β-peptides consist of β amino acids, which have their amino group bonded to the β carbon rather than the α carbon as in the 20 standard biological amino acids. The only commonly naturally occurring β amino acid is β-alanine; although it is used as a component of larger bioactive molecules, β-peptides in general do not appear in nature. For this reason β-peptide-based antibiotics are being explored as ways of evading antibiotic resistance.
Chemical structure and synthesis
In α amino acids (molecule at left in Figure 1), both the carboxylic acid group (red) and the amino group (blue) are bonded to the same carbon, termed the α carbon () because it is one atom away from the carboxylate group. In β amino acids, the amino group is bonded to the β carbon (), which is found in most of the 20 standard amino acids. Only glycine lacks a β carbon, which means that there is no β-glycine molecule.
The chemical synthesis of β amino acids can be challenging, especially given the diversity of functional groups bonded to the β carbon and the necessity of maintaining chirality. In the alanine molecule shown, the β carbon is achiral; however, most larger amino acids have a chiral atom. A number of synthesis mechanisms have been introduced to efficiently form β amino acids and their derivatives .
Because the backbones of β-peptides are longer than those of peptides that consist of α-amino acids, β-peptides form different secondary structures. The alkyl substituents at both the α and β positions in a β amino acid favor a gauche conformation about the bond between the α-carbon and β-carbon. This also affects the thermodynamic stability of the structure.
Many types of helix structures consisting of β-peptides have been reported. These conformation types are distinguished by the number of atoms in the hydrogen-bonded ring that is formed in solution; 8-helix, 10-helix, 12-helix, 14-helix, and 10/12-helix have been reported. Generally speaking, β-peptides form a more stable helix than α-peptides .
β-peptides are stable against proteolytic degradation in vitro and in vivo, an important advantage over natural peptides in the preparation of peptide-based drugs . β-peptides have been used to mimic natural peptide-based antibiotics such as magainins, which are extremely powerful but difficult to use as drugs because they are degraded by proteolytic enzymes in the body .
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