Phasing with mercury at 1 Å wavelength

Synchrotron sources provide a continuously tunable X-ray beam which makes it possible to optimize the anomalous contribution to phase determination using heavy-atom replacement. This method was used to solve two protein structures, those of Dictyostelium discoideum nucleoside diphosphate kinase and of lobster enolase. The first had 17 kDa of protein in the asymmetric unit, the second, 47 kDa. In both cases, a single mercury derivative yielded single isomorphous replacement with anomalous-scattering phases from which an interpretable electron-density map was derived by solvent flattening. The efficient solution of the X-ray structure was largely due to the large anomalous scattering of mercury at a wavelength shorter than the L_III absorption edge.