Molecular Biology: Protein Structure and Functions - Stepanov V.M. 2005

Peptides
Chemical properties of peptides

Like Amino Acids, Peptides containing free amine and carboxyl groups exist as zwitterions and possess an isoelectric point. Their solubility in Water varies significantly and is determined by both the peptide length and The Nature of its constituent amino acids. The acid-base properties of peptides differ somewhat from those of amino acids because the mutual Influence of the amine and carboxyl groups is much weaker than in α-amino acids, and in long peptides, it can be neglected altogether. In peptides, the pK_a of the α-carboxyl group is noticeably higher than in amino acids; for instance, it is 3.12 for alanyl-Alanine (cf. pK_a of 2.34 for the alanine carboxyl and 4.7 for acetic acid). For the carboxyl group of alanyl-alanyl-alanine, pK_a = 3.39; with further chain elongation, the value remains close to 3.4.

This slight increase in acidity compared to the carboxyl group of acetic acid is attributed to METABOLISM/18.html">The Influence of the adjacent peptide bond. Similarly, the α-amino group in peptides becomes less basic than in α-amino acids: the pK_a of the α-ammonium group is 9.69 for alanine; 8.30 for alanyl-alanine; and 8.03 for alanyl-alanyl-alanine, remaining unchanged with further chain lengthening.

In other respects, The chemical properties of the α-amine and carboxyl groups of peptides are qualitatively similar to those of amino acids. Specifically, they undergo the same reactions described for amino acids, except for those involving the simultaneous participation of both functional groups, which are atypical for peptides. An exception is the rather facile cyclization of esters and certain other dipeptide derivatives featuring an activated α-carboxyl group, which yields diketopiperazines:

Class="center">

In strongly alkaline solutions, peptides form highly complex colored complexes with copper ions, which is The basis of the so-called biuret test. Naturally, peptides can also undergo Reactions Involving the functional groups found in The amino acid side chains (see Chapter 9).