D-form, of amino acids

Table 7-1 Difference in Taste Between the L-and D-Forms of Amino Acids...

Analysis of Amino Acids. Differentiation of the l- and D-forms of amino acids is essential because they differ in their biological and physiological properties. Although chromatographic columns that effect separations of chiral compounds could be used for analysis of solutions of these acids, a combination of a reactor containing a stereoselective immobilized enzyme and a chromatographic system provides the necessary selectivity for such analyses. 

Biological systems depend on specific detailed recognition of molecules that distinguish between chiral forms. The translation machinery for protein synthesis has evolved to utilize only one of the chiral forms of amino acids, the L-form. All amino acids that occur in proteins therefore have the L-form. There is, however, no obvious reason why the L-form was chosen during evolution and not the D-form... 

A possible explanation for the preference of living systems for the L (levorotatory) over the D (dextrorotatory) optical isomer may be associated with the stereoselective properties of layered minerals. To test this hypothesis, the rates of L- and D-histidine intercalation into HT layered compound was investigated using the pressure-jump relaxation technique (21). The rate constants and interlayer spacing based on this investigation are summarized in Table V. As shown the slightly enhanced rate for L-histidine suggests that relative chemical reactivity may be associated with natural selection of the L-form of amino acids in nature. 

On the other hand, free amino acid analysis represents a powerful tool to characterize different foods and beverages, monitor proteolysis, assess freshness, detect adulterations, and safeguard consumer health. The occurrence of some potentially toxic nonprotein amino acids (some of which are neuroexcitatory) in commercially available seedlings has been reported by different authors [197-199]. Due to the incapability of humans to utilize the o-isomers of amino acids (some of which are thought to be toxic), the enantiomeric separation of d- and L-form of amino acids is also an area of growing interest [196]. 

There are two major approaches to achieve enantiomeric separation of d- and L-amino acids. The first involves precolumn derivatization with a chiral reagent, followed by RP-HPLC [226], while the second involves direct separation of underivatized enantiomers on a chiral bonded phase [227], Weiss et al. [209] determined d- and L-form of amino acids by applying derivatization with OPA and chiral /V-isobutyryl-L-cysteine. 

It has been reported that a variety of single-chain amphiphiies spontaneously produce stable, membrane-forming aggregates when dispersed in water 258 260). Dialky 1-amphiphile l-III (l or d means l- or D-configuration of amino acid unit in compound III, respectively), which was prepared by condensation of didodecyl L-glutamate and p-(4-bromobutoxy)benzoic acid and the subsequent quarterization with tri-methylamine, produces bilayer vesicles in water as probed by electron microscopy 251 >. 

A number of other racemases and epimerases may function by similar mechanisms. While some amino acid racemases depend upon pyridoxal phosphate (Chapter 14), several others function without this coenzyme. These include racemases for aspartate,113 glutamate,114-1153 proline, phenylalanine,116 and diamino-pimelate epimerase.117 Some spiders are able to interconvert d and l forms of amino acid residues in intact polypeptide chains.118119... 

Figure 2.10. (Bottom) Difference between l and d amino acids. The two naturally occurring forms of amino acids differ in the position of the R group with respect to the backbone. An L-amino acid has the R group on the left if viewed along the chain from the free carbonyl end to the free amino end as shown. If the R group is on the right, it is defined as the d form. The predominant form found in proteins is the l form, although some amino acids in the d form are present in proteins.

Nutritional Implications. The nutritive quality of any protein depends on three factors amino acid composition, digestibility, and utilization of the released amino acids. Bacemization brought about by processing can impair the nutritive value of proteins by (a) generating non-metabolizable forms of amino acids (D-enanticmers), (b) creating peptide bonds inaccessible to proteolytic enzymes, and (c) toxic action (or interaction) of specific D-enanticmers. Little is known concerning the health consequences of human consumption of racemized proteins. No study has specifically evaluated amino acid losses due to racemization within food proteins. 

D-Enantiomers of amino acids in living organisms are attributed to many important bioprocesses or can be markers of certain disorders. Therefore, accurate quantification of the low levels of the D-form in the presence of a large amount of L-form is of considerable interest. Branched D-amino acids in mammalian tissues and body fluids were quantified recently using a sophisticated two-dimensional HPLC system containing a narrow-bore reversed phase and t-BuCQN column for the enantioseparation of l- and D-enantiomers. Target analytes were determined as their fluorescent derivatives, precolumn labeled with 4-fluoro-7-nitro-2,1,3-benzoxadiazole. D-Val, alio-lie, lie, and Leu were determined at nmol/ml level, making this a valuable method for quantification of D-enantiomers of amino acids [80]. 

In subsequent process generations, peniciUin G acylase derived enzymes were also used to couple the synthetic side chains, such as D-phenylglycine (ampicillin, cephalexin) and D-p-hydroxyphenylglycine (amoxicillin, cephadroxil) in the form of amino acid amides or esters to 6-APA and 7-ADCA (Scheme 4.6D). Biotransformation routes to the n-phenylglycine and n-p-hydroxyphenylglycine side chains were also developed (Scheme 4.6C), but the enzymatic process towards n-phenyl-glycine amide has been substituted by a classical resolution. 

Chakrabarti, A. C. and Deamer, D. W. (1994). Permeation of membranes by the neutral form of amino acids and peptides relevance to the origin of peptide translocation. Journal of Molecular Evoution, 39, 1-5. 

In polar solvents, the structure of the acridine 13 involves some zwitterionic character 13 a [Eq. (7)] and the interior of the cleft become an intently polar microenvironment. On the periphery of the molecule a heavy lipophilic coating is provided by the hydrocarbon skeleton and methyl groups. A third domain, the large, flat aromatic surface is exposed by the acridine spacer unit. This unusual combination of ionic, hydrophobic and stacking opportunities endows these molecule with the ability to interact with the zwitterionic forms of amino acids which exist at neutral pH For example, the acridine diacids can extract zwitterionic phenylalanine from water into chloroform, and NMR evidence indicates the formation of 2 1 complexesiP such as were previously described for other -pl yl-ethylammonium salte. Similar behavior is seen with tryptophan 40 and tyrosine methyl ether 41. The structures lacking well-placed aromatics such as leucine or methionine ate not extracted to measureable d ees under these conditions. 

All the amino acids involved in protein structure have an L-configuration of the carbon atom. If supplied in the D-form, some amino acids can be converted to the L-form by deamination of the amino acid to the keto acid and reamination to the L-form (see Chapter 9). 

The conversion of L-amino acids in food proteins into D isomers generates nonutilizable forms of amino acids, creates peptide bonds resistant to proteolytic... 

If the racemic mixture consists of ionizable species, the addition of acid or base to the solution has been found to stabilize the solution (Asai and Ikegami, 1982). This has been found to be true for ionizable amino acids, e.g. the L and D forms of glutamic acid, the L and D forms of DOPA (3,4-dihydroxy-P-phenylalanine), etc. (Asai, 1985). Here both the free and the salt forms of the acid are racemic mixtures. But the salt forms are much more soluble. The reaction that takes place, for example, in the presence of NaOH is as follows ... 

Fig. 4.14 FUV absorption spectra of ammonium carboxylic acid and aminocarboxylate tmion forms of amino acids with aromatic side chain (a) His, (b) Phe, (c) Ttp, and (d) Tyr (From Ref. [73])...

The metabolite of 2-amino-4-phenylthiazole (used as an anaesthetic for fish) was identified (223) as 2-amino-4-phenylthiazole 2-N, -d-glucopyranosiduronic acid (71) (Scheme 50). The formation of this compound probably involves the reaction of the exocyclic nitrogen on the Open-chain form of the acid. The isolation of this metabolite is part of a very Systematic study by Japanese researchers related to the anaesthetic... 

Fig. 3. (a) Chemical stmcture of a synthetic cycHc peptide composed of an alternating sequence of D- and L-amino acids. The side chains of the amino acids have been chosen such that the peripheral functional groups of the dat rings are hydrophobic and allow insertion into Hpid bilayers, (b) Proposed stmcture of a self-assembled transmembrane pore comprised of hydrogen bonded cycHc peptides. The channel is stabilized by hydrogen bonds between the peptide backbones of the individual molecules. These synthetic pores have been demonstrated to form ion channels in Hpid bilayers (71). 

Enzymatic hydrolysis is also used for the preparation of L-amino acids. Racemic D- and L-amino acids and their acyl-derivatives obtained chemically can be resolved enzymatically to yield their natural L-forms. Aminoacylases such as that from Pispergillus OTj e specifically hydrolyze L-enantiomers of acyl-DL-amino acids. The resulting L-amino acid can be separated readily from the unchanged acyl-D form which is racemized and subjected to further hydrolysis. Several L-amino acids, eg, methionine [63-68-3], phenylalanine [63-91-2], tryptophan [73-22-3], and valine [72-18-4] have been manufactured by this process in Japan and production costs have been reduced by 40% through the appHcation of immobilized cell technology (75). Cyclohexane chloride, which is a by-product in nylon manufacture, is chemically converted to DL-amino-S-caprolactam [105-60-2] (23) which is resolved and/or racemized to (24)... 

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