Enantiomers of amino acids

Different optical enantiomers of amino acids also have different properties. L-asparagine, for example, tastes bitter while D-asparagine tastes sweet (see Figure 8.3). L-Phenylalanine is a constituent of the artificial sweetener aspartame (Figure 8.3). When one uses D-phenylalanine the same compound tastes bitter. These examples clearly demonstrate the importance of the use of homochiral compounds. 

By taking into account the latest results on the behaviour of systems far away from equilibrium, Kondepudi and Nelson (1985) were able to show by calculation that L-amino acids are slightly favoured. There is a very tiny stabilisation effect due to the weak interaction amplification mechanisms cause this effect to reach 98% of the probability that L-enantiomers of amino acids are favoured for incorporation into polymers. The amplification mechanisms are explained by the thermodynamics of irreversible systems. 

The rates of reaction of both enantiomers of amino-acid esters in the presence of (S)-[324] are the same, but with (S)-[323] they are in most cases different. The reactions of L-amino acid esters in the presence of (S)-[323] are faster than those in the presence of (R)-[323] by factors of 9.2 (R = i-Pr), 8.2 (R = C6H5CH2) and 6.0 (R = i-Bu). No difference in rates is observed for L-alanine p-nitrophenyl ester. The results were explained in terms of the formation of diastereomeric tetrahedral intermediates [325] and [326]. The bulk of the group R will determine how much the complex stability of the (D)-complex decreases relative to that of the (L)-complex, which difference is reflected in the activation energy of the rate-determining step. 

In simple experiments, particulate silica-supported CSPs having various cin-chonan carbamate selectors immobilized to the surface were employed in an enantioselective liquid-solid batch extraction process for the enantioselective enrichment of the weak binding enantiomer of amino acid derivatives in the liquid phase (methanol-0.1M ammonium acetate buffer pH 6) and the stronger binding enantiomer in the solid phase [64]. For example, when a CSP with the 6>-9-(tcrt-butylcarbamoyl)-6 -neopentoxy-cinchonidine selector was employed at an about 10-fold molar excess as related to the DNB-Leu selectand which was dissolved as a racemate in the liquid phase specified earlier, an enantiomeric excess of 89% could be measured in the supernatant after a single extraction step (i.e., a single equilibration step). This corresponds to an enantioselectivity factor of 17.7 (a-value in HPLC amounted to 31.7). Such a batch extraction method could serve as enrichment technique in hybrid processes such as in combination with, for example, crystallization. In the presented study, it was however used for screening of the enantiomer separation power of a series of CSPs. 

Ding, GS. etal.. Chiral separation of enantiomers of amino acid derivatives by high-performance liquid chromatography on a norvancomycin-bonded chiral stationary phase, Talanta, 62, 997, 2004. 

A remarkably wide range of different enzymes has been deployed in various processes for the production of pure L- or D-enantiomers of amino acids, and in a number of these processes several enzymes are used simultaneously. In the review that follows, processes are grouped as far as possible according to the enzyme principally responsible for chiral selection in the overall process. 

Monoamine oxidases are enzymes that catalyze the racemization of ot-amino acids 186). Both l- and D-selective monoamine oxidases are known (i.e., they catalyze the racemization of either S)- or (R)-enantiomers of amino acids). This property has been exploited to obtain enantiomerically pure (R) and S) amino acids by using an appropriate achiral reducing agent such as NaBH4, NaB(CN)H3, or H3N BH3 in combination with an l- or D-selective monoamine oxidase 187). 

A number of variations on this type of coating have been prepared and offer some improvement over the original phase. Figure 11.11 shows the volatile pentafluoropropionamide-trifluoroethyl ester (PFP-TFE) derivatives of L and D phenylalanine. Figure 11.12 shows the separation of PFP-TFE derivatives of the D and L enantiomers of the amino acids phenylalanine and p-tyrosine on a Chirasil Val column, the D(/ )-enantiomers elute first. Chirasil Val generally performs best for the separation of enantiomers of amino acids, for many other compounds it is not as effective. 

One of the classical approaches of liquid chromatography, paper chromatography, was used for chiral resolution about 50 years ago but is not part of modem practice. In paper chromatography, the stationary phase is water bonded to cellulose (paper material), which is of course chiral and hence provides a chiral surface for the enantiomers. However, some workers used chiral mobile phase additives also in paper chromatography [73,74]. In 1951 some research groups independently [73,75-77] resolved the enantiomers of amino acids. Simultaneously, numerous interesting publications on chiral resolution by paper chromatography appeared [70]. 

Machida et al. presented a method for the immobilization of CCE on silica gel in 1998 [45]. They reported the covalent binding of (+)-(18-crown-6)-tetracarboxylic acid to 3-aminopropylsilanized silica gel and the prepared CSP was tested to separate enantiomers of amino acids, amino alcohols and other drugs (containing primary amino group). In 1998, the same CSP was prepared by Hyun et al. [46]. The developed CSP was used extensively for the chiral resolution of a variety of racemic compounds having a primary amino group... 

Using the native cyclodextrin, the enantiomers of amino acid derivatives were enantioselectively complexed [21]. Further, for a more detailed analysis, zwitterionic tryptophan was employed [22]. For the complexation studies performed on this molecule the a-cyclodextrin was used, as its inner cavity is the smallest. The H NMR measurements showed that (R)-tryptophan formed a stronger complex with a-cyclodextrin compared with the (S) enantiomer. This is due to the number of hydrogen bonds which can be formed between each enantiomer and the host molecule. The NMR studies showed another very interesting fact the amino acid is very likely forming no intracavity complex. It has been suggested that it is coordinated near the rim of the cyclodextrin. 

R. Bhushan and S. Joshi, Resolution of enantiomers of amino acids by HPLC, Biomed. Chromatogr., 7 235 (1993). 

Copper chelates of amino acid enantiomers such as proline or phenylalanine have been used to resolve enantiomers of amino acids and structurally related compounds [241,245]. Other metals such as zinc and cadmium have also been used. Metal chelates have been used to resolve a-amino-a-hydroxy carboxy acids and a-methyl-a-amino acid enantiomers [246]. One example of pharmaceutical interest is the resolution of D-penicillamine from the L-antipod [247] and resolution of L,D-thyroxine [248]. 

Although these experiments did not provide the desired systems needed to amplify chirality, they were helpful in elucidating the stereochemical mechanism of the role played by additives in the early stages of crystal nucleation. This information was instrumental to the elaboration of appropriate model systems for the amplification of chirality, such as the generation of homochiral lysine via crystals of nickel/caprolactam [131] and the auto catalytic process of the spontaneous segregation of racemic enantiomers of amino acids in aqueous solutions assisted by centrosymmetric glycine crystals grown at interfaces. 

TTte A-tosyl derivative of verruculotoxin (294) was prepared by cyclization of the (S),(S)-enantiomer of amino acid 293 on the action of ethyl chloroformate in the presence of A-methylmorpholine (91TL1417). 2-tert-Butylperhydropyrido[l,2-a]pyrazin-3-one was prepared by reacting 2-(N-... 

Essentially two basic approaches to the separation of enantiomers of amino acids have been applied (i) the derivatives described in the preceding sections are chromatographed on optically active stationary phases, e.g., N-acyl alkyl esters or alkylamides of amino acids, ureides or N-acyl alkyl esters of dipeptides (ii) GC separation is performed on conventional stationary phases and the derivatives of amino acids are prepared by reaction... 

Some of the less common d enantiomers of amino acids are also found in nature. For example, D-glutamic acid is found in the cell walls of many bacteria, and D-serine is found in earthworms. Some naturally occurring amino acids are not a-amino acids y-Aminobutyric acid (GABA) is one of the neurotransmitters in the brain, and jS-alanine is a constituent of the vitamin pantothenic acid. 

Enzymatic resolution is also used to separate the enantiomers of amino acids. 

G. Dotsevi, Y. Sogah, and D. J. Cram, Host-guest complexation. 14. Host covalently bound to polystyrene resin for chromatographic resolution of enantiomers of amino acid and ester salts, / Am. Chem. Soc. 101 (1979), 3035. 

One potentiaUy powerful indicator of source and diagenetic state is the D/L-amino acid ratio within DOM. Although proteins in nature contain primarily the L-enantiomer of amino acids, certain compounds, many of bacterial origin, do contain D-amino acids. Proteins do not contain D-amino acids but peptidyl compounds such as peptidoglycan (a structural component of bacterial ceU waUs Osborn, 1969), peptide antibiotics (Bodanszky and Perlman, 1969) and peptide... 

If we use a chiral reagent to synthesize an amino acid, however, it is possible to favor the formation of the desired enantiomer over the other, without having to resort to a resolution. For example, single enantiomers of amino acids have been prepared by using enantioselective (or asymmetric) hydrogenation reactions. The success of this approach depends on finding a chiral catalyst, in much the same way that a chiral catalyst is used for the Sharpless asymmetric epoxidation (Section 12.15). 

Of course, enzymes use geometry to control not only positions of reaction in a substrate but also selectivity in the formation of chiral centers and selectivity among substrates. It is fair to say that chemists were inspired to develop ways to produce optically active compounds as single enantiomers by observing how nature produces single enantiomers of amino acids and sugars, and essentially every natural compound that can exist... 

A compound is chiral when it can occur in two forms that are mirror images of each other. The two forms (enantiomers) are very similar, hut not identical, for instance, like the right and the left hand of the same person. Classical synthesis produces both enantiomers in a 1 to 1 ratio. They cannot he separated hy normal physical means. Nature is, however, more selective. Here, only single enantiomers are formed. This can he used to separate D,L enantiomers of amino acids. The enzyme L-amylase produces selectively the L-amino acid from a mixture of the DL-acylamino acids. The D-acylamino acid remains unchanged and can he separated easily hy extraction or crystallization. 

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