Dipeptide, glycylalanine

Reaction of HAuCU with the dipeptide glycylalanine (H-Gly-Ala-OH) and the tripeptide glycylalanylalanine (H-Gly-Ala-Ala-OH) gives the corresponding complexes [Au(Gly-Ala-0)Cl] (66) and [Au(Gly-Ala-Ala-O)] (67) where the peptides are coordinated to gold through — NH2, amide(s) and G02 groups in a near square... 

In the dipeptide glycylalanine, which bonds of the backbone allow free rotation of the attached groups ... 

EXAMPLE 3.28 Distinguish between the dipeptides glycylalanine and alanylglycine (Fig. 3-55). 

Each amide group is called a peptide bond when it is formed by amino acids. A peptide bond is formed by a condensation reaction between the carboxyl group of one amino add and the amino group of another amino acid. Alanine and glycine, for example, form the dipeptide glycylalanine ... 

Write the chemical reaction equation for the hydrolysis of the dipeptide glycylalanine into the constituent amino acids. 

A peptide bond is an amide bond that forms when the —COO group of one amino acid reacts with the —NH3 group of the next amino acid. We can write the formation of the dipeptide glycylalanine (Gly-Ala, GA) between glycine and alanine. With a free... 

A peptide bond between the ionized structures of glycine and alanine as forms the dipeptide glycylalanine. 

It might be noticed that under acidic conditions (i.e., anhydrous trifluoro-acetic acid) the hydroxyl (protonated) becomes a leaving group, and so it is expected an amine (amide linkage) could also leave. Thus, the reaction of a protein with phenylisothiocyanate provides an important method for the determination of the N-terminal amino acid and subsequent determination of the amino acid sequence of the protein. Remembering that the monomeric units of the protein are connected by amide (so-called peptide) linkages, this may be illustrated for the simple dipeptide glycylalanine. 

First, specificity or sequential coupling of the amino acids must be achieved. Unwanted side reactions must be avoided. Consider, for example, the synthesis of the dipeptide glycylalanine (Gly-Ala). It is not possible to mix the two amino acids together and let them react as the desired sequence will not be the only product. For example, it is possible that glycine will... 

Consider even as simple a target as the dipeptide glycylalanine. Just heating glycine and alanine to make the peptide bond by dehydration would result in a complex mixture of di-, tti-, and higher peptides with random sequences. Because the two starting materials can form bonds either to their own kind or to each other, there is no way to prevent random oligomerization. 

Complete knowledge of peptides is not satisfied by merely knowing the kind or number of the component amino acids it must include the sequence in which they are linked. Notice that even the dipeptide glycylalanine is not the same as alanyl-glycine. 

The peptide bond is described by the structure in the enclosed area. For example, glycine and alanine react to form two different dipeptides, either glycylalanine or alanylglycine. 

FIGURE 25.8 The formation of two dipeptides from two different amino acids. Alanylglycine is different from glycylalanine in that in alanylglycine the amino and methyl groups are bonded to the same carbon atom. 

Note that two amino acids can form a dipeptide (a peptide composed of two units) in either of two ways. For example, glycine and alanine can form glycylalanine (gly-ala) or alanylglycine (ala-gly) ... 

Glycylalanine and alanylglycine are structural isomers of each other, and each has a unique set of properties. The reactions to form these and other dipeptides are more complex than Reactions 9.6 and 9.7 imply. In nature, these processes are carried out according to genetic information using a mechanism we shall examine in Section 11.8. 

In the condensation reaction, one molecule of water is eliminated between the carboxylic acid of one amino acid and the amine group of another. The result is a peptide bond (called an amide group in simpler molecules), and the molecule is a dipeptide. When two different amino acids are linked by amide bonds, two different combinations are possible, depending on which amine reacts with which acid group. For example, when glycine and alanine react, both glycylalanine and alanylglycine can be formed. Either end of the dipeptide can then react with another amino acid. 

In glycylalanine, glycine is the N-terminal amino acid, and alanine is the C-terminal amino acid. In alanylglycine, these roles are reversed. The two dipeptides are structural isomers. 

Treat the dipeptide with 2,4-dinitrofluorobenzene and then hydrolyze. If the dipeptide is alanylglycine, we will obtain DNP-alanine and glycine if the dipeptide is glycylalanine, we will get DNP-glycine and alanine. 

Armed with this knowledge, let us solve the problem of the synthesis of glycylalanine. We add amino-protected glycine to an alanyl ester in the presence of DCC. The resulting product is then deprotected to give the desired dipeptide. 

Two amino acids form a dipeptide, three a tripeptide, eight an octapeptide, etc. If a peptide is made up of not more than ten amino acids it is called an oligopeptide beyond that it is a polypeptide. Polypeptides become proteins when they are made up of over a hundred amino acids sometimes they are also called macropeptides. Since in systematic nomenclature peptides are acyl amino acids, the specific name for peptides is derived by attaching the ending -yl to that amino acid whose carboxyl group has undergone reaction e.g. glycylalanine and alanyl-leucyltyrosine. 

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