Assembly of the peptide chain

Acylation 3. Add activated amino acid derivative dissolved in DMF to resin 4. Agitate gently 30 7-12 

Test 5. Remove resin sample for colour test 6. Continue agitation if necessary  

The exceptions are peptides containing an N-terminal glutamine residue without side-chain protection and polymer-bound dipeptides—these should be stored Fmoc-protected, in order to prevent intramolecular cyclization. For a period of a few days, the peptidyl resin can be stored swollen in DMF at 5 C. For longer periods, the resin should be stored in the dried state such resins should be re-swollen in DMF for 18 h before use. 

---

The N-to-C assembly of the peptide chain is unfavorable for the chemical synthesis of peptides on solid supports. This strategy can be dismissed already for the single reason that repeated activation of the carboxyl ends on the growing peptide chain would lead to a much higher percentage of racemization. Several other more practical disadvantages also tend to disfavor this approach, and acid activation on the polymer support is usually only used in one-step fragment condensations (p. 241). 

Dintzis, H.M. (1961) Assembly of the peptide chains of hemoglobin. Proc. Natl. Acad. Sci. USA 47, 247-261. 

Fig. 13 Cryo-TEM images of (a) 16, (b) I7 (inset magnified view of representative fibers), and (c) schematic representation of the proposed formation of fibers of Ig. The benzenedithiol core of building block 1 is shown in yellow and the peptide chain in blue. Stacks of hexamer are held together by the assembly of the peptide chains into elongated cross-P sheets. (Reproduced from [55])...

The synthesis of a monocarba analogue of deaminooxytocin has been described in which incorporation of the cysteine moiety as a 5-3-propanoic acid aUyl ester allows, after assembling of the peptide chain, the selective deprotection and cyclization of the peptide.P ... 

Compared with O-glycopeptides, aecess to A-glycopeptides on the solid phase is more readily possible via two different pathways [42]—conventional sequential incorporation of preformed and suitably protected building blocks into the solid-phase synthesis or assembly of the peptide chain, selective removal of the aspartic acid side-chain protecting groups, and formation of the amide linkage between glycosyl amines and the peptide backbone [81] on the solid phase. 

After finishing the assembly of the peptide chain, add 10 mL of DCM, stir for 30 s, and then remove the solvent by vacuum filtration. Repeat this step four times. 

These methodologies have been reviewed (22). In both methods, synthesis involves assembly of protected peptide chains, deprotection, purification, and characterization. However, the soHd-phase method, pioneered by Merrifield, dominates the field of peptide chemistry (23). In SPPS, the C-terminal amino acid of the desired peptide is attached to a polymeric soHd support. The addition of amino acids (qv) requires a number of relatively simple steps that are easily automated. Therefore, SPPS contains a number of advantages compared to the solution approach, including fewer solubiUty problems, use of less specialized chemistry, potential for automation, and requirement of relatively less skilled operators (22). Additionally, intermediates are not isolated and purified, and therefore the steps can be carried out more rapidly. Moreover, the SPPS method has been shown to proceed without racemization, whereas in fragment synthesis there is always a potential for racemization. Solution synthesis provides peptides of relatively higher purity however, the addition of hplc methodologies allows for pure peptide products from SPPS as well. 

Cyclosporin A contains II amino acids, joined in a cyclic strncture by peptide bonds. The structure is also stabilized by intramolecular hydrogen bonds. Only two of the amino acids, i.e. alanine and valine, are typical of proteins. The compound contains several A-methylated amino acid residues, together with the even less common L-a-aminobutyric acid and an Ai-methylated butenylmethylthreonine. There is one o-amino acid, i.e. o-alanine, and the assembly of the polypeptide chain is known to start from this residue. Many of the other natural cyclosporin structures differ only with respect to a single amino acid (the a-aminobutyric acid residue) or the number of amino acids that have the extra Ai-methyl group. 

There are two principal modes for introducing the photophore into a peptide (1) Coupling a photoreactive N-protected amino acid during the stepwise assembly of the peptide e.g. in Scheme 3, the amino acid 4-benzoylphenylalanine (3) is incorporated into position 8 of substance P (9). (2) Post-synthetic modification of a fully assembled peptide, at either a free N-terminal a-amino group or a side-chain functionality, by the photophore in a site-specific or nonspecific manner e.g. in Scheme 3, treatment of cyclic R-G-D-containing peptide 10 with 4-benzoylbenzoic anhydride (11) gives the modified peptide 12. 

Formation of the three-dimensional network of bacterial peptidoglycan includes synthesis both of glycosidic and peptide linkages, but only the former type of reaction is discussed in this Section. The assembly of the carbohydrate chains of a peptidoglycan has been shown to occur through a block mechanism. The initial reaction consists in transfer of N-acetyl-muramyl-pentapeptide phosphate from the corresponding UDP derivative... 

A clever strategy has been reported by Moroder s group. 139 During the convergent assembly of three peptide chains containing various protected forms of Cys, thiol-thiol exchange reactions lead to formation of a cysteine knot core (Scheme 17). 

Despite the breakthrough associated with Merrifield s approach, there are several limitations such as the discontinuous nature of the reaction, the need for large excesses of reagent and the mechanical instability of the polymer matrix. An early solution to the restrictions imposed by Merrifield s polystyrene supported batch process was the use of commercially available benzyl alcohol-functionalized silica (used for H PLC columns). This was initially derivatized with the first member of the peptide chain to be propagated. The synthesis of a tetrapeptide in flow was completed in half the time required for the equivalent batch mode assembly and required significantly smaller excesses of the solution-phase reagent [92],... 

---