ESI-MS was used to determine monoisotopic masses of the intact protein for wild type Prdx6 and C91S Pdx6 in TBS (10 mM Tris, 100mM NaC1, pH7.4) and TBS+TCEP (5 mM). (A) Prdx6 WT protein showing a molecular mass of 24,886.14 Da in the non-reduced protein and 2.01 Da greater in the protein reduced with TCEP; the expected monoisotopic mass is 24,888.15 Da. (B) C91S Prdx6 protein showing a molecular mass of 24,870.15 Da in the non-reduced protein and 2.02 Da greater in the presence of TCEP; the expected monoisotopic mass is 24,872.17 Da.