On Wednesday, 17 May, 2017 20:03:42 Antonio Ariza wrote: > I haven't asked anything for a loooooong time, so here are a couple of > question "for the honourable members of the esteemed CCP4 bulletin board" ... > or as I'd usually say: "for y'all". ;) > > 1) I have this modified CYS in one of the structures I'm working on and ... > I'm quite unhappy with it (see attached pics). At first I thought it was > cacodylate ... but alas, no cacodylate was used during purification or > crystallisation. So I looked up possible modifications on CYS residues and I > came up with cysteine-s-sulfonic acid (CSU). This looks good in principle but > it doesn't quite fit the electron density as the bond between the two sulphur > atoms is too short. The average length for an S-S bond is about 2.05 Ang and > refmac refines this one to 1.97 Ang, but it looks like it should be at least > 2.5 Ang to sit correctly in the electron density. Also, there is some > negative density in there, suggesting that maybe it's something with fewer > electrons than a sulfonic acid ... or maybe it has less than 100% occupancy. > Any suggestions?
Could it be that a cloning error introduced a Cys->Arg mutation? -- Ethan A Merritt Biomolecular Structure Center, K-428 Health Sciences Bldg MS 357742, University of Washington, Seattle 98195-7742
