Mass-spectrometry assisted heavy-atom derivative screening of human Fc gamma RIII 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, Fc gamma RIII. Of these, HgCl(2), trimethyllead acetate (TMLA), KAu(CN)(2), K(2)PtCl(4) and PbAc(2) reacted with Fc gamma RIII in solution, whereas KAuCl(4), ethylmercuric thiosalicylate (EMTS) and Na(2)WO(4) did not. To validate the mass-spectrometry results, these heavy-atom compounds were also used to soak crystals of Fc gamma RIII and crystallographic data were collected after soaking. The calculated R(iso) indicated that HgCl(2), TMLA, K(2)PtCl(4) and PbAc(2) were likely to form derivatives, whereas KAu(CN)(2) and Na(2)WO(4) were not. The anomalous difference Patterson maps calculated for the HgCl(2) 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.