Mass-spectrometry assisted heavy-atom derivative screening of human FcγRIII crystals

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 R_iso 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.