Hi Jonathan, Not sure if it's been observed in a crystal structure before, but a Cys-Lys bond (sulfenamide) has been proposed to occur with functional relevance in the bleach-sensing transcription factor, NemR. (See Antioxid Redox Signal. 2015 Sep 20; 23(9): 747–754. doi:10.1089/ars.2015.6346) I think sulfenamides are more regularly reported between the Cys sulfur and its backbone amide, but the Cys-Lys sulfenamide should still be perfectly feasible as far as I understand. Cheers,
David
