Beta-peptide

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. Pioneering studies in this field were published in 1996 by the group of Dieter Seebach ^[1] and that of Gellman ^[2].

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Chemical structure and synthesis

In α amino acids (molecule at left), both the carboxylic acid group (red) and the amino group (blue) are bonded to the same carbon, termed the α carbon (C^α) because it is one atom away from the carboxylate group. In β amino acids, the amino group is bonded to the β carbon (C^β), 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 C^β atom. A number of synthesis mechanisms have been introduced to efficiently form β amino acids and their derivatives^[3][4] notably those based on the Arndt-Eistert synthesis.

Two main types of β-peptides exist: those with the organic residue (R) next to the amine are called β³-peptides and those with position next to the carbonyl group are called β²-peptides ^[5].

Secondary Structure

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 ^[6].

The β-peptide zwit-1F with a fully described quaternary structure ^[7] is called a β-protein because it has many characteristics of an actual protein. Eight 12-helix units self-assemble in water to a superstructure with a hydrophobic inner core.

Clinical potential

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 ^[8]. 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 ^[9].

References

• Daniels DS, Petersson EJ, Qiu JX, Schepartz A (2007). "High-Resolution Structure of a β-Peptide Bundle". J. Am. Chem. Soc. 129,: 1532–1533. doi:doi:10.1021/ja068678n.

See also

• Peptidomimetic