Fmoc-Ala-Wang resin

Fmoc and Boc group are the two main groups used to protect the α-amino function in Solid-Phase Peptide Synthesis (SPPS). In this regard, the use of the Mmsb linker allows the combination of these two groups. Peptide-O-Mmsb-Resin is stable to the piperidine and trifluoroacetic acid (TFA) treatment used to remove Fmoc and Boc, respectively. The peptide is detached in a two-step protocol, namely reduction of the sulfoxide to the sulfide with Me3SiCl and Ph3P, and then treatment with TFA. The advantage of this strategy has been demonstrated by the following: preparation of peptide with no diketopiperazine formation in sequences prone to this side reaction; on-resin cyclization without the concourse of common organic reagents such as Pd(0) but of difficult use in a biological laboratory; and on-resin disulfide formation in a total side-chain unprotected peptide. The use of Mmsb linker together with Msib (4-(methylsulfinyl)benzyl) and Msbh (4,4'-bis(methylsulfinyl)benzhydryl) described in the accompanying manuscript add a fourth dimension to the SPPS protecting group scheme.

This chapter describes the basic protocols for solid-phase peptide synthesis using the Fmoc group as the N (α)-protecting group (Fmoc-SPPS). The chapter introduces resins and their handling, choice of linkers, and the most common methods for peptide chain assembly. The proper choice of resins and linkers for solid-phase synthesis is a key parameter for successful peptide synthesis. This chapter provides an overview of the most common and useful resins and linkers for the synthesis of peptides with C-terminal amides, carboxylic acids, and more. The chapter finishes with robust protocols for general solid-phase peptide synthesis, i.e., the standard operations.

We have been engaged in the microwave-solid phase peptide synthesis (SPPS) synthesis of the phenylglycine (Phg)-containing pentapeptide H-Ala-Val-Pro-Phg-Tyr-NH(2) (1) previously demonstrated to bind to the so-called BIR3 domain of the anti-apoptotic protein XIAP. Analysis of the target peptide by a combination of RP-HPLC, ESI-MS, and NMR revealed the presence of two diastereoisomers arising out of the racemisation of the Phg residue, with the percentage of the LLLDL component assessed as 49%. We performed the synthesis of peptide (1) using different microwave and conventional stepwise SPPS conditions in attempts to reduce the level of racemisation of the Phg residue and to determine at which part of the synthetic cycle the epimerization had occurred. We determined that racemisation occurred mainly during the Fmoc-group removal and, to a much lesser extent, during activation/coupling of the Fmoc-Phg-OH residue. We were able to obtain the desired peptide with a 71% diastereomeric purity (29% LLLDL as impurity) by utilizing microwave-assisted SPPS at 50 °C and power 22 Watts, when the triazine-derived coupling reagent DMTMM-BF(4) was used, together with NMM as an activator base, for the incorporation of this residue and 20% piperidine as an Fmoc-deprotection base. In contrast, the phenylalanine analogue of the above peptide, H-Ala-Val-Pro-Phe-Tyr-NH(2) (2), was always obtained as a single diastereoisomer by using a range of standard coupling and deprotection conditions. Our findings suggest that the racemisation of Fmoc-Phg-OH, under both microwave-SPPS and stepwise conventional SPPS syntheses conditions, is very facile but can be limited through the use of the above stated conditions.