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Chemical Synthesis of Proteins with Base-labile Posttranslational Modifications Enabled by a Boc-SPPS Based General Strategy Towards Peptide C-terminal Salicylaldehyde Esters.

Chemical synthesis of proteins bearing base-labile post-translational modifications (PTMs) is a challenging task; for instance, O -acetylation and S -palmitoylation PTMs that cannot survive Fmoc removal conditions during Fmoc-solid phase peptide synthesis. In this work, we developed a new Boc-SPPS-based strategy for the synthesis of peptide C-terminal salicylaldehyde (SAL) esters, which are the key reaction partners in Ser/Thr ligation and Cys/Pen ligation. The strategy utilized the semicarbazone-modified AM (aminomethyl) resin, which could support the Boc-SPPS and release the peptide SAL ester upon treatment with TFA/H 2 O and pyruvic acid. The non-oxidative aldehyde regeneration was fully compatible with all the canonical amino acids. Armed with this strategy, we finished the syntheses of the O -acetylated protein histone H3(S10ac, T22ac) and the hydrophobic S -palmitoylated peptide derived from caveolin-1.