Fig. 2
From: Accelerated microfluidic native chemical ligation at difficult amino acids toward cyclic peptides
Evidence for the enhanced reactivity of SEAE peptide thioesters. a Principle of the thiol-thioester exchange between MPA peptide thioester 1a and MPAA. b Principle of the thiol-thioester exchange between SEAE peptide thioester 5a and MPAA. c Evidence for the high reactivity of model SEAE peptide 5a (circles filled with magenta) featuring a C-terminal difficult amino acid (valine) in comparison with the classical MPA peptide thioester analog 1a (filled triangles). The peptides were reacted in 0.2 M sodium phosphate buffer (pH 7.0) containing 50 mM TCEP and 400 mM MPAA at 37 °C. The exchange reaction was monitored by HPLC (UV detection at 215 nm). d Same reactions as in c under the optimal microfluidic conditions described in Fig. 4 (1a: black triangles, 5a: magenta circles). e The high reactivity of SEAE peptide thioesters is potentially a consequence of an intramolecular acid-catalysis that facilitates departure of thiol 6 from the tetrahedral intermediate (step 2). This step is rate-limiting for simple alkylthiols such as MPA. R’ and R” correspond to the amino acid side-chain (see Fig. 1)
