Figure 2:

Mass chromatograms of the Cys(Acm) protected peptide under various conditions. The arrow points to the observed m/z whereas the number inside the parentheses denotes the theoretical m/z value. (A) Mass chromatogram of the Acm-test peptide after 12 h incubation in neat TFA at 37 ºC. The MS data show that nearly all of the peptide exists in the Cys(Acm) form, in agreement with HPLC analysis. (B) Mass chromatogram of the Acm-test peptide after 12 h incubation in TFA/TIS (98/2) at 37 ºC. The MS data show that inclusion of 2% TIS in the reaction mixture drives deprotection to the Cys-SH (m/z = 691.4) and disulfide (m/z = 690.46) forms in support of our HPLC analysis. Please note that the value m/z = 690.46 for the peptide disulfide correspond to the doubly charged ion (z = 2).