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peptide bond[′pep‚tīd ‚bänd]
(also peptide linkage), an amide bond that arises during the formation of proteins and peptides when the α-amino group (—NH2) of one amino acid interacts with the α-carboxyl group (—COOH) of another amino acid:
where R1 and R2 are molecular parts that differ from one amino acid to another. The —CO—NH— group in proteins exhibits a keto-enol tautomerism.
The presence of peptide bonds in proteins and peptides was first proposed by A. Ia. Danilevskii and E. Fischer and later confirmed by chemical and physical data. Owing to tautomerism, the peptide bond exhibits a partial (40-percent) similarity to the double (conjugate) bond, which is indicated by the shorter length (1.32 A) of the peptide bond than of the single —C—N— bond (1.47 Å). The partially conjugate nature of the —C—N— bond determines the flat configuration of the —CO—NH— group (all four atoms are coplanar) and the existence of the trans and cis forms:
The greater stability of the trans form has been proved experimentally.
The enzymatic formation of peptide bonds in living cells occurs during protein biosynthesis. Laboratory methods have been developed for the chemical and enzymatic synthesis and cleavage of peptide bonds, thereby making it possible to synthesize a number of biologically active peptides and to determine the amino-acid sequence for many proteins and peptides.
N. N. ZAITSEVA