L -Isoleucine

Diamide Chiral Separations. The first chiral stationary phase for gas chromatography was reported by GH-Av and co-workers in 1966 (113) and was based on A/-trifluoroacetyl (A/-TFA) L-isoleucine lauryl ester coated on an inert packing material. It was used to resolve the tritiuoroacetylated derivatives of amino acids. Related chiral selectors used by other workers included -dodecanoyl-L-valine-/-butylamide and... 

The asterisk signifies an asymmetric carbon. AH of the amino acids, except glycine, have two optically active isomers designated D- or L-. Isoleucine and threonine also have centers of asymmetry at their P-carbon atoms (1,10). Protein amino acids are of the L-a-form (1,10) as illustrated in Table 1. 

Purification. The objective of crystallization also can be purification of a chemical species. For example, L-isoleucine (an essential amino acid) is separated by crystallization from a fermentation broth that has been filtered and subjected to ion exchange. The recovered crystals contain impurities deleterious to use of the product, and these crystals are, therefore, redissolved and recrystalHzed to enhance purity. 

By measuring the L-isoleucine/o-alloisoleucine ratio in the protein isolated from the eggshells of an extinct Australian bird, a team of scientists recently determined that this bird lived approximately 50,000 years ago. Radiocarbon ( " C) dating is not accurate for samples older than about 35,000 years, so AAR is a useful addition to the tools available to paleontologists. 

The first partial chiral resolution reported in CCC dates from 1982 [120]. The separation of the two enantiomers of norephedrine was partially achieved, in almost 4 days, using (/ ,/ )-di-5-nonyltartrate as a chiral selector in the organic stationary phase. In 1984, the complete resolution of d,l-isoleucine was described, with N-dodecyl-L-proline as a selector in a two-phase buffered n-butanol/water system containing a copper (II) salt, in approximately 2 days [121]. A few partial resolutions of amino acids and dmg enantiomers with proteic selectors were also published [122, 123]. 

Chemical structure. The structure of the free base of Cypridina luciferin (C22H27ON7, Mr 405.50) was determined by Kishi et al. (1966a,b) as shown below (A) its sec-butyl group is in the same configuration as in L-isoleucine. The structure of oxyluciferin reported by the same authors contained an error, and the structure was corrected later as shown in Fig. 3.1.8 (McCapra and Chang, 1967 Stone, 1968). 

Figure 30-21. Subsequent catabolism of the tiglyl-CoA formed from L-isoleucine.

Much was unknown for the halogenation for unreactive substrates until very recently, when the biosynthesis of the cyclopropyl amino acid side chain of coronatine was elucidated. This intriguing pathway, which involves /-chlorination of an enzyme-bound L-isoleucine followed by chloride displacement by the a-carbon, yields the cyclopropanated precursor... 

Although not of fatty acid origin another group of scarab beetles utilizes amino acid derivatives as pheromones [119]. The large black chafer, Holotrichia parallela, uses L-isoleucine methyl ester [ 120] and the cranberry white grub, Phyllophaga anxia, uses both L-isoleucine and i.-valine methyl esters [121]. More recently L-isoleucine methyl ester, N-formyl L-isoleucine methyl ester, and N-acetyl L-isoleucine methyl ester were identified in the scarab beetle Phyllophaga elenans [ 122]. These pheromone components are obviously derived from the amino acids isoleucine and valine. 

Similar to the work described by Spohn et al. [34], a trienzyme sensor was developed recently for the determination of branched-chain amino acids (L-valine, L-leucine, and L-isoleucine). Leucine dehydrogenase, NADH oxidase, and peroxidase were coimmobilized covalently on tresylate-hydrophylic vinyl polymer beads and packed into a transparent PILL tube (20 cm X 1.0 id), which was used as flow cell. The sensor was free of interferences from protein and NH4+ and it was stable for 2 weeks. The sensor system was applied to the determination of branched-chain amino acids in plasma with recoveries ranging from 98 to 100% [36],... 

Gramicidin A consists of a pair of congeners containing primarily L-valine gramicidin A (80-95%) but also containing L-isoleucine gramicidin A (5-20%)33 3I. ... 

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