The Pos i t ion of A n o m a l o u s Scatterers in Prote in Crystals

383 hexagonal phases, contains within it empty hexagonal tunnels (Magndli, 1949b). It is interesting to note that this has been identified in needles grown from tungsten metal in an electron-diffraction camera, (Hashimoto, Tanaka, Yoda & Araki, 1958) in which the high vacuum could prevent the formation of an A X8 type oxide. 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.