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Chapter 24: Q-24-32P (page 1298)

Suggest a method for the synthesis of the natural L enantiomer of alanine from the readily available L enantiomer of lactic acid.

Short Answer

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L-Lactic acid D-alanine

Step by step solution

01

Step-1. Enantiomers of alanine:

Alanine exists in two enantiomeric forms, that is, L- and D-alanine. D-alanine is the D-enantiomer of alanine. It is a metabolite found in E. coli strain. L-enantiomer is a pyruvate family amino acid and has (S)-configuration whereas D-alanine has (R)-configuration.

02

Step-2. Synthesis of D-alanine from L-Lactic acid:

Firstly, L-Lactic acid is made to react with tosyl chloride and pyridine, in which pyridine acts as a base and abstracts proton from hydroxyl group of lactic acid and tosyl group gets attached to the oxygen and good leaving group in the form of tosylate is obtained. On treatment with excess ammonia in next step, tosylate leaves and amide ion acts as a nucleophile and attacks at carbon to which tosylate is attached. After intramolecular proton exchange with acid, we get our required product, that is, D-alanine.

L-Lactic acid D-alanine

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Most popular questions from this chapter

Lipoic acid is often found near the active sites of enzymes, usually bound to the peptide by a long, flexible amide linkage with a lysine residue.

(a) Is lipoic acid a mild oxidizing agent or a mild reducing agent? Draw it in both its oxidized and reduced forms.

(b) Show how lipoic acid might react with two Cys residues to form a disulfide bridge.

(c) Give a balanced equation for the hypothetical oxidation or reduction, as you predicted in part (a), of an aldehyde by lipoic acid.

Draw the complete structure of the following peptide Met-Ser-Gln-NH2

Show how the following amino acids might be formed in the laboratory by reductive amination of the appropriate α-ketoacid.

Phenylalanine (b) Cysteine (c) Serine (d) Alanine

A molecular weight determination has shown that an unknown peptide is a pentapeptide, and an amino acid analysis shows that it contains the following residues: one Gly, two Ala, one Met, one Phe. Treatment of the original pentapeptide with carboxypeptidase gives alanine as the first free amino acid released. Sequential treatment of the pentapeptide with phenyl isothiocyanate followed by mild hydrolysis gives the following derivatives:

Propose a structure for the unknown pentapeptide.

Complete hydrolysis of an unknown basic decapeptide gives Gly, Ala, Leu, Ile, Phe, Tyr, Glu, Arg, Lys, and Ser. Terminal residue analysis shows that the N terminus is Ala, and the C terminus is Ile. Incubation of the decapeptide with chymotrypsin gives two tripeptides, A and B, and a tetrapeptide, C. Amino acid analysis shows that peptide A contains Gly, Glu, Tyr, and; peptide B contains Ala, Phe, and Lys; and peptide C contains Leu, Ile, Ser, and Arg.Terminal residue analysis gives the following results.

Incubation of the decapeptide with trypsin gives a dipeptide D, a pentapeptide E, and a tripeptide F. Terminal residue analysis of F shows that the N terminus is Ser and the C terminus is Ile. Propose a structure for the decapeptide and for fragments A through F.

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