Chemical Protein Synthesis by Unknown

Author:Unknown
Language: eng
Format: epub
ISBN: 9781071624890
Publisher: Springer US

The side-chain anchoring strategy utilizes the inherent functionality present on the side chain of a number of proteinogenic amino acids as a “handle” to immobilize the residue to the solid support, either directly or through a suitable linker. The C-terminal carboxylic acid is usually protected with an allyl group, due to its orthogonality with Fmoc-solid-phase peptide synthesis (SPPS). After elongation of the desired peptide, allyl deprotection and thio/selenoesterification steps can be conveniently performed on solid support, streamlining the overall process. Since the Fmoc deprotection cannot be performed in the presence of a thio/selenoester, a Boc-protected amino acid is usually coupled at the N-terminus of the peptide. Importantly, this method not only circumvents the solubility issues encountered during solution-phase thio/selenoesterification, but also reduces the epimerization to <1%. The lack of epimerization can be attributed to the proximity of the carboxylate moiety to the solid support blocking the oxazolone pathway [33]. Unfortunately, this method is only applicable for systems featuring a side-chain anchorable amino acid at the C-terminus. These residues (provided in one-letter code) include C, K, N, Q, R, S, T, and Y. However, the applicability of this approach can be improved by implementing a dipeptide-based loading, wherein the C-terminal amino acid can be any amino acid bearing a C-terminal allyl ester and the penultimate residue presents a side-chain anchoring handle, such as C, D, E, K, N, Q, R, S, T, and Y [32–34]. Importantly, these strategies also enable access to bifunctional peptide fragments which are valuable for the synthesis of large proteins involving ligations in either the C → N or N → C direction. If the N-terminal amino acid is incompatible with the thio/selenoesterification conditions, it can be introduced in the end using coupling conditions that do not lead to the hydrolysis of the thio/selenoester moiety [35]. Detailed procedures to prepare peptide thioesters and selenoesters based on aforementioned side-chain anchoring strategies are outlined below.

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