[ccp4bb] Solvent-exposed Cys and an unidentified ligand

[Lynn Gregory-Pauron]

My structure has a Cys that is solvent-exposed and an unidentified 
'blob' of density which is connected to the sulfur of the Cys. The 
protein was crystallized in presence of DTT, and DTT does fit the 
density. However, I do not know if it is oxidized or not, as the density 
does not appear to accommodate the ring conformation of oxidized DTT.  
Does anyone know of an article that  talks about the chemistry of DTT, 
reducing agents in crystallographic structures or have any suggestions 
as to what is going on?
Thank you,
Lynn