Human Biochemistry, Volume 1 - Murray R. 1993

Structure and Function of Proteins and Enzymes
Peptides
Ionic Forms of Peptides

The peptide (amide) group remains uncharged across all physiological pH values. The formation of a peptide from Amino Acids at pH 7.4 is accompanied by the loss of one negative and one positive charge for every peptide bond formed. Nevertheless, Peptides carry a net charge at physiological pH because the C- and N-terminal groups, as well as the charged side chains attached to the α-carbon atoms, remain charged (Table 3.1).

Polypeptides, much like Amino Acids and other charged molecules, can be isolated using Methods based on charge Separation (such as Electrophoresis and Ion-exchange Chromatography). The pK value of the C-terminal carboxyl group in a polypeptide is higher than that of the α-carboxyl group in the corresponding free amino acid (meaning the peptide COOH group is a weaker acid). Conversely, the protonated N-terminal amino group is a stronger acid (exhibiting a lower pK value) than the amino group of the corresponding free amino acid from which it was derived (Table 4.1).

Class="center">Table 4.1. pK values for Glycine and glycine peptides

pK(COOH)

pK(NH+3)

Gly

2.34

9.60

Gly-Gly

3.12

8.17

Gly-Gly-Gly

3.26

7.91


Last update: 06/08/2026

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