Abstract: During a study aimed at generating a bispecific molecule between BN antagonist (d-Trp6,Leu13-ψ[CH2NH]-Phe14)BN6-14 (Antag1) and mAb22 (anti-FcγRI), we attempted to cross-link the two molecules by introducing a thiol group into Antag1 via 2-iminothiolane (2-IT, Traut's reagent). We found that reaction of Antag1 with 2-IT, when observed using HPLC, affords two products, but that the later eluting peptide is rapidly transformed into the earlier eluting peptide. To understand what was occurring we synthesized a model peptide, d-Trp-Gln-Trp-NH2 (TP1), the N-terminal tripeptide of Antag1. Reaction of TP1 with 2-IT for 5 min gave products 1a and 3a; the concentration of 1a decreased with reaction time, whereas that of 3a increased. Thiol 1a, the expected Traut product, was identified by collecting it in a vial containing N-methylmaleimide and then isolating the resultant Michael addition product 2a, which was confirmed by mass spectrometry. Thiol 1a is stable at acidic pH, but is unstable at pH 7.8, cyclizes and loses NH3 to give N-TP1-2-iminothiolane (3a), ES-MS (m/z) [602.1 (M+H)+], as well as regenerating TP1. Repeat reaction with Antag1 and 2-IT allowed us to isolate N-Antag1–2-iminothiolane (3b), FAB-MS (m/z) [1212.8 (M+H)+] and trap the normal Traut product 1b as its N-methylmaleimide Michael addition product 2b, ES-MS (m/z) [1340.8 (M+H)+]. Thiol 1b is also stable at acidic pH, but when neutralized is unstable and cyclizes, forming 3b and Antag1.