Dear CCP4BB, I've recently solved a structure to 1.15 Angstrom which has extremely clear density for what appears to be a lysine covalently bound to a cysteine (S-N). The distance between the cys-S and lys-N is 1.8 A. I thought initially that there may be a methyl group between the two (based on this paper http://doi.org/10.1002/pro.2958) but there definitely isn't enough space.
Some more info: The protein was purified from recombinant E. coli expression, standard buffers (Tris, NaCl). Crystallisation condition contains sodium acetate and PEG. The residues in question are not part of an active-site. I have two questions: Has anyone seen a cys that is involved in a sulfanamide bond with a lys? How would one go about modeling this bond in a way that doesn't upset coot, refmac or PDB validation? Thanks in advance! Jonathan
