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The behavior of mercury-derivatized triclinic crystals of a 60 kDa protein target from the New York Structural GenomiX Research Consortium provides novel insights into the mechanism of heavy-atom-specific radiation damage and its potential exploitation for de novo structure solution. Despite significant anomalous signal, structure solution by classic SAD and MAD phasing approaches was not successful. A detailed analysis revealed that significant isomorphic variation of the diffracted intensities was induced by X-ray irradiation. These intensity changes allowed the crystal structure to be solved by the radiation-damage-induced phasing (RIP) technique. Inspection of the crystal structure and electron-density maps demonstrated that the covalent S-­Hg bonds at all four derivatized cysteine sites were much more susceptible to radiation-induced cleavage than other bonds typically present in native proteins. A simple diagnostic is described to identify the fingerprint of such decay at the time of data collection/processing. The rapid radiation-induced decomposition of mercury adducts is consistent with the difficulties frequently associated with the experimental phasing of mercury derivatives and suggests a straightforward solution to overcome this limitation by radiation-damage-induced phasing with anomalous scattering (RIPAS). These results indicate that historically recalcitrant and newly emerging difficulties associated with Hg phasing should be revisited.

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