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Chemistry · Glossary

What is Amino acids?

Also known as: alpha-amino acid · side chain · zwitterion · isoelectric point

Definition 30.1 University Chemistry — Year 2 · Chapter 30 — Biomolecules: Amino Acids, Sugars, Lipids and Nucleotides

An α\alpha-amino acid HX2N−CHR−COOH\ce{H2N-CHR-COOH} carries an amino group and a carboxylic acid group on the same carbon; R\ce{R} is its side chain. In water near neutral pH it exists as a zwitterion, HX3NX+−CHR−COOX−\ce{H3N+-CHR-COO-}, carrying a positive and a negative charge. The isoelectric point pI is the pH at which its average net charge is zero.

Left: net charge of three amino acids against pH, from their pK_a at 25 C; each curve crosses zero at the pI. Right: the three forms of glycine, H3N+CH2COOH, H3N+CH2COO- and H2NCH2COO-; the zwitterion dominates from about pH 3 to pH 9. Left: net charge of three amino acids against pH, from their pK_a at 25 C; each curve crosses zero at the pI. Right: the three forms of glycine, H3N+CH2COOH, H3N+CH2COO- and H2NCH2COO-; the zwitterion dominates from about pH 3 to pH 9.
Left: net charge of three amino acids against pH, from their pKa\mathrm pK_a at 25∘C25{}^{\circ}\mathrm{C}; each curve crosses zero at the pI. Right: the three forms of glycine, HX3NX+CHX2COOH\ce{H3N+CH2COOH}, HX3NX+CHX2COOX−\ce{H3N+CH2COO-} and HX2NCHX2COOX−\ce{H2NCH2COO-}; the zwitterion dominates from about pH 3 to pH 9.
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