β amino acids
According to the different positions of amino and carboxyl groups in the amino acid molecular structure, it can be divided into α-amino acids, β-amino acids and γ-amino acids. As an important amino acid, β-amino acids are similar to α-amino acids in structure, which both contain amino terminus and carboxyl terminus. However, as shown in Fig.1a, two carbon atoms separate these functional termini. Fig.1b shows the four stereoisomers that may be produced by mono-substituted β-amino acids. This also proves that there are much more isomers of β-amino acids than the corresponding isomers of α-amino acids. Di- and poly-substitutions further increase the number of amino acids.