Oxygen radicals, radical strength

Table 12 Radical strength of oxygen radicals (Y0-) in terms of their YO-H bond formation free energies, and dissociative bond energies for H-R molecules...

Table 5-1 Radical Strength of Oxygen Radicals (YO ) in Terms of Their YO-H Bond-Formation Free Energies (-AGbf)/ and Dissociative Bond Energies (AHdbe) for H-R Molecules...

OH free radicals react with almost all amino-acids. For aliphatic residues, rate constants are correlated with the strength of the X-H bond(X = S, C or N) (1). Thus the reaction is relatively slow with glycine (k = 1.7 x 10 mol 1 s ) and fast with the -SH function of cysteine (k = 1.9 x 10 mol M s i). The thiyl radical formed upon oxidation of cysteine, whose spectral properties are in table 3, is formed but a carbon-centered radical is also present (50, 51). In the presence of oxygen, thiyl radical fixes O2 giving peroxy radicals (52). These radicals may photoisomerize into sulfonyl radicals RS02 (53). In small molecules, disulfide groups can also be oxidized. This reaction was not demonstrated in proteins, but cannot be neglected. A disulfide radical cation is formed (54). Final compounds are not known. 

Before a discussion of specific reactions, let s briefly consider the nature of the carbonyl excited state. For ,ir states, a good model for the carbonyl excited state is the biradical-like structure shown in Eq. 16.54, especially for the triplet state. What should we expect from such a structure There are two reactive centers, a carbon-based radical and an oxygen radical. To first order they are independent, one electron in the plane and the other in the it system (see Figure 16.7). The oxygen radical should be extremely reactive. Recall the high bond strength of the O-H bond discussed in Chapter 2, and how this makes hydroxyl radical a... 

We discussed how the n,TT states of carbonyls, especially the triplets, have considerable radical character. In addition, oxygen-centered radicals are very reactive because of the high strengths of O-H and O-C bonds. Hydrogen atom abstraction should be expected, and indeed it is a common reaction. A typical reaction involves a carbonyl n,TT state undergoing a bimolecular hydrogen atom abstraction, and that is the basis for the common process called photoreduction. 

Two of the many products of ethylene radiolysis—methane and propane—show no or only negligible variation with field strength. Methane is produced by a molecular elimination process, as evidenced by the inability of oxygen or nitric oxide to quench its formation even when these additives are present in 65 mole % concentration (34). Propane is completely eliminated by trace amounts of the above scavengers, suggesting methyl and ethyl radicals as precursors ... 

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