Amino acids stereoisomers

Two amino acid stereoisomers protected by benzyloxycarbonyl groups were deprotected in different ways. One isomer was hydrogenolyzed on 5% Pd/C (0.05 mol Pd/mol) in AcOEt-MeOH for 16 hours at ambient pressure and temperature. The other isomer was dissolved in MeOH, and cyclohexene (10 mol/mol) was added under nitrogen followed by 5% Pd/C (0.18 mol Pd/mol). The temperature was immediately raised to reflux, and stirring was continued for 7 minutes (Scheme 4.60).266... 

Another major site of peptide metabolism is the blood and especially blood serum and plasma. From an extensive compilation of peptide tm values (over 100 peptides, plus derivatized and cyclized analogues, D-amino acid stereoisomers, peptide bond isosteres, etc.), it appears that the differences between serum, heparinized plasma, and whole blood are fairly limited [161]. Interspecies differences are larger, particularly between humans and rats, with most human/rat t1/2 ratios ranging from 1 1 to 25 1 ... 

Berkecz, R. et al., LC enantioseparation of -lactam and P-amino acid stereoisomers and a comparison of macrocyclic glycopeptide and P-cyclodextrin-based columns, Chromatographia, 63, S37, 2006. 

J. Florance, High-performance liquid chromatographic separation of peptides and amino acid stereoisomers, J. Chromatogr., 414313 (1987). 

Florance J., Galdes A., Konteatis Z., Kosarych Z., Langer K., Martucci C., High Performance Liquid Chromatographic Separation of Peptide and Amino Acid Stereoisomers, Journal of Chromatography. 414 (1987), 313-323. 

Amino Acid Classes Biologically Active Amino Acids Modified Amino Acids in Proteins Amino Acid Stereoisomers Titration of Amino Acids Amino Acid Reactions PEPTIDES PROTEINS... 

Proteins are long chains of amino acids covalently bonded together by amide bonds formed between the carboxyl group of one amino acid to the amine group of another amino acid. Because they are made from pure amino acid stereoisomers, protein themselves are single stereoisomers despite having several hundred or more chiral centers. 

Berkecz R, Torok R, Disz I, Forro E, Fulop F, Armstrong DW, Peter A (2006) LC enan-tioseparation of p-lactam and p-amino acids stereoisomers and a comparison of macrocychc glycopeptides and p-cyclodextrin based columns. Chromatographia 63 S37-S43... 

If you synthesized the tnpeptide Leu Phe Ser from amino acids prepared by the Strecker synthesis how many stereoisomers would you expect to be formed ... 

It is understood that a-amino acids occur as their l stereoisomers unless otherwise indicated. The d notation is explicitly shown when a D amino acid is present, and a racemic amino acid is identified by the prefix dl. 

The four stereoisomers of this amino acid include the D- and L-forms of a-methylisoleucine and a-methylalloisoleucine. Cronin, J.R., and Pizzarello, S., 1997. Enantiomeric excesses in meteoritic amino acids.. Science 275 951—955. 

Since most often the selective formation of just one stereoisomer is desired, it is of great importance to develop highly selective methods. For example the second step, the aldol reaction, can be carried out in the presence of a chiral auxiliary—e.g. a chiral base—to yield a product with high enantiomeric excess. This has been demonstrated for example for the reaction of 2-methylcyclopenta-1,3-dione with methyl vinyl ketone in the presence of a chiral amine or a-amino acid. By using either enantiomer of the amino acid proline—i.e. (S)-(-)-proline or (/ )-(+)-proline—as chiral auxiliary, either enantiomer of the annulation product 7a-methyl-5,6,7,7a-tetrahydroindan-l,5-dione could be obtained with high enantiomeric excess. a-Substituted ketones, e.g. 2-methylcyclohexanone 9, usually add with the higher substituted a-carbon to the Michael acceptor ... 

Molecules like lactic acid, alanine, and glyceraldehyde are relatively simple because each has only one chirality center and only two stereoisomers. The situation becomes more complex, however, with molecules that have more than one chirality center. As a general rule, a molecule with n chirality centers can have up to 2n stereoisomers (although it may have fewer, as we ll see shortly). Take the amino acid threonine (2-amino-3-hydroxybutanoic acid), for example. Since threonine has two chirality centers (C2 and C3), there are four possible stereoisomers, as shown in Figure 9.10. Check for yourself that the R,S configurations are correct. 

Two appendices are included at the end of this chapter. The first is intended to serve as a reminder, for those of you who might need it, of the nomendature and representation of stereoisomers. The second appendix contains descriptions of various chemo-enzymatic methods of amino acid production. This appendix has been constructed largely from the recent primary literature and includes many new advances in the field. It is not necessary for you to consult the appendix to satisfy the learning objectives of the chapter, rather the information is provided to illustrate the extensive range of methodology assodated with chemo-enzymatic approaches to amino add production. It is therefore available for those of you who may wish to extend your knowledge in this area. Where available, data derived from die literature are used to illustrate methods and to discuss economic aspects of large-scale production. 

Several classes of enzymes have been used to separate stereoisomers of a-H-and a-disubstituted amino acids, eg amidases, nitrilases, hydantoinases, acylases and esterases. 

Alternatively, to avoid difficult separation of diastereomers 18a and epi-18a, the Fmoc-y9 -amino acid of unlike configuration 26 can be obtained as a single dia-stereoisomer in a three-step reaction sequence via conjugate addition of the Li-amide derived from (S)-N-benzyl-l-phenylethylamine (Davies methodology [113]) to tert-butyl tiglate [105] (Scheme 2.3). 

For betaxanthins, partial synthesis is quite common and presents a viable tool for identification by co-injection experiments. - Starting from a red beet extract or semi-purified betanin-isobetanin blend, alkaline hydrolysis by addition of 32% ammonia is initiated. Spectrophotometric monitoring at 424 nm allows the release of betalamic acid to be followed. Betaxanthins are obtained through the addition of the respective amino acid or amine in at least 20-fold molar excess followed by careful evaporation. Since the starting material most often consists of a racemic betacyanin mixture, the resulting betaxanthin will also consist of two stereoisomers that may not easily be separated by HPLC. ... 

Asymmetric syntheses of (3- amino acids result from the addition of chiral enolates (399) to nitrone (400) via A-acyloxyiminium ion formation (642, 643). Regioselective convergence is obtained in the reactions of chiral boron- and titanium- enolates (399a,b), (401), and (402). This methodology was used in preparing four stereoisomers of a-methyl- 3-phenylalanine (403) in enantiomeric pure form (Scheme 2.179) (644). 

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