Side Chain Cyclization
Side chain (SC) cyclization is a method utilizing functional groups such as amines, carboxylic acids or thiols, or the amino or carboxyl termini of the peptide to form the cyclization through amide or disulfide bonds.
Both the cyclization types have their respective advantages: The backbone cyclization (BC) peptide is more active and stable, probably due to the benefit of N-alkylation. But SC cyclic peptides are easier to synthesize. To a certain extent, the BC scaffold can be replaced by the SC one. It is essential for drug design to compare the properties of SC versus BC peptides, since cyclization is one of the most frequently used methods for peptido- and proteinomimetics. The synthetic disadvantages of BC make this method sometimes difficult to approach, which needs the improvement such as the coupling methods and building blocks accessibility. For SC cyclization, the minimum length of the peptide is 4 amino acid units, but the maximum length is not limited.
Side chain cyclization is the classical ones that are limited to cyclization through the side chains and/or the amino or carboxyl terminal groups. Conformationally restricted peptides which contain medium- and long-range cyclization have been mainly prepared following the same modes of cyclization of homodetic and heterodetic natural peptides. Common methods of side chain cyclization are:
- Cys-Cys cyclization (for more information, please visit Disulfide-rich Cyclic Peptides)
- Amide bond cyclization
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- Side chain to side chain cyclization (usually the formation of a lactam ring and/or an -S-S-bond through cyclization of functional groups already present in the native sequence or by substitution of other amino acids with Glu and Lys or Cys respectively)
- Side chain to tail cyclization
- Side chain to backbone cyclization
1. Hayouka, Z., Levin, A., Hurevich, M., Shalev, D. E., Loyter, A., Gilon, C., & Friedler, A. (2012). A comparative study of backbone versus side chain peptide cyclization: Application for HIV-1 integrase inhibitors. Bioorganic & medicinal chemistry, 20(10), 3317-3322.
2. Gilon, C., Halle, D., Chorev, M., Selincer, Z., & Byk, G. (1991). Backbone cyclization: a new method for conferring conformational constraint on peptides. Biopolymers: Original Research on Biomolecules, 31(6), 745-750.