A peptide consists of two or more amino acids linked by an amide bond, to form a chain of amino acids typically 2 – 70 amino-acids long. Peptides are distinguished from proteins by not requiring to be folded for biological activity. Peptides occur endogenously as peptide hormones, such as angiotension, LHRH, enkephalin, and as toxins in plants and animals. Peptides are of great interest as lead compounds for drug discovery and as drugs in their own right. They also find applications in vaccines, biomaterials, histological probes and are used in large numbers as antigens to generate antibodies.
Peptides are synthesized chemically either in solution or on a solid phase. The process involves directed and selective formation of an amide bond between an N-protected amino acid and an amino acid bearing a free amino group and protected carboxylic acid. In solid phase synthesis, the carboxyl protecting group is linked to a polymer support. Following bond formation, the amino-protecting group of the dipeptide is removed, and the next N-protected amino-acid is coupled.
Figure 2Side-chain protecting groups for Boc solid-phase peptide synthesis (SPPS)
Solid-phase peptide synthesis (SSPS) is the most frequently used method of peptide synthesis due to its efficiency, simplicity, speed, and ease of parallelization. SPPS involves sequential addition of amino and side-chain protected amino acid residues to an amino acid or peptide attached to an insoluble polymeric support (Figure 1).
Either an acid-labile Boc group (Boc SPPS) or base-labile Fmoc-group (Fmoc SPPS) is used for N-α-protection. After removal of this protecting group, the next protected amino acid is added using either a coupling reagent or pre-activated protected amino acid derivative. The C-terminal amino acid is anchored to the resin via a linker, the nature of which determines the conditions required to release the peptide from the support after chain extension. Side-chain protecting groups are often chosen so as to be cleaved simultaneously with detachment of the peptide from the resin (Figure 2 and 3).
Figure 3.Side-chain protecting groups for Fmoc solid-phase peptide synthesis (SPPS)
Most peptides are prepared by the Fmoc method as the final cleavage and deprotection is carried by treatment with trifluoroacetic acid as opposed to the Boc method which requires use of highly toxic, corrosive liquid anhydrous HF in specialist equipment.
Peptides of 50 amino acids can be routinely prepared although the synthesis of proteins of over 100 amino acid are commonly reported. Longer proteins can be made by native chemical ligation of fully deprotected peptides in solution. With this method, it is possible to synthesize natural peptides that are difficult to express in bacteria, to incorporate unnatural or D-amino acids, and to generate cyclic, branched, labelled, and post-translationally modified peptides.
Liquid-phase peptide synthesis, usually utilizing Boc or Z-amino protection, has been superseded by solid-phase peptide synthesis except for existing processes of large-scale synthesis of peptides for industrial purposes.