Hi Vinson Beyond the possibility for another type of residue as already suggested by Phil and Mark, there is also the possibility of O-linked glycosylation of the serine and threonine, if your protein undergoes such post-translational modification and it has been expressed via an expression system that processes the protein in that way. Ser/Thr tandems are well known targets for O-glycosylation (http://www.cbs.dtu.dk/databases/OGLYCBASE/).
best regards Savvas ---- Savvas Savvides Unit for Structural Biology @ L-ProBE Ghent University K.L. Ledeganckstraat 35, 9000 Ghent, Belgium Ph. +32 (0)472 928 519 http://www.LProBE.ugent.be/xray.html On 24 Nov 2010, at 13:10, Vinson LIANG wrote: > Dear all, > > I'm refining a structure and find some strange triangle density on the oxygen > of Ser and Thr at the C terminus. One picture of the strange density is > attached here. Could anyone please give me some suggestions on what this > could be? > > The buffer used during purification is PBS, Tris and NaCl. And > crystallization condition contains PEG3,350 and Mg(NO3)2. > > Thank you all in advance for any suggestion. > > Best, > > Vinson Liang > > > > <triangle_density.gif>
