Amino acids (aa, AA, amino acids), amino acids – a group of organic chemical compounds containing a basic amino group and a carboxylic acid group -COOH, or more generally, any acid group.

An example of an amino acid containing a sulfonic acid group – SO3H is taurine (2-aminoethanesulfonic acid), while a phosphonic acid containing -C-PO3H2 is ciliatin (2-aminoethane phosphonic acid).

The amino group can be primary (-NH2), secondary (-NHR), tertiary (-NR2) or quaternary ammonium (-NH3 +). Amino acids with a quaternary ammonium group are compounds from the betain group, and their representative is N, N, N-trimethylglycine (betaine), (CH3) 3N + CH2COO−.

Since amino acids contain both acidic and basic groups, they undergo an intramolecular acid-base reaction and these compounds exist mainly in the form of zwitterions. The zwitterions of amino acids are a type of internal salts (ampholytes), which is why they have many salt-typical properties: they are crystalline substances with high melting points, show high dipole moments, are soluble in water, but insoluble in hydrocarbons.

Criteria for the division of amino acids

Carboxylic amino acids can be divided into:
• natural
1. protein
 primary (standard, encoded) – 20 (21 including selenocysteine) amino acids that are substrates in ribosomal protein synthesis
– endogenous
– exogenous
 secondary – resulting from post-translational modification (e.g. 4-hydroxyproline)
Tertiary – arising in the process of post-translational modification as a result of reactions leading to the formation of non-peptide covalent bonds between amino acid residues included in the protein e.g. cystine
2. non-protein (e.g. γ-aminobutyric acid (GABA))
• synthetic (e.g. β-methylphenylalanine)

Taking into account the chemical structure, carboxylic amino acids can be divided according to:
• position of the amino group relative to the carboxyl group: α, β, γ, δ and ε-amino acids; the Greek letter indicates the position of the amino group at a given carbon atom in the carboxylic acid main chain. In α-amino acids, both these functional groups are located on the same carbon atom (α).
• side chain polarity, for standard amino acids:
• with non-polar side chain (glycine, alanine, valine, leucine, isoleucine, methionine, proline, phenylalanine, tryptophan)
• with a polar side chain but not having a formal charge (serine, threonine, asparagine, glutamine, tyrosine, cysteine)
• with a polar side chain having a formal charge in the physiological pH range:
 positive (lysine, arginine, histidine)
 negative (aspartic acid, glutamic acid)

Natural amino acids
Over 300 naturally occurring amino acids are known. The protein of all living organisms consists mainly of 20 amino acids, which are α-amino acids containing an asymmetric carbon atom of the L configuration (except achiral glycine), as well as small amounts of many others, mostly derived from basic amino acids. The side chain (R) may contain structural elements such as an aromatic ring, aliphatic chain, sulfur (thiol or thioether group), hydroxyl group, additional amino or carboxyl group. They are the basic building blocks of peptides and proteins. They are usually soluble in water and under ionic conditions under physiological pH.

The type of amino acid residues and their order in the polypeptide chain (sequence) depend on the genetic code stored in the DNA. Subsequent amino acid residues in the polypeptide chain are joined together by peptide bonds.