Separation, amino acids into optical isomers

Alanine, aspartic acid, glutamic acid, tyrosine, leucine, phenylalanine and also a-amino-n-caproic acid and a-aminobutyric acid have in this way been separated by Fischer into their optically active isomers. To these must be added ornithine which was synthesised by Sorensen in 1903, and separated into d- and 1-ornithine in 1905. 

By combining a-aminophenylacetic acid with d-camphorsulphonic acid Betti and Mayer in 1908 separated it into its isomers. This seems to be the first case in which the basic function of an amino acid has been requisitioned for purposes of separation in all the above cases, the acidic function, by combination with optically active bases, has been made use of... 

The production of a-amino acids by chemical synthesis yields a mixture of DL forms. The D-fomi of glutamic acid has no flavor-enhancing properties and thus requires transformation into the optically active form insofar as monosodium glutamate is concerned. The three methods for separating the optical isomers are (1) preferential inoculation method ... 

The circulins—As early as 1949, Peterson and Reineke characterized circulin as its sulphate. Total hydrolysis yielded D-leucine, L-threonine and L-K,y-diaminobutyric acid together with an optically active isomer of pelargonic acid. The existence of two components, found by Peterson and Reineke was later confirmed by the chromatographic separation of crude circulin into two major components, named circulin A and circulin B. In addition there was evidence for at least three other ninhydrin-positive, biologically active entities. In the hydrolysate of circulin A, L-isoleucine was found besides the amino acids previously reported . Quantitative amino acid analysis showed circulin A and B to be composed of L-a,y-diamino-butyric acid, L-threonine, D-leucine, L-isoleucine and ( + )-6-methyloctanoic acid in the molar ratio 6 2 1 1 1. After partial acid hydrolysis, fractionation and structure determination of the resulting peptides, circulin A and circulin B were formulated as cyclodecapeptides . Very recently, however, Japanese workers have revised the structure of circulin A. According to them, circulin A differs from colistin A only by a replacement of L-leucine in the latter by L-isoleucine Figure 1.7). 

Impregnated TLC has been, for the first time, reported by Bhushan and Parshad [8] for the resolution of ( )-ibuprofen into its enantiomers. Two-dimensional TLC on silica gel plates impregnated with optically pure L-arginine (0.5%) as chiral selector, using acetonitrile/methanol/water (5 1 1, v/v) as the solvent system, has been successful (Table 13.1). As L-arginine has a pi of 10.8 a few drops of acetic acid were added. This allowed to maintain pH below the isoelectric point and to keep the amino acid in the cationic form. Separate plates were run for one- and two-dimensional modes. In the one-dimensional mode the spots of ( )-ibuprofen and the (-l-)-isomer were applied side by side on the same plate. In the two-dimensional mode, first the spot of ( )-ibuprofen was applied to the L-arginine impregnated plate and then the spot of the (-l-)-isomer was applied after the first run at the side of the former spot. The developed plates were dried and the spots were located in iodine chamber. 

The racemic mixture are separated into their individual, substantially optically pure isomers through well-known techniques such as, for example, the separation of diastereomeric salts formed with optically active adjuncts, e.g. acids or bases followed by conversion back to the optically active substances. So the 4-acethylamino-5-amino-3-(l-ethyl-propoxy)-cyclohex-l-ene-l-carboxyllic acid ethyl ester,(3a,4p,5a) was obtained. 

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