L-a-amino acids

The nonprotein amino acid, 1-aminocyclopropane-l-carboxylic acid, is an intermediate of ethylene biosynthesis in plants. This amino acid is synthesized from the L-a-amino acid methionine through the intermediate 5 -adenosyl-L-methionine (SAM) (Scheme 8).  

Both D-amino acids and non-a-amino acids occur in nature, but only L-a-amino acids are present in proteins. 

Figure 1 Stereochemical drawing and Fischer projection of an L-a-amino acid, where R is the side chain of the amino acid.

There is only a small selection of nonprotein amino acids that contain carbonyl groups in the form of ketone, aldehyde, and carboxylic acid moieties, as part of the side chain. The examples given in Table 6 are components of nonribosomal peptides isolated from bacteria or fungi and siderophores from bacteria. The biosynthesis of these amino acids is not clear however, some of the amino acids with carboxylic acid side chains may be traced back to the L-a-amino acids aspartic acid and glutamic acid. 

Using this procedure, D- and L-a-amino acids have been enantiodifferentiated in the gas phase. ESI of hydroalcoholic solutions of the amino acid and CUCI2 into the source of an ion trap mass spectrometer reveals the presence of singly charged, covalently bound dimeric and trimeric ions. Table 10 reports the CID results of the diastereomeric complexes [A/ -Cu -(ref)2-H] and [As-Cu (ref)2-H].  

Protein a polymer formed from a family of 20 common L-a-amino acids. 

For the purpose of this chapter, all amino acids containing side chains that differ from the L-a-amino acids will be designated as either unusual or nonprotein amino acids. This is due to the observation that these unusual amino acids are generally not found to be the constituents of proteins. The scope of this chapter is limited to nonprotein L-a-amino acids. The definition of nonprotein amino acid is not absolute and, therefore, the 

Nature utilizes the shikimate pathway for the biosynthesis of amino acids with aryl side chains. These nonprotein amino acids are often synthesized through intermediates found in the shikimate pathway. In many cases, L-a-amino acids are functionalized at different sites to yield nonprotein amino acids. These modifications include oxidation, hydroxylation, halogenation, methylation, and thiolation. In addition to these modifications, nature also utilizes modified biosynthetic pathways to produce compounds that are structurally more complex. When analyzing the structures of these nonprotein amino acids, one can generally identify the structural similarities to one of the L-a-amino acids with aromatic side chains. 

Fig. 9.6 Mirror image behaviour of enantiomeric molecules. The left-handed L-a-amino acid is converted to the right-handed D-a-amino acid by reflection 

In a closely related study, Tung and Sun discussed the microwave-assisted liquid-phase synthesis of chiral quinoxalines [80], Various L-a-amino acid methyl ester hydrochlorides were coupled to MeOPEG-bound ortho-fluoronitrobenzene by the aforementioned ipso-fluoro displacement method. Reduction under microwave irradiation resulted in spontaneous synchronous intramolecular cyclization to the corresponding l,2,3,4-tetrahydroquinoxalin-2-ones (Scheme 7.71). Retention of the chiral moiety could not be monitored during the reaction, but after release of the desired products it was found that about 10% of the product had undergone racemization. 

Scheme 10.7 1,3-Dipolar cycloadditions of nitrones with 1,1-diethoxypropene catalysed by oxazaborolidines derived A-tosyl-L-a-amino acids.

The extension of the term isotactic to condensation polymers was made by Natta in his first article discussing poly-L-a-amino acids (22). In itself the term isotactic is redundant here as the configuration of the repeating unit is sufficient to identify the macromolecular stmcture. It is, however, useful to distinguish a system, racemic or not, in which each macromolecule is composed of only l or only D residues from a mixture of macromolecules made up of random or alternate sequences of L or d units. Similarly the term syndiotactic serves in the identification of oligopeptides or polypeptides composed of alternate sequences of D and L units, like those synthesized by Lorenzi and Tomasic (77). 

Naturally air-fused DKPs are thermodynamically more stable, compared with their trawr-fused counterparts. This seems logical considering their biosynthetic origin, usually from two proteinogenic L-a-amino acids. Some other cis- and rntwr-functional DKPs are derived from nonproteinogenic D-a-amino acids. Naturally 

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