Peptides fully unprotected

Fig. 4. Scheme of NCL. The mechanism allows the straightforward preparation of small proteins with native backbone structures from fully unprotected synthetic peptide building blocks. The initial tran -thioesterification step includes the chemo-selective reaction between one peptide with a C-terminal a-thioester group (peptide 1) and second peptide with an N-terminal cysteine residue (peptide 2). Generated thio-ester-linked intermediate spontaneously rearranges to form a native peptide bond at the site of ligation. 

A conceptually different approach to assemble fully unprotected peptides is to use an enzyme to attain both specificity and catalysis of the amide bond formation. This strategy has been developed using proteases, enzymes that cleave peptide backbone amide bonds. Following the principle of microscopic reversibility, any enzyme can be coerced to catalyze a reaction not only in the forward direction but also in the reverse direction. Such reverse proteolysis methods typically use substrates containing activated C-termini,... 

Starting from Fmoc-L-phenylalanine bound to Wang resin (1216), standard solid-phase peptide synthesis using the Fmoc-strategy was carried out, imtil the desired heptapeptide 1217 was obtained. Deprotection and cleavage from the solid phase yielded fully unprotected heptapeptide 1218, and consequent cyclization delivered unguisin A (1219) in 81% yield. 

Lei went on to develop solid-supported sulfur ylide linker 5 that has become the standard practice in our group for preparing C-terminal peptide ot-ketoacids (Scheme 10) [24]. Once the linker is loaded onto the resin, standard Fmoc-based solid-phase peptide synthesis can be used to grow peptide chains without difficulty. Deprotection and cleavage from the resin gives the fully unprotected peptide sulfur ylides that are easily handled and purified by standard methods. Exposure of these peptides to Oxone or DMDO gives the peptide a-ketoacids, which are usually isolated by preparative HPLC and lyophilized. Just one example out of many is shown in Scheme 10. 

The removal of side-chain protecting groups and release of the peptide from the support can either occur in one or two steps. While the one-step protocol in Fmoc-SPPS combines an acidolytic handle with acid-labile side-chain protecting groups for the convenient production of fully unprotected peptides, the two-step protocol enables regioselective chemistry on the peptide, for example, the synthesis of branched peptide structures. 

A similar sequence of 12- and 10-membered turns is present in the structure of Boc-protected /S //S -peptides 96 and 97, the C=0 of the Boc group being engaged in the first 12-membered ring with NH of residue 3. The pattern of 10- and 12-membered turns is reversed for the fully protected / // -peptide 94 as well as the unprotected /S //S -dodecapeptide 98 which thus folds into a 10/12-helix, with the NH of residues 1 and 2, respectively being involved in the formation of an N-ter-minal 10-membered turn. 

The incorporation of unprotected side-chain derivatives of Asn and Gin derivatives has also been investigated using HATU. In the SPPS of AGP no cyano derivatives of Asn and Gin were detected, however, the incorporation of Asn was not complete, possibly due to the low solubility of Fmoc-Asn-OH in DMF.P l Due to this low solubility of Fmoc-Asn-OH and the unfavorable effects of unprotected side chains on the aggregation behavior of the growing peptide chain, the use of side-chain protection is reconunended (Section 2.3.3).f l For reasons still not clear, even side-chain protection of Asn does not fully eliminate deficiencies in phosphonium- or uronium-mediated couplings. Thus, the incorporation of Fmoc-Asn(Trt)-OH is more satisfactory if one equivalent of HOAt is present during HATU-mediated couplings.P ... 

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