The position of anomalous scatterers in protein crystals

A Patterson sunnnation with {IF(hkl)l-IF(~kZ)I} ~ coefficients gives peaks at the end of vectors relating the anomalous scatterers in the structure. Application to two mercury derivatives of horse haemoglobin, using Cu Ka diffraction data, shows that the ends of vectors between mercury atoms can be recognized easily, and the mercury positions determined, without resorting to isomorphous replacement. Interactions between the iron atoms of the haem groups and the mercury atoms are lost in the background, but can nevertheless be located by removing the spurious background peaks with Buerger's Minimum Function.