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Classification of synthetic peptide methods
The synthesis of polypeptides is mainly divided into two pathways: chemical synthesis of polypeptides and biosynthetic peptides.
Chemical synthesis is mainly carried out in the form of condensation between amino acids and amino acids. In the synthesis of a polypeptide containing a specific sequence, since the polypeptide synthesis raw material contains an amino acid monomer having a functionality of more than 2, the polypeptide should be temporarily protected by a group which does not require a reaction, and the peptide reaction can be carried out, thereby ensuring the polypeptide. The directionality of the target product is synthesized. The chemical synthesis of polypeptides is further divided into liquid phase synthesis and solid phase synthesis.
The liquid phase synthesis of peptides is mainly divided into two strategies: stepwise synthesis and fragment combination. The step-by-step synthesis is simple and rapid, and can be used for the synthesis of various biologically active polypeptide fragments. Fragment combination methods mainly include natural chemical linkages and Staudinger linkages. In recent years, peptide liquid phase synthesis has developed rapidly, and major breakthroughs have been made in the field of peptide and protein synthesis. In the polypeptide fragment synthesis method, depending on the chemical specificity or chemoselectivity of the polypeptide fragment, the polypeptide fragment can be spontaneously ligated to obtain the target polypeptide. Because polypeptide fragments contain relatively few amino acid residues, they are relatively pure and easy to purify.
Biosynthesis methods of peptides mainly include fermentation and enzymatic hydrolysis. With the development of bioengineering technology, genetic engineering, which is dominated by DNA recombination technology, has also been applied to the synthesis of peptides. 
Principle of synthetic peptides
Polypeptide synthesis is how to connect various amino acid units according to the amino acid arrangement order and connection mode of natural products. Since amino acids are present in the intramolecular zwitterionic form (H3+NCH(R)COO-) under neutral conditions, the direct condensation of amino acids to form an amide bond is difficult under normal conditions.
The amino acid ester has a high reactivity. The peptide ester can be polymerized by heating at 100 ° C or at room temperature for a long time, but the reaction is not oriented. The esters of the two amino acids a1 and a2 will form a1a2..., a1a1..., a2a1, etc. during polymerization. a mixture of sequences.
In order to obtain a synthetic polypeptide having a specific sequence, it is not feasible to employ any polymerization method, and only a stepwise condensation-directed polypeptide synthesis method can be employed. Generally, it is represented by the following formula, in which an amino group or a carboxyl group which does not require a reaction is temporarily protected with a suitable group, and then a ligation reaction is carried out to ensure the orientation of the synthesis of the polypeptide.
X and Q in the formula are a protecting group for an amino group and a carboxyl group, respectively, which not only prevent the occurrence of a side-by-side reaction, but also have the effect of eliminating the zwitterionic form of the amino acid and making it soluble in an organic solvent.
In some cases, Q may not be a covalently bonded group, but an organic cation composed of an organic strong base such as triethylamine and a carboxyl hydrogen ion of an amino acid. Y is a strong electron withdrawing group which activates the carboxyl group and favors the free amino group of another amino acid, and nucleophilic attack on the carboxyl carbon atom of the activated carboxyl group to form an amide bond.
The ligation product thus obtained is a protective peptide having a protective group at both the N-terminus and the C-terminus, and a free peptide can be obtained after the protective group is removed. If the peptide chain does not end here, but it is also necessary to extend the peptide chain from the N-terminus or the C-terminus, then X or Q can be selectively removed first, followed by a new N-protected amino acid (or peptide) or C-protection. The amino acid (or peptide) is ligated a second time and is repeated in sequence until the desired length of the peptide chain.
For peptide synthesis of long peptides, there are generally two ways of gradual growth and fragment condensation of two elongated peptide chains, the former starting with the starting amino acid (or peptide). Each time it is ligated, one amino acid is lengthened, and the latter is a new long peptide chain obtained by condensing the N-protected peptide with the C-protected peptide to obtain the length of both.
For peptides containing amino acids with side chain functional groups such as glutamic acid, aspartic acid, lysine, arginine, histidine, cysteine, etc., in order to avoid side chain function Side reactions caused by the group generally require temporary protection of the side chain groups with appropriate protecting groups. 
Method and principle of synthesizing polypeptide
Synthetic polypeptides are compounds in which alpha-amino acids are linked together by peptide chains, which are also intermediates in protein hydrolysis. A compound obtained by dehydration condensation of two amino acid molecules is called a dipeptide, and analogously, a tripeptide, a tetrapeptide, a pentapeptide, and the like.