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.
β-amino acids are like most amino acids, which can be dissolved in strong acids and strong alkaline solutions and are difficult to dissolve in ethanol and ether. There are five main methods for synthesizing β-amino acids: chemical resolution, chiral chromatography, Amdt-Eistert reaction, asymmetric synthesis and enzyme-catalyzed synthesis.
The structure of β-amino acids and four possible isomers of mono-substituted β-amino acids.Fig. 1 The structure of β-amino acids and four possible isomers of mono-substituted β-amino acids.
Applications
Due to the diversity of β-amino acids structure, molecular design of β-amino acids is a promising peptidomimetic method recently proposed. The combination of β-amino acids has been successfully synthesized peptidomimetics, which not only have strong biological activity, but also prevent protein hydrolysis.
Receptor agonists and antagonists: Potential selective receptor agonists and antagonists are an important class of potential therapeutic targets. Many β-amino acids are now used to design new receptor binding ligands. The application of β-amino acid substitutions in the study of the structure and function of adrenocorticotropin[1], angiotensin II [2, 3], gastrin [4] and oxytocin [5]. Several types of ligands have been studied, including neuropeptides, platelet aggregation factors, lipid transport systems, opioids, and taste ligands.
Protease inhibitors: α-peptides containing β-amino acids are also potential lead compounds in the development of enzyme inhibitors for treatment, because proteolytically resistant peptides containing b-amino acids have a significant affinity for their target enzymes.
MHC-binding peptides: The application of β-peptides has also been studied to develop peptide-based vaccines and T cell receptor antagonists to prevent T cell responses in MHC-related autoimmune diseases [6].
DNA-Binding peptides: The combination of β-Gly and hairpin polyamides increases the flexibility of the molecule and further promotes the bonding [7,8]. Through the incorporation of β-Ser (D and L types), β-amino-Gly and α-fluoro-β-Gly [9], the feasibility of changing the recognition performance of polyamide molecules was further explored. The results showed that the incorporation of these residues significantly changed the binding properties of DNA, indicating that the β-amino acids have the ability to regulate DNA binding selectivity, and these results have great potential in inhibiting transcription and gene expression.
