Peptide synthesis, also known as peptide chain synthesis, is a solid-phase synthesis sequence, generally from C-terminal (carboxyl end) to N-terminal (amino terminal) synthesis. In the past, peptide synthesis was carried out in solution, called liquid phase synthesis. There are two ways to synthesize peptides: chemical synthetic peptides and biosynthetic peptides.
Principle of peptide synthesis
Peptide synthesis is how to connect various amino acid units according to the amino acid sequence and connection mode of natural substances. Because amino acids exist in the form of intramolecular zwitterions (H3 + NCH (R) COO -) under neutral conditions, the direct condensation of amino acids to form amide bonds is difficult under general conditions.
The reaction activity of amino acid ester was higher. When heated at 100 ℃ or stored at room temperature for a long time, the peptide ester can be polymerized to form peptide ester, but the reaction is not directional 、a1a1… 、a2a1… And all kinds of mixtures in any order.
In order to obtain synthetic polypeptides with a specific sequence, it is not feasible to use arbitrary polymerization method, but to adopt the directional peptide synthesis method of gradual condensation. It is generally shown in the following formula: firstly, the amino or carboxyl groups which do not need to be reacted are temporarily protected with appropriate groups, and then the linking reaction is carried out to ensure the directional progress of peptide synthesis.
In the formula, X and Q are the protective groups of amino group and carboxyl group respectively, which can not only prevent the occurrence of side reaction of random connection, but also eliminate the zwitterionic form of amino acid and make it easily soluble in organic solvent.
In some cases, Q may not be a covalently linked group, but an organic cation composed of strong organic bases (such as triethylamine) and carboxyl hydrogen ions of amino acids. Y is a strong electron withdrawing group, which can activate the carboxyl group, and is in favor of the free amino group of another amino acid. The carboxyl carbon atom of the activated carboxyl group is nucleophilic to form amide bond.
In order to protect the peptide, the peptide with N-terminal was protected. If the peptide chain does not stop here, but needs to extend the peptide chain from the N-terminal or the C-terminal, X or Q can be selectively removed first, and then the new N-protected amino acid (or peptide) or the C-protected amino acid (or peptide) can be connected for the second time, and repeated continuously until the required length of the peptide chain is reached.
For the synthesis of long peptides, there are two ways to lengthen the peptide chain: gradual growth and fragment condensation. The former starts from the initial amino acid (or peptide). Each time, an amino acid is lengthened, and the latter is a new long peptide chain added by the condensation of N protective peptide and C protective peptide.
For peptides containing amino acids with side chain functional groups, such as glutamic acid, aspartic acid, lysine, arginine, histidine, cysteine and so on, in order to avoid the side effects caused by side chain functional groups, it is generally necessary to protect the side chain groups with appropriate protection groups temporarily.
Classification of peptide synthesis methods
There are two ways to synthesize peptides: chemical synthetic peptides and biosynthetic peptides.
Chemical synthesis is mainly in the form of condensation between amino acids. In the synthesis of polypeptides with a specific sequence, due to the amino acid monomers with more than 2 functionality in the raw materials of peptide synthesis, the groups that do not need to be reacted should be protected temporarily during peptide synthesis, so as to ensure the directionality of the target product of peptide synthesis. The chemical synthesis of peptides can be divided into liquid phase synthesis and solid phase synthesis.
The liquid phase synthesis of polypeptide can be divided into two strategies: Step-by-Step synthesis and fragment combination. It can be used for the synthesis of various bioactive peptides. The fragment combination method mainly includes natural chemical connection and stoudinger connection. In recent years, liquid-phase peptide fragment synthesis has developed rapidly, and has made a great breakthrough in the field of peptide and protein synthesis. In the peptide synthesis method, according to the chemical specificity or selectivity of the polypeptide fragment, the peptide fragment can connect spontaneously to obtain the target peptide. Because of the relatively small amount of amino acid residues, the polypeptide has high purity and is easy to purify.
With the development of bioengineering technology, genetic engineering method dominated by DNA recombination technology has also been applied to peptide synthesis.
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