- Principle of peptide synthesis
- In order to obtain synthetic peptides with specific order, arbitrary polymerization method is not feasible, but the directional peptide synthesis method of gradual condensation can only be used.
- 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.
Peptide synthesis, also known as peptide chain synthesis, is a solid-phase synthesis followed by ordinary synthesis from N-terminal (amino terminal) to C-terminal (carboxyl end). In the past, peptide synthesis was stopped in solution, which is 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 way of zwitterions (H3 + NCH (R) COO -) in neutral condition, the reaction of direct condensation between amino acids to form amide bond is difficult to stop under ordinary conditions.
The reaction activity of amino acid ester was higher. When heated at 100 ℃ or stored at room temperature for a long time, the reaction is not directional. The ester of two amino acids A1 and A2 will produce A1A2 、a1a1… 、a2a1… And all kinds of arbitrary mixture.
In order to obtain synthetic peptides with specific order, arbitrary polymerization method is not feasible, but the directional peptide synthesis method of gradual condensation can only be used.
Generally, it is shown in the following formula: firstly, the amino or carboxyl groups that do not need reaction are temporarily maintained with appropriate groups, and then the linking reaction is stopped to ensure the directional termination of peptide synthesis.
In the formula, X and Q are the maintenance groups of amino group and carboxyl group respectively, which can not only prevent the occurrence of side effects of random connection, but also eliminate the zwitterionic mode of amino acids and make them easily soluble in organic solvents.
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 beneficial to the free amino group of another amino acid. It stops the nucleophilic attack on the carboxyl carbon atom of the activated carboxyl group to form amide bond.
The resulting conjugation product is a maintenance peptide with both N-terminal and C-terminal maintenance groups, and the free peptide can be obtained only after the maintenance group is removed.
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 second connection with the new n-maintained amino acid (or peptide) or the c-maintained amino acid (or peptide) can be terminated, and repeated from time to time until the required length of the peptide chain is reached.
As 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. In the latter, n-maintenance peptide is condensed with c-maintenance peptide to obtain a new long peptide chain.
As 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 necessary to maintain the side chain groups temporarily with appropriate maintenance groups.
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