Radical aromatic amino acid

The hydroxyl radical reacts at nearly a diffusion limited rate (k 1 x 10 s to 5 X 10 s ) with the aromatic amino acids, as well as with me-... 

The COs radical anion is a strong one-electron oxidant ( 7-1.7 V vs NHE [15]) that oxidizes appropriate electron donors via electron transfer mechanisms [103]. Detailed pulse radiolysis studies have shown that carbonate radicals can rapidly abstract electrons from aromatic amino acids (tyrosine and tryptophan). However, reactions of CO3 with S-containing methionine and cysteine are less efficient [104-106]. Hydrogen atom abstraction by carbonate radicals is generally very slow [103] and their reactivities with other amino acids are negligible [104-106]. 

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). 

Affinities between NOSs and BH4 are stronger than those between aromatic amino acid hydroxylases and BH4, so the purified NOS from animal tissues still contain 0.2-0.5 BH4 molecules per heme moiety [128]. BH4 tightly binds to endothelial and neural NOSs with dissociation constants in the nanomolar range, and this binding is reported to stabilize the dimeric structure of NOS [129-131], whereas aromatic amino acid hydroxylases do not have BH4 in the proteins. BH4 functions as a one electron donor to a heme-dioxy enzyme intermediate. The BH4 radical remains bound in NOS and is subsequently reduced back to BH4 by an electron provided by the NOS reductase domain [128]. 

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...

Eberlein GA, Bruice TC, Lazarus RA, et al. The interconversion of the 5,6,7,8-tetrahydro-, 6,7,8-dihydro, and radical forms of 6,6,7,7-tetramethyldihydropterin. A model for the biopterin center of aromatic amino acid mixed function oxidases./Am Chem Soc 106 7916-7924,1984. 

It is necessary that an antioxidant protects cells at all stages of oxidative stress, and therefore an antioxidant should be able to scavenge the secondary radicals produced by the reaction of primary radicals with biomolecules. Radiation chemists designed methods to study reactions of secondary radicals from amino acids of proteins and base and sugar radicals of DNA with antioxidants.The most commonly employed aromatic amino acid radicals generated by radiation chemical experiments are the indolyl radicals of tryptophan (TRP ), the... 

Metallothionein (MT) is an ubiquitous cytosolic, cysteine-rich, metal- and free radical-binding protein. Since the discovery of MT four decades ago, extensive efforts have been directed to define the physiological role of MT. The first extensive chemical characterization of MT demonstrated that it contained 5.9% Cd2+, 2.2% Zn2+ and 8.5% S (w/w), and that 95% of the S was present as sulfhydryl groups of cysteine residues40. The sulfhydryl groups were all found to be involved in cation binding. Aromatic amino acids and histidine were absent and the protein had little or no absorbance at 280 nm. 

Addition of HS to double bonds of the aromatic amino acids (Reaction 7) will result in the formation of a new carbon radical. This... 

Debye and Edwards noted two components in the decay of low temperature protein phosphorescence. The greater part of the decay was exponential in character having lifetimes on the order of several seconds. A weak, and much longer-lived component, however, was reported to have the same emission spectrum but a non-exponential decay. Debye and Edwards claimed that this emission was a result of radical recombination following photoejection of an aromatic amino acid electron into... 

Along with the optically excited triplet states, EPR also results in the detection of free radicals formed upon irradiation of frozen solutions of the aromatic amino acids 74,76). In all aqueous and alcoholic media examined solvent radicals were induced by the presence of an aromatic solute. Once formed the radicals are stable after extinction of the exciting light until the solution is warmed to temperatures at which solvent reorientation begins to take place 74). 

The major interest in such radical EPR studies lies in identifying the role of the triplet states in photobiological processes. The establishment of triplet state intermediates has now been confirmed in a wide variety of photoionization and sensitization reactions 76,80,81) in frozen solutions of nucleic acid bases, porphyrins including chlorophyll and, of course, the aromatic amino acids 82). Comparisons of the photo-induced products formed in these systems with observations in vivo provides strong evidence that the triplet state is an essential intermediate in many photobiological processes. 

In peptides, amino acids are joined by peptide bonds to form the polypeptide chain. The e q, OH, and H radicals react with the amino acids and with the polypeptide chain with high rate coefficients that are close to the diffusion limit. OH radicals and, less frequently, H atoms abstract a hydrogen atom from the polypeptide chain (from the carbon atoms) or from the side chains, but they can also add to the rings of the aromatic amino acids. The radical site produced on the main chain may move along the chain by an internal hydrogen abstraction mechanism. If the H atom is abstracted from an S-H bond the radical migration stops, since the S-H bond is much weaker than the G-H bonds. This reaction is the basis of repair mechanism. The radicals may terminate in radical-radical reactions. 

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