Title:
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Novel protecting group strategies for the aspartyl residue in peptide synthesis
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An investigation into novel protecting group strategies for the prevention of imide formation at the Asp-X segment of a peptide sequence was undertaken. Research centred around the esterification of the aspartyl side chain with an acid labile protecting group which exerted an extreme steric influence on the system. Three protecting group strategies were extensively studied with respect to the aspartyl residue; namely derivatised trityl systems, the 2,2,4,4-tetramethylpent-3-yl (Tmp) system and the 2,4-dimethyl-2,4-bis(4'-methoxyphenyl)pent-3-yl (Dmbp) system. The characteristics of the trityl β-aspartates indicated that they possessed the requirements sought in a successful protecting group, but orthogonal synthesis of such derivatives applicable to automated SPPS proved elusive. The 9-fluorenylmethyloxycarbonyl (Fmoc) amine β-protected aspartyl residue was successfully synthesised with the side chain protected with Tmp and Dmbp groups. The respective β-derivatised aspartyl residues were incorporated into peptide sequences which were known to be susceptible to aspartimide formation, namely the partial sequence of coat protein phage MS2 and ferrodoxin, and tested for their efficacy in preventing rearrangement during Fmoc-based SPPS. The β-aspartyl-protected peptides were subjected to a variety of acidic deprotection protocols and the merits of each were assessed with respect to aspartimide formation.
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