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Hi Sebastiano, Free Cys' are crying out for Mercury derivatives. Try 2-3 with varying sizes of additional groups - HgCl2, K2HgI4, PCMB, from the "Magic seven" would be a good place to start. Watch out for problems with your low pH, though. Some salts will form insoluble hydroxide salts at pH 8.8. (HgCl2 & K2HgI4 should be okay) Bart Hazes used to have an excellent set of notes online about heavy atom derivatisation - I can't seem to find the URL right now... HTH, David 2009/7/15 Sebastiano Pasqualato <[log in to unmask]>: > Hi all, > I've got crystals of a protein of ca 200 residues, with 2 free cysteines, 5 > histidines, 2 methionines. > We have nice diffraction for the native crystals, that grow in 150 mM KSCN, > 17% PEG 3350, bis tris propane pH 8.8. > We are crystallising the SeMet derivative, but I'm not completely sure I > will be able to have nice crystals by saturday, when we have tunable time at > the ESRF. > I was thinking of trying with some heavy atom soaks, but only have like 30 > crystals, so limited trials allowed! > Which compound would you advice as more likely to work, and thus worth > testing? > Thanks in advance for the suggestions, > ciao > s > > > > -- > Sebastiano Pasqualato, PhD > IFOM-IEO Campus > Dipartimento di Oncologia Sperimentale > Istituto Europeo di Oncologia > via Adamello, 16 > 20139 - Milano > Italy > > tel +39 02 9437 5094 > -- ============================ David C. Briggs PhD Father & Crystallographer http://drdavidcbriggs.googlepages.com/home Skype: DocDCB ============================