β-peptides consist of β amino acids, which have their amino group bonded to the
β carbonrather 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["β-Peptides: Synthesis by Arndt-Eistert homologation with concomitant peptide coupling. Structure determination by NMR and CD spectroscopy and by X-ray crystallography. Helical secondary structure of a -hexapeptide in solution and its stability towards pepsin" Helvetica Chimica ActaVolume 79, Issue 4, Date: 26 Juni 1996, Pages: 913-941 Dieter Seebach, Mark Overhand, Florian N. M. Kühnle, Bruno Martinoni, Lukas Oberer, Ulrich Hommel, Hans Widmer DOI|10.1002/hlca.19960790402] and that of Gellman ["β-Peptide Foldamers: Robust Helix Formation in a New Family of -Amino Acid Oligomers" Appella, D. H.; Christianson, L. A.; Karle, I. L.; Powell, D. R.; Gellman, S. H. J. Am. Chem. Soc.; (Communication); 1996; 118(51); 13071-13072. DOI|10.1021/ja963290l ] .
Chemical structure and synthesis
In α amino acids (molecule at left), both the
carboxylic acidgroup (red) and the aminogroup (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 glycinelacks 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 alaninemolecule 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 [Basler B, Schuster O, Bach T. (2005). "Conformationally constrained beta-amino acid derivatives by intramolecular [2 + 2] -photocycloaddition of a tetronic acid amide and subsequent lactone ring opening". J. Org. Chem.70(24):9798-808. 2005 DOI|10.1021/jo0515226 [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=PureSearch&db=pubmed&details_term=16292808%5BUID%5D] .] [Murray JK, Farooqi B, Sadowsky JD, Scalf M, Freund WA, Smith LM, Chen J, Gellman SH. (2005). "Efficient synthesis of a beta-peptide combinatorial library with microwave irradiation". J. Am. Chem. Soc.127(38):13271-80. 2005 DOI|10.1021/ja052733v [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=PureSearch&db=pubmed&details_term=16173757%5BUID%5D] ] 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 ["β-Peptides: a surprise at every turn"
Dieter Seebachand Jennifer L. Matthews Chem. Commun., 1997, (21),2015-2022 DOI|10.1039/a704933a ] .
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 conformationabout 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 [Gademann K, Hintermann T, Schreiber JV. (1999). "Beta-peptides: twisting and turning.", "Curr Med Chem" Oct;6(10):905-25. [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=PureSearch&db=pubmed&details_term=10519905%5BUID%5D] .] .
The β-peptide "zwit-1F" with a fully described
quaternary structure["High-Resolution Structure of a β-Peptide Bundle Daniels", D.S., Petersson, E.J., Qiu, J.X., and Schepartz, A. J. Am. Chem. Soc., 129, 6, 1532 - 1533, 2007, DOI|10.1021/ja068678n ] 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.
β-peptides are stable against proteolytic degradation
in vitroand in vivo, an important advantage over natural peptides in the preparation of peptide-based drugs [Beke T, Somlai C, Perczel A. (2006). "Toward a rational design of beta-peptide structures.", "J Comp Chem" Jan 15;27(1):20-38. [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=PureSearch&db=pubmed&details_term=16247761%5BUID%5D] .] . 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 [Porter EA, Weisblum B, Gellman SH. (2002). "Mimicry of host-defense peptides by unnatural oligomers: antimicrobial beta-peptides". J. Am. Chem. Soc.124(25):7324-30. DOI|10.1021/ja0260871] .
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