Hello everyone

I have used ethyl mercuric phosphate as one of the components in my crystallisation buffer (native crystals ). After solving the structure  at (3Angstrom ) I was able to see strong positive peak around one of the Cysteine residues. My question is  how to link  mercuric ion (here CH3CH2Hg+ ion)  to sulfhydryl group of cysteine as I cannot see other residues  close enough to link them to Hg ion. Is one cysteine is enough to link to Hg or it has some specific pattern or geometry ? I have shown the possibilities which may arise in case two ion binds to Cys residue (as it appears in my case). I have attached the screen shot from the Coot. My second question is about the optimum distance of Hg with respect to sulfhydryl group of cysteine as i could see variation in the various entries of protein data bank (ranging from 2.05 A to even 2.5A). At this resolution of 3A if i bring it closer to SH group then ethyl moiety can easily fit  into the electron density other wise it is only mercury which can be placed inside the density blob while the ethyl group remains flanking outside.

Faisal