Stages of Peptide Synthesis

One starts with individual amino adds or with peptides and tries to achieve the regioselective formation of a new amide bond. In its most general form such syntheses of peptides involve the following stages  

H a NH arginine guanidinium cation N,Nbis (adamantyl-oxycarbonyl) = Adoc pH 10 iQA. H+ 

The phenolic hydroxyl group of tyrosine, the imidazole moiety of histidine, and the amide groups of asparagine and glutamine are often not protected in peptide synthesis, since it is usually unnecessary. The protection of the hydroxyl group in serine and threonine (O-acetylation or O-benzylation) is not needed in the azide condensation procedure but may become important when other activation methods are used. 

With the dicyclohexylcarbodiimide (DCQ reagent racemization is more pronounced in polar solvents such as DMF than in CHjCl2, for example. An efficient method for reduction of racemization in coupling with DCC is to use additives such as N-hydroxysuccinimide or l-hydroxybenzotriazole. A possible explanation for this effect of nucleophilic additives is that they compete with the amino component for the acyl group to form active esters, which in turn reaa without racemization. There are some other condensation agents (e.g. 2-ethyl-7-hydroxybenz[d]isoxazolium and l-ethoxycarbonyl-2-ethoxy-l,2-dihydroquinoline) that have been found not to lead to significant racemization. They have, however, not been widely tested in peptide synthesis. 

The protecting group Y of the amine is generally an alkoxycarbonyl derivative since their nucleophilicity is low. Benzyloxy- or tert-butoxycarbonyl derivatives usually do not undergo azlactone formation. 

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The central event in an epimerization reaction is the removal of a proton from the chiral a-carbon of an amino acid residue. This event generates an enolate with a trigonal planar carbon atom. Replacement of the proton on the face opposite from which it was abstracted results in the inversion of the configuration of the a-carbon. In theory, this event can occur at any stage of peptide synthesis. In practice, however, epimerization is observed almost exclusively during the amide-bond-formation step. This discussion will be confined to that type of epimerization. 

The last stage of peptide chain synthesis is termination. The genetic code specifies three stop codons, indicating the termination of a coding sequence. When the ribosome encounters one of these stop codons on the mRNA, certain release factors... 

Difficulties that have been encountered in divergent synthesis can be attributed to the clustered nature and sequences of peptide dendrimers that form interchain hydrogen bonds and occlude coupling reactions. Solvent combinations such as DMSO and NMP, as well as coupling at elevated temperatures (50-60 °C),[94] have been used to overcome these side reactions and should be used if a single coupling fails to complete the reaction. It is best not to dry the peptide resin at any stage of the synthesis because re-solvation of dried peptide dendrimer resin is difficult. 

This technique relies on the clean coupling of amino acids in peptide synthesis, the ability to easily remove reactants and solvents and wash the products between each stage of the synthesis and the ability to protect and deprotect reactive groups on the sohd support as necessary. 

In the Merrifield method an amino acid is attached to an insoluble polymer. Amino acids are sequentially added, one at a time, thereby forming successive peptide bonds. Because impurities and by-products are not attached to the polymer chain, they are removed simply by washing them away with a solvent at each stage of the synthesis. 

For the stereoselective formation of diastereomeric products, it was recognized at an early stage that the U-4CR can proceed stereoselectively if chiral primary amines and aldehydes are employed as the reactive components. The peptide derivatives 17 can only be formed if the amino component 14 contains an alkyl group that can be replaced by a proton in 16 producing 17 (Scheme 3).P l The essential problems were first solved separately by a variety of experimental investigations.It was found that chiral, a-ferrocenyl-substituted alkyl-amines 18 are able to fulfill all the requirements of peptide synthesis by stereoselective U-4CRs (Scheme 4).hi New, efficient methods were developed for the preparation of alkyl-amines in order to investigate their role in the synthesis of peptide derivatives by the... 

Three key steps in the elongation stage of protein synthesis are required for the addition of each amino acid. Because these steps are repeated for each peptide bond formed, this is sometimes called the elongation cycle. The central theme in elongation is that the fully assembled ribosomal complex functions as a ribonucleoprotein machine which rapidly moves 50 to 30 down the mRNA, much like a ratchet. At the center of this complex are two binding sites which line up over a pair of triplet codons, as shown in Figure 26.11. These two sites are called the P site, for peptidyl (or polypeptide), and the A site, for aminoacyl (or acceptor). A third site, called the E site for tRNA exit site, is also a functional component of the ribosome, but for reasons of clarity, it is not included in the figures. 

Within three years of the first publication Merrifield built a machine to automate the synthesis, and in six years he had synthesized ribonuclease A, an enzyme with 124 amino acid residues, having partial activity of the naturally isolated enzyme. The key features of the Merrifield method that have led to its widespread use are 1) At each stage of the synthesis the polymer-bound peptide can be separated from all other components of the reaction mixture by filtration. This makes possible the use of a large excess of the soluble N-protected amino acid to drive each coupling step to high conversion. 2) The method can be automated. 

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