Radicals peptide

The major fragmentation processes correspond to several competing radical-driven pathways that must necessarily proceed from different tautomeric forms of the peptide radical cation. (From Barlow et ah, 2006)... 

M(IV)-peptide radicals, and decay to final products have been determined (96). Interestingly, the rate constants for both the formation and decay of the high-valent intermediate are much larger than the corresponding values in the chromium system in Scheme 1. Thus the lifetimes of both the hydroperoxo species and M(IV)-peptide radicals are much shorter in the enzymatic system than for the chromium macrocycles. 

Fig. 4.6.5. Assay for spectroscopic investigation of how aromatic amino acids modulate DNA-mediated HT. The peptide radicals generated by DNA-mediated HT have different...

Keywords Amino acids Carbohydrates Peptides Radical reactions... 

Reaction of Peptide Radicals. On the basis of the kind of physicochemical evidence presented here and of the chemical evidence described elsewhere (46,47), it is apparent that the various radicals formed initially in the irradiation of peptides and proteins convert to other radicals that subsequently feact to form products. Conversion of cation radicals has already been mentioned. Conversions of the peptide a-carbon radicals are especially important to understanding the radiolysis of proteins, so some illustrative examples will be given. The eventual reaction of the a-carbon radicals is not well understood, but certain assumptions can be made relevant to the systems studied. 

Abstraction of H from the Peptide by Carbon Radicals. The carbon radicals derived from deamination, deamidation, and decarboxylation react subsequently with the peptides to abstract hydrogen, forming the more stable a-carbon peptide radicals (63). This reaction can be demonstrated in irradiated ices containing acetylamino acids. Continuing with the N-acetylalanine example, one can also see in Figure 7 that upon further annealing to — 50 °C, another radical appears corresponding to reaction 19 ... 

Figure 7. ESR spectra of radicals derived from N-acetyl-iL-alanine in an irradiated D20 ice plug. Sample contained 190 mg/ml of N-acetylalanine and was irradiated to 5 kGy at —196°C. Spectrum a was recorded at —196°C spectra b and c at —135°C. All spectra have markers from Fremys salt superimposed. (a) Composite spectrum of radicals present after annealing at —153°C (approximately 65% corresponds to the anion), (b) Composite spectrum of the radicals formed upon annealing the ice plug to — 80°C, of which 55% corresponds to the fatty acid radical, -CH(CHS)C02. (c) Spectrum of the peptide radical, CHsC0NDC(CHs)C02, formed upon further annealing the ice plug...

Reactions initiated by H and OH would lead to these and other radicals. H could react at the backbone C-H and at the carbonyl group, forming the peptide radical, but is more likely to add to aromatic or heterocyclic amino acid moieties, forming side chain radicals. OH could also react by abstraction or, more likely, by addition. The formation of addition radicals facilitates crosslinking through the side chains. Studies with polyphenylalanine peptides confirm such crosslinking and show how crosslinked products with and without hydroxyl groups can be produced [15, 16]. 

Figure 5. Decay of the ESR signal of the peptide radical in meat at -10°C.

These reactions show that dimers can be formed (and have been observed) if either peptide radicals or side chain radicals on the globin surface react intermolecularly. They also show that if these radicals are close enough to the iron center, electron transfer can occur leading to either reduction or oxidation, depending on the valence state, and to an associated color change. 

Turecek, R N-C bond dissociation energies and kinetics in amide and peptide radicals. Is the dissociation a non-ergodic process J. Am. Chem. Soc. 2003,125, 5954—5963. 

Formula RCO-NH(CH2)3N"(CH3)2CH2CHOHCH2.X-, RCO = coconut oil fatty acid, X = peptide radical derived from hydrolyzed collagen Properties Amphoteric... 

Asakawa, D. and Takayama, M. (2012) Fragmentation processes of hydrogen-deficient peptide radicals in matrix-assisted laser desorption/ionization in-source decay mass spectrometry. J. Phys. Chem. B, 116,4016 23. 

Mormann, M. Paulsen, H. Peter-Katahnic, J. Electron capture dissociation of O-glycosylated peptides Radical site-indnced fragmentation of glycosidic bonds. European Journal of Mass Spectrometry 2005,11, 497-511. 

The formation and dissociation of phosphotylated peptide radical cations [pM] has been studied. Phosphoserine- and phospho-threonine-containing peptide radical cations were formed by low-energy collision-induced dissociation (CID) of the ternaiy metal-ligand... 

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