An Introduction of Peptide Synthesis.
Peptide synthesis service is the production of peptides, which are organic compounds in which multiple amino acids are linked via amide bonds, also known as peptide bonds. The biological process of producing long peptides (proteins) is known as protein biosynthesis.
Peptides are synthesized by coupling the carboxyl group of one amino acid to the amino group of another amino acid molecule. Due to the possibility of unintended reactions, protecting groups are usually necessary. Chemical peptide synthesis most commonly starts at the carboxyl end of the peptide, and proceeds toward the amino-terminus. This is the opposite direction of protein biosynthesis.
The classical approaches to peptides production are called liquid-phase peptide synthesis and solid-phase peptide synthesis (SPPS). These two methods can be combined in a process called native chemical ligation. Our standard peptide synthesis process involves the solid phase. The liquid-phase approach is used for the synthesis of short peptides, such as di- and tripeptides, and C-terminally modified peptides, such as enzyme substrates. The controlled peptide synthesis requires selective protection and deprotection of the various functional groups: the amino group, the -carboxyl group or the side chain functional groups. The side group gives each amino acid its distinctive properties and helps to dictate the folding of the protein.
Amino acids have reactive Alaph-carboxylic acid and Alaph-amine groups that allow for their linking into polymers, but that also complicate the aim coupling specific pairs of amino acids, in precise order. In addition, many amino acids have reactive side chain functional groups, which can also react in a variety of ways, including with free α-carboxylic and α-amino groups during peptide synthesis (given the very reactive reagents present), "side-reactions" that would negatively influence yield and purity. Chemical groups have been developed to facilitate the synthesis of peptides with precise amino acid sequences, with minimal side reactions, groups that block or "protect" all functional groups present in amino acids except that pair whose coupling is desired. These protecting groups, while very many in practice, can be described in three groups: alaph-carboxylic acid (C-terminal) protecting groups, alaph-amino (N-terminal) protecting groups, and side chain protecting groups:
- Liquid-phase synthesis
Liquid-phase peptide synthesis is a classical approach to peptide synthesis. It has been replaced in most labs by solid-phase synthesis (see below). However, it retains usefulness in large-scale production of peptides for industrial purposes.
- Solid-phase synthesis
Solid-phase peptide synthesis (SPPS) allows for the synthesis of natural peptides which are difficult to express in bacteria, the incorporation of unnatural amino acids, peptide/protein backbone modification, and the synthesis of D-proteins, which consist of D-amino acids. Small porous beads are treated with functional units ('linkers') on which peptide chains can be built. The peptide will remain covalently attached to the bead until cleaved from it by a reagent such as anhydrous hydrogen fluoride or trifluoroacetic acid. The peptide is thus 'immobilized' on the solid-phase and can be retained during a filtration process while liquid-phase reagents and by-products of synthesis are flushed away.
The general principle of SPPS is one of repeated cycles of deprotection-wash-coupling-wash. The free N-terminal amine of a solid-phase attached peptide is coupled to a single N-protected amino acid unit. This unit is then deprotected, revealing a new N-terminal amine to which a further amino acid may be attached. The superiority of this technique partially lies in the ability to perform wash cycles after each reaction, removing excess reagent with all of the growing peptide of interest remaining covalently attached to the insoluble resin.
There are two majorly used forms of SPPS - Fmoc and Boc. Unlike ribosome protein synthesis, solid-phase peptide synthesis proceeds in a C-terminal to N-terminal fashion. The N-termini of amino acid monomers is protected by either of these two groups and added onto a deprotected amino acid chain. Automated synthesizers are available for both techniques, though many research groups continue to perform SPPS manually.
Microwave-assisted peptide synthesis
In peptide synthesis, microwave irradiation has been used to complete long peptide sequences with high degrees of yield and low degrees of racemization. Microwave irradiation during the coupling of amino acids to a growing polypeptide chain is catalyzed not only by the increase in temperature but also by the alternating electric field of the microwave. This is because the polar N-terminal amine group and peptide backbone continuously try to align with the alternating electric field, thus helping prevent aggregation and increasing access to the solid phase reaction matrix. This increases yields of the final peptide products. There is however no clear evidence that microwave is better than simple heating and some peptide laboratories regard microwave just as a convenient method for rapid heating of the peptidyl resin. Heating to above 50-55 degrees Celsius also prevents aggregation and accelerates the coupling.
- C-terminal protecting groups
- N-terminal protecting groups
- Tert-butyloxycarbonyl (t-Boc) protection
- 9H-fluoren-9-ylmethoxycarbonyl (Fmoc) protection
- Alloc and miscellaneous groups
- Side chain protecting groups