Figure 3. MALDI-TOF mass spectra for unlabeled S64.

(left) and S67 (right). The predicted monoisotopic masses for fully oxidized S64 are 3558.3 Da for the [M+H]⁺ ion and 1779.69 Da for the [M+2H]²⁺ ion. Likewise, the predicted mass for fully oxidized S67 is 2121.41 Da for the [M+2H]²⁺ ion. The appearance of peaks at these masses indicates that the cysteines are oxidized for both S64 and S67. The additional peaks seen to the left of the [M+2H]²⁺ peak in the S67 mass spectrum correspond to ions that have dehydrated. These peaks are also present in the S64 spectrum but are difficult to discern due to the much wider scale used for this spectrum.