L-aminoacids

A very attractive and efficient method for the synthesis of L-aminoacids via DKR has been reported by Turner et al. [41a,b]. They employed enzyme-catalyzed ring opening of 5(4H)-oxazolones in combination with a catalytic amount of Et3N. The relatively low pKa of the C-4 proton (8.9) of oxazolones facilitates racemization. Hydrolysis of the ester obtained through DKR, followed by debenzoylation, yields L-aminoacids in excellent enantiomeric excess (99.5%) (Figure 4.16). In their initial studies, they employed Rhizomucor miehei lipase (Lipozyme) as the biocatalyst [41]. More recently, they have obtained excellent results employing CALB [41bj. This method has also been employed by Bevinakatti [41c,d] and Sih [41e,fj. 

Figure 3.8 The two enantiomers of a-aminoacids. Here we follow the classic nomenclature of l- and D-aminoacids for indicating the two chiral forms. In terms of the S, R nomenclature, L-aminoacids correspond to the S absolute configuration -except for cystein, which is R.

The question of the origin of life is usually linked to the question of the origin of molecular asymmetry, specifically why only L-aminoacids (with extremely few exceptions) are present in nature (see Figure 3.8). 

Different syntheses of y-amino-/3-ketoeter derivatives from iV-protected L-aminoacids by Al,Al -carbonyldiimidazole activation and treatment with the magnesium enolate of hydrogen ethyl malonate are described. These compounds are useful intermediates in the preparation of active amino acid analogues, as illustrated and summarized in equation 103. 

An amazing selectivity and efficiency characterizes chemical reactions carried out in Nature that are catalyzed by specific enzymes. The fact that living organisms are built almost exclusively (see, however, [15]) from L-aminoacids... 

Production of antibiotics (Industrial scale) J. [4] Production of L-aminoacids (Industrial scale) [5] Production of L-aspartic acid (Industrial scale) [6] Production of L-alanine (Industrial scale) [7]... 

Other important applications in the food industry running at a large scale are the production of L-aspartic add with Escherichia coli entrapped in polyacrilamides [6], the immobilization of thermolysin for the production of aspartame [14], The production of L-alanine by Tanabe Seiyaku [7], the production of frudose concen-centrated syrup [3], the production of L-malic acid by the use of Brevibacterium ammoniagenens immobilized in polyacrilamide by entrapment immobilization methods [11] and L-aminoacids production by immobilized aminoacylase [5],... 

Fig. 3 Different handedness and supramolecular chirality of DNA (A, B and Z forms) arising from the same D-nucleotides, and the triple right-handed helix of polypeptide natural collagen made of natural L-aminoacids...

The microcystins are a group of over 70 structurally related monocyclic heptapeptides containing seven peptide-linked aminoacids with the general structure of Cyclo-(D-alanine -X -D-MeAsp -Y" -Adda -D-glutamate - Mdha ), in which X and Y are variable L aminoacids, D-MeAsp is D-erytro- - methylaspartic acid and Mdha is N-methyldehydroalanine (Fig. 14.1). 

The most important of the peptidic phosphatase inhibitors are the microcystins and nodularin. Mycrocystins are heptapeptides characterised by the sequence cyclo(D-Ala-X-D-e/7t/i/ o-P-methylisoAsp-Y-Adda-D-isoGlu-iV-methyldehydroAla), where X and Y are different L-aminoacids, and Adda is the abbreviation of the P-aminoacid [25,35,85,95]-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4(E),6(E)-dienoic acid. In the most common microcystin, namely microcystin-LR, X is Leu and Y is Arg. This kind of compounds was considered to be the highly hepatotoxic principle of the cyanobacteria genera Microcystis, Artabaena and Oscillatoria. Apart Jfrom the variations represented by X and Y, other differences arising jfrom the demethylation of aminoacids, lead to the existence to more than fifty microcystins. The rare acid Adda is also... 

Carboxypeptidase A (from bovine pancreas, peptidyl-L-aminoacid lyase) [11075-17-5] Mr 34,600 [EC 3.4.17.1]. Carboxypeptidase A is purified by DEAE-ceUulose chromatography, activation with... 

J.C. Cooper, J. Danzer, H.-L. Schmidt, Enhanced selectivity in flow-injection analysis for L-aminoacids using electrodialysis with amino acid oxidation, Anal. Chim. Acta 282 (1993) 369. 

Aminoacylase stereospecifically cleaves N-acetyl-L-aminoacids to acetate and l-aminoacid, a reaction well known for the production of optically pure L-aminoacids (Eq. (2)). 

The equilibrium conversion for hydrolysis of the acetyl-L-aminoacid depends on the initial oncentration. Only at zero concentration may one hundred percent conversion be attained (Fig. 7-6). 

It is very important to assay the essential amino acids in food and feeds for their nutritional value. To obtain the most reliable analytical information, an electrometric method based on amperometric biosensors122123 was adopted. Because of the reliability assured, the amperometric biosensors can be integrated in an amperometric FIA system.123 For the assay of aminoacids it is very important to discriminate the l- and D-amino acids. Constructed amperometric biosensors based on L-aminoacid oxidase and D-amino acid oxidase can discriminate the l- and D-amino acids.124 125 For this purpose, these electrometric methods assure the best reliability of the analytical information as a result of the simplicity of sampling process, and because of the possibility of assaying the enantiomers without prior separation from the matrix. 

Enzymatic phosphoryl transfer reactions are ubiquitous in nature and play significant roles in ATP hydrolysis and protein phosphorylation processess. As previously described, pentacoordinate phosphorus species have been assumed as transient intermediates or transition states in these pathways and their structural and electronic properties are undoubtedly related to the outcome of the process. Therefore, to aid understanding of the phosphorus-catalyzed biological routes, many model pentacoordinated phosphoranes have been synthesized. While most studies have focused on aspects of apicophilicity, anti-apicophilicity or Berry pseudorotation, there have been limited investigations on the stereochemistry of pentacoordinated spirophosphoranes with a chiral phosphorus atom. In the past year, much attention has been paid to the synthesis and determination of absolute configuration of several chiral pentacoordinate spirophosphoranes derived from D- and L-aminoacids. Some significant achievements in this area will be discussed in this section. 

D, L-N-trifluoro aoeM-a-methylaminoacid L-aminoacid D-N-lrifluoro ace -a-nwthyl amino acid... 

This reactivation reminds the process described by Curti et al. (8) for the flavoprotein L-aminoacid oxidase from venom this enzyme suffers a deactivation process by freezing that involves changes in its tertiary structure. The process is probed to be reversible in the presence of the addecuate buffer, hi the case of the FNR, the decreasing in the binding constant for the electrostatic complex between the reactive FNR ferredoxin, that resembles to the value found for the constant in the complex of the native enzyme with ferredoxin (table 2 ), also shows that some change in the tertiary stmcture of the FNR has taken place. 

Amino acylases catalyse the hydrolysis of N-acyl-L-aminoacids (Figure 6.28) but not of the corresponding D-isomers. This reaction is comparable to that catalysed by penicillin acylase which in fact shows a similar selectivity when hydrolysing amides other than penicillin G. If the racemic mixture of an amino acid is first acylated and is then treated with the acylase, the product will be a mixture of optically pure L-amino acid and predominantly D-acyl amino acid. These are easily separated on the basis of their solubilities at the isoelectric point of the amino acid. The pure L-amino acid can be crystallized from the mixture leaving the residual acyl amino acid to be racemized and recycled through the process. 

The a-hydroxy-acids are first oxidized to keto-acids with a mixture of l- and D-a-hydroxyacid dehydrogenases. The dehydrogenases use NAD" " as cosubstrate, and this is reduced to NADH. In the second stage of the reaction this NADH is used to synthesize an L-amino acid from ammonium ions and the ketoacid in a reaction catalysed by an amino acid dehydrogenase. For the synthesis of leucine the l- and D-hydroxyacid dehydrogenases are obtained from Lactobacillus confusus and L. casei respectively, while the L-aminoacid dehydrogenase is obtained from Bacillus sphaericus (Figure 6.30). ... 

Similar results were obtained by Furukawa et al (20) when using diethylzinc-L-aminoacid systems in polymerization of methyl oxirane. 

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