Peptide synthesis
Peptide synthesis process for producing the treated biochemically important class of peptides.
Description
Defined peptides can not be prepared by direct condensation of amino acids. Already in the chemical reaction of two different unmodified amino acids can occur four different dipeptides. Therefore, for a targeted synthesis of defined peptides both the amino group of one amino acid and the carboxyl group of another amino acid must be temporarily blocked respectively by suitable protecting group. In addition, activation of the carboxyl group is required, which is to react with the amino group of the second amino acid of the peptide bond as carboxylic acids reacting with amines with salt formation usually only. The formation of the peptide bond must take place under conditions where no racemization can occur when enantiomerically pure products are to be built. For protecting the carboxyl group, for example, the first amino acid in the presence of sulfuric acid of the amino acid tert- butyl ester are reacted with isobutene to form:
In some cases, such as in asparagine, this method fails and the tert-butyl ester can be obtained in the presence of strong acids only by transesterification with acetic acid tert-butyl ester.
For the protection of the amino group of the second amino acid is the amino group, for example, with benzyl chloroformate (abbreviation: Z- Cl or Cbz -Cl, R ¹ = benzyl ) Flurenyloxymethylchlorid (abbreviation: Fmoc -Cl, R ¹ = fluorenyl -9- methyl group ), or with di-tert -butyl dicarbonate ( BOC anhydride ) to give the corresponding urethanes:
After the formation of the peptide bond, the tert-butyl ester can be represented by acids such as trifluoroacetic acid, with elimination of tertiary butanol to hydrolyze, wherein the carboxylic acid is formed again. The cleavage of the urethane is carried out either by reaction with hydrogen chloride / acetic acid ( cleavage of the tert - Butylurethans ) by a solution of piperidine ( cleavage of fluorenyloxycarbonyl ) or by hydrogenolysis ( cleavage of Benzylurethans ) each with release of the amine function.
Activation of the carboxy group is achieved for example through the conversion into a carboxylic acid chloride, a carboxylic acid azide or a 4 -nitrophenyl ester. Particularly frequent activation of the carboxy group is carried out by the use of dicyclohexylcarbodiimide (DCC) as a condensing agent. Very common and active esters such as N- hydroxybenzotriazole ( HOBt) or N-hydroxysuccinimide are ( HOSu ) which are either of the condensation agents N, N- diisopropylcarbodiimide (DIC) or 3 -ethyl-1 ( N, N- dimethyl) aminopropylcarbodiimid ( EDCI ) were prepared will
An elegant method for peptide synthesis is the solid-phase synthesis according to Merrifield. In the peptide synthesis after each synthesis cycle, the uncoupled peptides with acetic anhydride are acetylated (English capping, bekappen ' ) so that they do not couple in the following cycles, because they would otherwise be exposed to one cycle and would be wrong.
The products of a peptide synthesis are usually subsequently purified, eg by precipitation or by HPLC. By use of a polymerizable molecule in the last coupling step of peptide synthesis, the synthesized peptides may be correctly separated from the acetylated and erroneous coupling products by a subsequent polymerization.
Importance of synthetic peptides in biochemistry
Synthetic peptides are used as inhibitors or substrates of enzymes or act as antigens in immunology.