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?
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Ethan A Merritt
Biomolecular Structure Center, K-428 Health Sciences Bldg
MS 357742, University of Washington, Seattle 98195-7742
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