The nature and configuration of the mercuri-iodide ion in the seal myoglobin derivative

Mercuri-iodide, an ion derived from K 2HgI 4, has been used successfully with seal myoglobin, as well as with other crystalline proteins, in preparing isomorphous derivatives, but the nature of the substituent group is not known. The following work was undertaken with the object of determining the structure of the group which becomes attached to the molecule of seal myoglobin. The investigation is confined to the centro-symmetric projection extending to 2 Å resolution. Three sets of h0 l data were employed, measured respectively from crystals of native protein, of the mercuri-iodide and of the gold chloride derivative. The method of isomorphous replacement has been used in a twofold way. (a) From the gold chloride derivative, first refined, preliminary signs were allotted to the structure amplitudes of the protein, and (b) from these protein signs and the observed intensity changes produced by the mercuri-iodide derivative signs could be given to the mercuri-iodide structure factors without any assumptions about the nature of the group. The resulting electron density map favoured a plane trigonal arrangement of one mercury and three iodines projected end-on. Approximate positions of the individual atoms of the group were refined by least squares and were used in a ( F observed — F calculated) synthesis which revealed the existence of a second site of substitution in the myoglobin molecule. Extensive refinement showed the mean occupancies of the two groups to be 0·54 and 0·17, respectively. The group at the main site appears to be almost planar and is oriented so that its projection is very nearly parallel to the projected plane of the haem group in the myoglobin molecule. The second group, because of the lower occupancy, is less accurately determined but it appears to have a similar configuration.