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A heavy-atom screening method aided by mass spectrometry is described here. Using mass spectrometry, several heavy-atom compounds have been screened in order to obtain potential phasing derivatives for the crystals of a human immunoglobulin Fc receptor, FcγRIII. Of these, HgCl2, trimethyllead acetate (TMLA), KAu(CN)2, K2PtCl4 and PbAc2 reacted with FcγRIII in solution, whereas KAuCl4, ethylmercuric thiosalicylate (EMTS) and Na2WO4 did not. To validate the mass-spectrometry results, these heavy-atom compounds were also used to soak crystals of FcγRIII and crystallographic data were collected after soaking. The calculated Riso indicated that HgCl2, TMLA, K2PtCl4 and PbAc2 were likely to form derivatives, whereas KAu(CN)2 and Na2WO4 were not. The anomalous difference Patterson maps calculated for the HgCl2 and TMLA derivative data sets were of good quality and can readily be interpreted by hand. In general, the number of binding sites obtained from the crystallographic phase refinement of the derivatives agrees with those obtained from the mass spectrometry, suggesting that mass spectrometry can be applied for rapid searching of suitable heavy-atom derivatives for X-ray crystallography.

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