Peptide backbone radical

This commonality in the radiolysis was established not only for cooked meats irradiated to high doses at -40°C, but also for raw meats irradiated to lower doses at 77 K [29]. The ESR spectra for raw pork, beef sirloin, and chicken breast (Figure 14) show the singlets associated with radicals formed by electron addition to the carbonyl groups, the yields of which linearly increased with dose. After annealing at -78°C, the spectral features changed to the predominant asymmetric doublet associated with the peptide backbone radical. Moreover, a direct comparison of spectra at -78°C for raw and roasted turkey breasts irradiated to 3.8 kGy showed no differences, indicating that native and denatured conformers of the protein respond radiolytically in similar ways [29]. 

The electron is accepted by an amide-associated proton on the peptide backbone. This very unstable radical reacts very quickly to cleave the peptide bond at the site of reaction. 

Moreover, formation of radical transients with S.-.O bonds is kinetically preferred, but on longer time scale they convert into transients with S.-.N bonds in a pH dependent manner. Ultimately transients with S.-.N bonds transform intramolecularly into C-centred radicals located on the C moiety of the peptide backbone. Another type of C-centred radicals located in the side chain of Met-residue, a-(aikylthio)alkyl radicals, are formed via deprotonation of MetS +. C-centred radicals are precursors for peroxyl radicals (ROO ) that might be involved in chain reactions of peptide and/or protein oxidation. Stabilization of MetS +through formation of S.-.O- and S.-.N-bonded radicals might potentially accelerate oxidation and autooxidation processes of Met in peptides and proteins. Considering that methionine sulfoxide, which is the final product coming from all radicals centred on sulphur, is restored by the enzyme methionine sulfoxide reductase into MetS, stabilization of MetS +appears as a protection against an eventual peroxidation chain that would develop from a carbon centred radical. 

Oxidation of the amino acid moieties in irradiated aqueous systems by reaction with OH is well established for fluid systems, but it is not likely to be encountered in frozen systems. Being a strong oxidant, the OH reacts by electron transfer. It also adds readily to double bonds and abstracts H from C—H, N—H, and S—H bonds, but with lower reaction rate constants. A compendium of rate constants for aqueous solution has been published (52) and a few representative values for amino acids are shown in Table I. As discussed by Simic (53), the predominant sites for reaction in amino acids and peptides can be inferred from these values, which indicate that the ring groups are favored, while abstraction from the peptide backbone is less likely. Hydroxylation of the phenylalanine ring also occurs as was found for the prototype reaction with benzene (54). Formation of phenoxyl radical following OH addition to tyrosine should be similar to the mechanism established for phenol (55) in which elimination of water occurs as is shown in reaction 12 ... 

Tervalent copper and nickel are involved in the autoxidation reactions of [Cu(H 3G4)] and [Ni(H 3G4)] respectively. In the case of nickel, decomposition of [Ni(H 3G4)] proceeds by decarboxylation of the terminal carboxy-group adjacent to the peptide nitrogen. - With copper, decomposition of [Cu-(H sG4)] proceeds through a carbon-centred free radical produced by abstraction of a hydrogen atom from the peptide backbone. Bulky carbon substituents assist the stabilization of the higher-oxidation state ions, and a study of the stabilities of leucyl tripeptide complexes with copper(ii) and nickel(u) has been reported. Copper(iii) and nickel(iii) tripeptide complexes of a-aminoisobutyric acid are thermally stable but are readily decomposed by photochemical pathways. Resonance Raman and other studies with copper(iii) peptide complexes have also been reported. ... 

Several peroxidative reactions initiated by OH radical that may be generated by either y- and X-radiolysis of aqueous solutions or by transition metal-catalyzed reduction of H2O2 have been identified in free amino acids and short peptides. In this respect we may distinguish oxidizing reactions that involve the polypeptide backbone on the one hand... 

Peptides derived from fish proteins have shown the ability of exerting potent antioxidative activities (Table 15.1) in different oxidative systems (Rajapakse et ah, 2005). Currently, an increasing interest exists to explore natural antioxidative substances without side effects and the identified antioxidative activities have potential to develop safe and nonhazardous natural antioxidants for the complications arose from oxidation of biomolecules. Je et ah (2007) purified an antioxidative peptide from tuna backbone protein and identified as VKAGFAWTANQQLS (1519Da) which was very important regarding the functional foods. It was reported that the derived peptides were good radical scavengers and antioxidant. 

All the constituent amino acid sidechains in proteins are susceptible to attack by oxidants and free radicals, but some are more vulnerable than others. Thus, exposure of proteins to free radical-generating systems may induce tertiary structural changes as a consequence of modifications to individual amino acid sidechains. As secondary structure is stabilised by hydrogen bonding between peptide groups, interactions of radical species with the polypeptide backbone and interference with the functional groups of the peptide bonds may cause secondary structural modifications. Disruption of the secondary structure may also occur under certain conditions of free radical attack at the a-carbon atom of the peptide bond [20],... 

Abstract This review provides an overview of some of the more recent work directed to exploit radical-based chemistry for the modification of some of Natures most important biomolecules, such as amino acids, peptides, and carbohydrates. Radical reactions are particularly advantageous for carrying out a variety of structural modifications on biomolecules as the reaction conditions are typically compatible with a wide variety of functional groups and solvents. An array of effective synthetic transformations will he discussed including selective side chain and backbone modifications of amino acids and peptides, along with methods for the transformation of carbohydrate substituents, as well as fragmentation and cyclizations reactions for the preparation of either structurally modified carbohydrates or chiral building blocks. 

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