence of a radical scavenger (diphenylpicrylhydrazine) plotted against the scavenger concentration and also the most probable distance between scavenger molecules. Radicals are usually termed free when they are essentially uncorrelated with any other radical (i.e. radical—radical distances >100nm). At shorter radical—radical distances, the correlation becomes strong and these are termed secondary pairs or primary pairs if formed as an encounter pair. Taken from Hammond et al. [290] for the recombination of cyanocyclohexyl radicals in chlorobenzene at 80 C.

The direct charge transfer to dichloroacetate proposed in reaction (7.21) requires that the scavenging molecules are adsorbed on the Ti02 surface prior to the adsorption of the photon. Otherwise, this reaction could not compete with the normal hole-trapping reactions (7.9) and (7.10). So the adsorption of the model compound DCA on the titanium dioxide surface prior to the bandgap excitation appears to be a prerequisite for an efficient hole scavenging. 

Free radical damage is considered to be a causative factor in the development of cancer and inflammatory and chronic diseases. Therefore, free radical scavenging molecules (antioxidants) may play a beneficial role in these conditions. With repect to CVD, the oxidation of low-density lipoprotein (LDL) is believed to be a critical process in the development of atherosclerosis (Berliner et al., 1995 Navab et al., 1995). The presence of oxidized LDL in the intima of an artery leads to the production of macrophage-derived foam cells, the main cell type present in fatty streaks that are believed to be the earliest lesion of atherosclerosis (Fuster, 1994). Therefore, the use of antioxidants as dietary supplements to protect against LDL oxidation may reduce both the development and progression of atherosclerosis (Gey, 1995). 

In addition, a large number of free radical-scavenging molecules, including glutathione, are found in cells. 

An exception is technetium sulfide (TcjSy), known as " Tc-sulfur colloid (Stern et al. 1966). Scavenging molecules like phosphinimine (R3P = N-SiMe3) have been reported to incorporate TcO, producing organic molecules containing Tc(VII) (Katti et al. 1993 Singh et al. 1995). 

Maxwell et al. identified carboxylesterase as another enzyme with the potential to be a good anti-organophosphorous scavenger molecule (summarized in Table 7.4). °... 

Removal of carbon radicals is slow when irradiated ribonuclease is treated with molecules larger than H2S such as chloroform, isopropyl cyanide, and isoamyl mercaptan. For isoamyl mercaptan (15 torr, room temp.) less than half of the carbon radicals react in 3 hours (Figure 2). However, after 144 hours all radicals have reacted. The use of large radical scavenger molecules for studying the conformation of proteins in the dry state is discussed later. 

The rate constants, kj2, of the forward reaction (12) are an order of magnitude lower than those of the class (i) reactions, though some of the holetrapping solutes have comparably low adiabatic IPs. The values of kiz did not correlate with the observed ACP of reaction (12). An explanation was proposed that the rate constants are controlled by the height of the activation barrier determined by the difference in the vertical IP of the solute and the adiabatic IP of the solvent [11]. This suggests that electron transfer to the rapidly-migrating solvent hole (as it passes by the scavenger molecule) is much faster than the relaxation time of the solute radical cations. 

Neither of these expressions resembles the low scavenger concentration form [eqn. (172)] at all closely. In the exponential function of eqn. (176a), appropriate for small initial ion-pair separation distances, rg, the term k c ro/SDrc appears. Recasting this as (47rrgCs/3) ks/4 TTrJ)) shows this to be the product of the number of scavenger molecules within a sphere of radius rg and the ratio of the (homogeneous) rate of reaction between an ion and scavenger and the ion-pair. 

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