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.

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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>