Amino Acids-Peptides

AMINO ACIDS AND PEPTIDES THEORY

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Amino acids are organic compounds that contain carboxylic acid and amino functions. Two amino acids can be joined under suitable conditions, through an amide bond, forming a dipeptide. The dipeptide can incorporate a third amino acid, forming a tripeptide. Chains with more than 50 amino acids are called proteins.

The bond that joins the two amino acids to form the dipeptide is called a peptide bond.

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The 20 amino acids that make up proteins are: Serine (Ser,S), Threonine (Thr,T), Cysteine (Cys,C), Asparagine (Asn,N), Glutamine (Gln,Q) and Tyrosine (Tyr, Y), Glycine (Gly,G), Alanine (Ala,A), Valine (Val,V), Leucine (Leu,L), Isoleucine (Ile,I), Methionine (Met,M), Proline (Pro,P ), Phenylalanine (Phe,F) and Tryptophan (Trp,W), Aspartic Acid (Asp,D) and Glutamic Acid (Glu,E), Lysine (Lys,K), Arginine (Arg,R) and Histidine (His, h)

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isoelectric-point The presence of acid (-COOH) and basic (-NH ₂ ) groups gives amino acids characteristic acid-base properties.

In strong acid media, both the amino group and the acid group are protonated and the amino acid has the following form:

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Amino acids can be obtained by halogenation of carboxylic acids, followed by nucleophilic substitution with ammonia. The halogenation of the a position of the carboxylic acid is carried out with the Hell-Volhard-Zelinsky reaction.

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In this synthesis, the phthalimide salt is reacted with the halogenated malonic ester. In later stages it is alkylated, hydrolyzed and decarboxylated, obtaining the amino acid.

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This is a widely used method for preparing amino acids in the laboratory. Ethanamide amidate [1] with the halogenated malonic ester [2] , obtaining the compound [3] that is alkylated and hydrolyzed giving a diacid [6] , which decarboxylates to form the amino acid [7] .

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The Strecker synthesis allows obtaining amino acids from aldehydes or ketones.

The aldehyde [1] reacts with ammonia in an acid medium to form an imine, which adds cyanide, forming a-aminonitrile [2] , which is hydrolyzed to carboxylic acid in the last stage.

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Two amino acids join together under the right conditions to form a dipetide. If we react glycine (gly) with alanine (ala) a mixture of 4 products is obtained: gly-ala, ala-gly, gly-gly and ala-ala.

To exclusively obtain the gly-ala dipeptide, it is necessary to protect the amino group of glycine and the acid group of alanine. In this way, the reaction will take place exclusively between the acid group of glycine and the amino group of alanine.

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The amino group is protected to prevent its reaction with the acid group of the second amino acid. Once the peptide bond is formed, deprotection is performed, again leaving the amino group free.

Benzyl chloroformate (CBZ-Cl) is used as a protective group for the amino, transforming it into an amide.

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The acid group is protected by transforming it into an ester by reaction with an alcohol. Methyl, ethyl or tert -butyl esters are prepared by esterification and hydrolyzed (deprotected) in a basic medium.

Protection of alanine with tert-butanol in an acid medium.

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The synthesis of the dipeptide Glycine-Alanine (Gly-Ala) takes place through the formation of the amide (peptide) bond between the acid group of Glycine and the amino group of alanine. Therefore, it is necessary to protect the amino group of Glycine and the acid of alanine.

Stage 1. Protection of the amino group of Glycine.