Amino acids chiral" pairs

Treatment of a number of covalent polymers substituted with molecular recognition capability with suitable guests leads to chiral induction. The same crown ether-amino acid complementary pair described for the rosettes above was employed in the form of crown-ether pendant cis-transoidai poly(phenylacetylene). When the achiral polymer is treated with amino acids (in the form of their hydroperchlorate salts in acetonitdle) a large induced CD signal is observed in the backbone of the polymer. The polymer is sensitive to small enantiomeric excesses in the amino acid, as little as 0.005% enantiomeric excess of alanine can be detected. In a similar vein, c/5-transoidal poly(carboxyphenylacetylene) shows induced circular dichroism when treated with nonracemic chiral amines In addition, the system displays chiral memory, in that treatment of the complex with achiral amino alcohols results in retention of the chiral polymer backbone. 

However, it was not until the beginning of 1994 that a rapid (<1.5 h) total resolution of two pairs of racemic amino acid derivatives with a CPC device was published [124]. The chiral selector was A-dodecanoyl-L-proline-3,5-dimethylanilide (1) and the system of solvents used was constituted by a mixture of heptane/ethyl acetate/methanol/water (3 1 3 1). Although the amounts of sample resolved were small (2 ml of a 10 inM solution of the amino acid derivatives), this separation demonstrated the feasibility and the potential of the technique for chiral separations. Thus, a number of publications appeared subsequently. Firstly, the same chiral selector was utilized for the resolution of 1 g of ( )-A-(3,5-dinitrobenzoyl)leucine with a modified system of solvents, where the substitution of water by an acidified solution... 

An approach, similar to that employed in the analysis of tartrate mixtures, has been used for the chiral discrimination of amino acid (M/j/s) mixtures, using an amino acid of defined configuration as reference (S). The proton-bound trimers [S2-M H]+ form [S M H]+ and [S2H]+ fragments upon CID or MIKE decay (equations (9)-(12)). With two independent measurements of the fragmentation ratio [S-M-H] /[S2H] from either [S2-M -H] and [52-M5-H]" , the differences in binding energies can be determined. The relative gas phase basicities (GB) of the molecular pairs [S-M] and [S2] can be derived from equations (13) and (14). 

Polymers containing chiral groups are useful for resolving racemic mixtures into the individual enantiomers [Kiniwa et al., 1987 Mathur et al., 1980 Wulff et al., 1980]. For example, the copper(II) complex of XXXXIII (either the R- or S-enantiomer) resolves racemates of amino acids [Sugden et al., 1980], The separation is based on the formation of a pair of diastereomeric complexes from the reaction of the polymer reagent with the two enantiomers. One of the enantiomers is complexed more strongly than the other and this achieves separation of the enantiomers. 

There are numerous chiral stationary phases available commercially, which is a reflection of how difficult chiral separations can be and there is no universal phase which will separate all types of enantiomeric pair. Perhaps the most versatile phases are the Pirkle phases, which are based on an amino acid linked to aminopropyl silica gel via its carboxyl group and via its amino group to (a-naphthyl)ethylamine in the process of the condensation a substituted urea is generated. There is a range of these type of phases. As can be seen in Figure 12.23, the interactions with phase are complex but are essentially related to the three points of contact model. Figure 12.24 shows the separation of the two pairs of enantiomers (RR, SS, and RS, S,R) present in labetalol (see Ch. 2 p. 36) on Chirex 3020. 

The a-carbon of each amino acid is attached to four different chemi cal groups and is, therefore, a chiral or optically active carbon atom. Glycine is the exception because its a-carbon has two hydro gen substituents and, therefore, is optically inactive. [Note Amino acids that have an asymmetric center at the a-carbon can exist in two forms, designated D and L, that are mirror images of each other (Figure 1.8). The two forms in each pair are termed stereoisomers, optical isomers, or enantiomers.] All amino acids found in proteins are of the L-configuration. However, D-amino acids are found in some antibiotics and in bacterial cell walls. (See p. 250 for a discus sion of D-amino acid metabolism.)... 

While a collection of molecules that are all of the same chirality (e.g., a D- or L-amino acid or a naturally occurring protein) must form a chiral crystal, inherently nonchiral molecules are not barred from doing so, if they crystallize in one of the 11 pairs of enantiomorphous space groups. In that event, which is rather rare, there will, of course, be an equal probability of forming either enantiomorph and a batch of crystals will normally contain both. A couple of real examples are (NH4)3Tc2C18 3H20 (P3,21 and P3 >21) and SntTa Cl (P6 22 and P6522). 

Armstrong et al. [54] resolved the enantiomers of some amino acids and their derivatives on x-CD-based CSPs using 1% aqueous triethylammonium acetate (pH 5.1). The same authors also tested a /LCD CSP for the chiral resolution of amino acids [55]. In addition, they evaluated a y-CD phase for the enantiomeric resolution of some dansyl amino acids and other drugs. The mobile phase was 38% methanol with 1% triethylammonium acetate [58]. In another study, the same authors reported the chiral resolution of 25 pairs of amino acids in less than 30 min [63]. The enantiomers of some /i-adrcncrgic blockers were resolved on a /LCD stationary phase, with 1% aqueous triethylammonium acetate, containing methanol, as the mobile phase [9,48]. 

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