What is Peptide Synthesis?

 

Peptide synthesis is the process of making short sequences of polypeptides by adding one amino acid at a time. This process is useful for creating specific sequences that represent epitopes of certain protein domains that may or may not be modified by moieties, such as phosphate groups. These short sequences are then used for injection into animals for the purposes of antibody production against that polypeptide.

The Advantage of Solid Phase Peptide Synthesis (SPPS)

Ease of Removing Unwanted Soluble Compounds

Polypeptides made by the solid phase method are ones in which the first amino acids in the sequence is attached to a polymer substrate. Anything attached to the polymer becomes insoluble, making it easier to separate them from soluble molecules that can be washed away. A chain of peptides that is anchored to a polymer also makes the desired chemical reactions between free floating amino acids and the existing chain easier. The chain is held in place, which reduces the randomness within the aqueous system.

The Order of Reactions Can Be Controlled

The free amino acids are modified by chemical groups that protect them from reacting with other molecules in the system. Different chemical groups are used, each of which can be removed by distinct chemical treatments. This allows the scientist to control what amino acids are made to be reactive first, which ones second, and so on and so forth. This approach ensures that only the desired amino acid that is next in the sequence connects to the anchored chain.

The General Principle of Solid Phase Peptide Synthesis

Solid phase synthesis requires that the polypeptide chain be attached to a solid polymer, which serves as an anchor and at catalytic surface. The polymer only reacts with the first amino acid in the sequence, but remains inert when it comes to the addition of subsequent amino acids. The bond between the first amino acid and the polymer is cleavage, usually by treatment with a weak acid. Thus, the solid polymer is only temporarily attached to the peptide chain.

The free floating amino acids cannot react with each other or the growing chain that is attached to the polymer. This is because the reactive sites of the amino acids are capped by protecting groups, common ones being Fomc and Boc. These protecting groups minimize unwanted side reactions that interfere with the production of a specifically ordered sequence of amino acids. The protecting groups are removed using distinct chemical methods, which allows scientists to activate only one amino acid in a mixture at a time.

After the peptide chain has completely formed, it needs to be released from the polymer by digestion. But before digestion happens, the system is washed to remove unwanted soluble compounds. This results in a purer solution of the desired polypeptide chain once the chain is separated from the polymer resin.

Conjugating the Peptide to Enhance Immunogenicity

Polypeptide chains may be too small to induce an immune response after injection. Thus, peptide chains need to be conjugated to a larger polymer that is sure to activate antigen presenting cells. Common conjugation substrates include bovine serum albumin (BSA), ovalalbumin (OA), and keyhole limpet hemocyanin (KLH).