Supplementary Material (ESI) for Chemical Communications This journal is (c) The Royal Society of Chemistry 2010 Reactions of Medicinally Relevant Gold Compounds with the C terminal Motif of Thioredoxin Reductase elucidated by MS analysis Alessandro Pratesi, a Chiara Gabbiani, b Mauro Ginanneschi a and Luigi Messori* b a Laboratory of Chemistry and Biology of Peptides and Proteins, Department of Chemistry “Ugo Schiff”, University of Florence, Via della Lastruccia 13, 50019 Sesto Fiorentino, Firenze, Italy. b Laboratory of “Metals in Medicine” (METMED), Department of Chemistry “Ugo Schiff”, University of Florence, Via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy. E-mail address: luigi.messori@unifi.it (L. Messori) Fax: +39 055 4573385; Tel: +39 055 4573388 - Peptide syntesis Selenocysteine was prepared starting from Selenocystine, in accordance with Moroder S1 and Hondal, S2 Fmoc-Sec(Mob)-OH was obtained with a bidimensional orthogonal protection scheme suitable for solid phase peptide synthesis. The peptide Ac-Gly-[Cys-Sec]-Gly-NH 2 (1) was synthesized starting from Rink Amide MBHA resin (500 mg; 0,4 mmol/g). All the reactions were performed on a semi automatic synthesizer (MultiSyn Tech - Germany) following the standard SPPS method and using the orthogonal Fmoc/Trt/t-Bu protection scheme. The resin was swelled in DMF for 20 minutes and the removal of the Fmoc amino acids protecting group was performed by 20% piperidine in DMF (2 × 15 min). The amino acids Fmoc-Gly-OH, Fmoc-Sec(Mob)-OH, Fmoc-Cys(Mob)-OH; according to the reported sequences, were introduced following the TBTU/HOBt/NMM method with formation of active esters. The coupling reactions were performed by using an excess of the amino acids, of the activating agents HOBt and TBTU (2.5 mole equiv.) and of NMM (5 mole equiv.) in DMF, vortexing for 40 min at room temperature. After each coupling, the resin was washed with DMF (3 × 5 ml) and DCM (2 × 5 ml). Coupling reactions efficiency was controlled by the Kaiser test. S3 The linear peptide was deprotected with 20% piperidine in DMF and the -NH 2 terminal group was acetylated with a solution of acetic anhydride (20 eq) and NMM (20 eq). The resin-containing mixture was then swollen at room temperature for 30 min. The reaction was repeated once again