a luciferin product. We herein report an alternative reaction pathway leading to an N-
terminal amidine rendering the side chain thiol available for further modification.
Examination of peptide sequence dependence of this amidine conjugation reveals a
tripeptide tag CIS that allows facile N, S-double labeling of a protein of interest with> 90%
yield. This alternative reaction pathway of CBT-NCys condensation presents a significant …