1 Supporting Information Radical Cascades in Electron Transfer Dissociation (ETD) – Implications for Characterizing Peptide Disulfide Regio-Isomers Lei Tan, Kirt L. Durand, Xiaoxiao Ma, Yu Xia* Department of Chemistry, Purdue University, West Lafayette, IN, 47907-2084, USA Address reprint requests to: Dr. Yu Xia Department of Chemistry Purdue University West Lafayette, IN, USA 47907-2084 Phone: (765) 494-1142 Fax: (765) 494-9421 E-mail: yxia@purdue.edu Electronic Supplementary Material (ESI) for Analyst This journal is © The Royal Society of Chemistry 2013