Hydrogen abstraction from amino acids

Hydrogen atom abstraction from amino acid derivatives is especially facile, since the resulting captodative radicals are highly stabilized. Treatment of an alanine derivative with DTBP led to a mixture of an a-methylated product and a diastereomeric mixture... 

Examples of photoreduction processes, brought about by hydrogen abstraction from solvents are the reduction of azobenzene to hydrazo-benzene,81 of 2,3-diphenyl-5,8-quinoxalinedione (CXXXIII) to its hexahydro derivative,148 of pteroxyl-L-glutamic acid to dihydro-2-amino-4-hydroxypteridine-6-aldehyde,80 and of the blue gem-chloro-nitroso compounds CXXXIV-CXXXVI to the corresponding ketox-imes.189... 

In biological systems, free radicals can react with cellular macromolecules in a variety of ways, the most important of which is hydrogen abstraction from DNA leading to chain scission or cross-linking. In proteins, tryptophan is the amino acid residue most susceptible to free radical attack. Lipid peroxidation by free radicals in turn is liable to cause alteration in cell membranes. 

The diamagnetic products are observed and can be characterized very well because the detection method is high-resolution NMR spectroscopy. On the other hand, the signal enhancement by the CIDNP effect mitigates the inherent low sensitivity of NMR. In consequence, even diamagnetic species that are unstable and thus present only in low concentration can be captured. An early example is the enol of acetophenone formed in the photoreaction of acetophenone with phenol [45], others are vinylamines in photoinduced hydrogen abstractions from aliphatic amines [46] (see Section V.B) and in the sensitized photoreactions of amino acids (Section V.G.l). 

Two major chemical modifications of proteins that occur during alkaline treatment are crosslinking and racemlzation. Lysine, ornithine (via arginine), cystine and 0-substituted serine can participate in base-catalyzed reactions forming the crossllnked amino acids lysinoalanine, ornithinoalanine and lanthionine Under the same conditions, inversion can occur when the or hydrogen of an amino acid residue is abstracted by the base, resulting in a planar, optically inactive carbanion ( ), as illustrated in Figure 1. The carbanion may be reprotonated from either face of the plane, which causes inversion when this occurs from the opposite face. 

Scheme 60). Griesbeck et al. assume that in a non-polar solvent such as benzene the intramolecular electron transfer from the methionic sulfur group is much faster than the abstraction of hydrogen from the hydroxyl group of the unprotected amino acid. C-Hydrogen abstraction leads to 313, whereas previous lactonization of the zwitterionic biradical 311 yields 314. Since the cis-hydroxy acid is not detected it is conceivable that it cyclizes immediately to the lactone 314. Photolysis of the corresponding methyl ester under the same conditions attains improved yields (84% combined) of two diastereomeric tricyclic products in a ratio of 48 52. 

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]. 

Figure 6. Relationship between yields of low molar mass nitrogen-containing products resulting from elimination of a section of the main chain and hydrogen abstraction radical from the irradiation of poly(amino acid)s at 25° C. (A) tyr, (A) phe, ( ) gly, (O) ala, ) val.

Fig. 9.6 Abstraction of a hydrogen atom from the a-C of an amino acid by hydroxyl radical has been investigated computationally...

The generation of such a radical was effected through hydrogen atom abstraction from N-acetylglycine ethyl ester by a photoexcited acetone molecule. This radical can add to olefins yielding the corresponding derivatives of the higher alpha amino acids (70),... 

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