Fig. 3: Insights into the conversion of SetCys to a Cys residue.
From: A cysteine selenosulfide redox switch for protein chemical synthesis
a Proposed mechanism for the loss of the N-selenoethyl appendage of N-terminal SetCys peptides. b Experimental pH–rate profile of the conversion of N-terminal SetCys peptide to the corresponding N-terminal cysteinyl peptide (magenta diamonds). Each of the 10 data points represents the value of the reaction rate k1 obtained from different reactions conducted at the considered pH (1 mM SetCys peptide 1 in 0.1 M phosphate buffer, 6 M Gn·HCl, 200 mM MPAA, 100 mM TCEP, 100 mM sodium ascorbate, 37 °C). Data were fitted by chi2 minimization. Associated standard errors (SE) were estimated based upon the covariance matrix resulting from nonlinear regression fitting (Supplementary Table 3). Data are presented as mean values ± SE. The data were then fitted to a Gaussian (green curve) to determine the pH values for the inflexion points (pH 4.8 and 7.3). c Different ionization states for the SetCys residue in open form. The numbers on the arrows correspond to the inflexion points determined in Fig. 3b and to pKa values calculated using ACDLabs® software (in parenthesis). d Rate of 2-selenoethyl limb cleavage in SetCys (filled circles, k1 = 2.42 × 10−3 min−1) and SetAla (open circles, kSetAla = 2.96 × 10−4 min−1) model peptides at pH 6. SetAla peptide was produced in situ from the corresponding diselenide by reduction in the presence of TCEP (1 mM SetCys or SetAla peptide in 0.1 M phosphate buffer, 6 M Gn·HCl, 200 mM MPAA, 100 mM TCEP, 100 mM sodium ascorbate, 37 °C).
