Glycine radicals structures

Rega N, Cossi M, Barone V (1997) Intrinsic and environmental effects in the structure and magnetic properties of glycine radical in aqueous solution. J Am Chem Soc 119 12962—12967... 

This enzyme catalyzes the conversion of pyruvate to formate and acetyl CoA and is a key enzyme in the anaerobic degradation of carbohydrates in some Enterobacteriaceae. Using an enzyme selectively C-labeled with glycine, it was shown by EPR that the reaction involves production of a free radical at C-2 of glycine (Wagner et al. 1992). This was confirmed by destruction of the radical with O2, and determination of part of the structure of the small protein that contained an oxalyl residue originating from gly-734. 

Amino Acid vs. Peptide Yields. The peptides show greater yield reductions at low temperatures than do the corresponding amino acids, suggesting a stabilizing effect on the radicals produced at low temperature. Yields from glycylmethionine more closely resemble the yields from methionine than from glycine. This is consistent with the results of an electron spin resonance study of crystalline glycylglycine (0), which showed the structure of the radical produced at 20°C. to be ... 

Flash photolysis and laser flash photolysis are probably the most versatile of the methods in the above list they have been particularly useful in identifying very short-lived intermediates such as radicals, radical cations and anions, triplet states, carbenium ions and carbanions. They provide a wealth of structural, kinetic and thermodynamic information, and a simplified generic experimental arrangement of a system suitable for studying very fast and ultrafast processes is shown in Fig. 3.8. Examples of applications include the keton-isation of acetophenone enol in aqueous buffer solutions [35], kinetic and thermodynamic characterisation of the aminium radical cation and aminyl radical derived from N-phenyl-glycine [36] and phenylureas [37], and the first direct observation of a radical cation derived from an enol ether [38],... 

Superoxide radicals are another factor in oxidative damage. They can be determined with nitrobluetetrazolium (NBT), which then forms the colourless formazan. When melanoidins scavenge the superoxide radicals, the colour of the NBT persists.490,491 The activity of a glucose-glycine melanoidin on superoxide radicals is equivalent to the effect of 16 units of superoxide dismutase. The effect of the HMM and LMM fractions of this melanoidin is almost the same. The reaction rate constant of the melanoidin was markedly higher than that of ascorbic acid. If this were due to the reductone structures embedded in the melanodin, it is difficult to explain why the reducing power of the melanoidins is only 0.7 that of ascorbic acid.490... 

So far, all AFGP analogs prepared in our laboratory have utilized the L-lysine-glycine-glycine tripeptide unit that is radically different in structure than the core repeating tripeptide unit found in native AFGP. The threonine residue in native AFGP was replaced with lysine for two reasons. Firstly, recent work has demonstrated that an alanine-lysine rich polypeptide possessed weak antifreeze protein-specific activity (26). 

FIGURE 6.2 Structures of glycine and glycyl radicals. The orientations of the principal axes of the g tensor are also shown. 

For the first two peptides, CysS radicals abstract hydrogen atoms from the or-carbon of glycine with 7 = (1.0 to 1.1) x 10 s , while the reverse reaction proceeds with = (8.0 to 8.9) x 10 s . For the latter peptide, CysS radicals abstract hydrogen atoms from the ce-carbon of alanine with = (0.9 to 1.0) x 10 s while the reverse reaction proceeds with k -j = 1.0 x 10 s" The order of reactivity, Gly > Ala, is in accordance with previous studies on intermolecular reactions of thiyl radicals with these amino acids. The fact that < k y suggests that some secondary structure prevents the adoption of extended conformations for which calculations of homolytic bond dissociation energies would have predicted k j > k y. 

Type III enzymes operate anaerobically, and rather than a2 32 quaternary structure of the Type I enzymes, have an a2 + 32 structure, in which the permanent radical site, presumably stabilised capto-datively, lies on a main chain glycine residue of the dimeric a unit. The site is generated by a separate small iron-sulfur protein ((32 subunit) which brings about a homo-olytic cleavage of 5 -adenosylmethionine, whose fragments generate the glycyl radical. 

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