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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

Expert verified

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

Draw three-dimensional representation of the following amino acids:

(a) L-alanine

(b) L-Leucine

(c) D-serine

(d) D-glutamine

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.

Draw the structure of the phenylthiohydantoin derivatives of

(a) Alanine (b) Tryptophan (c) Lysine (d) proline

The Sanger method for N-terminus determination is a less common alternative to the Edman degradation. In the Sanger method, the peptide is treated with the Sanger reagent, 2,4-dinitrofluorobenzene, and then hydrolyzed by reaction with 6 M aqueous HCl. The N-terminal amino acid is recovered as its 2,4-dinitrophenyl derivative and identified.

(a)Propose a mechanism for the reaction of the N terminus of the peptide with 2,4-dinitrofluorobenzene.

(b) Explain why the Edman degradation is usually preferred over the Sanger method.

Show where trypsin and chymotrypsin would cleave the following peptide.

Tyr-Ile-Gln-Arg-Leu-Gly-Phe-Lys-Asn-Trp-Phe-Gly-Ala-Lys-Gly-Gln-Gln.NH2
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