The enzyme hen egg-white lysozyme was one of the first macromolecular structures to be solved using
X-ray diffraction techniques in the 60's [14] [13] [12]. The results of
extensive studies on the binding of heavy atom complexes to lysozyme, myoglobin and hemoglobin have
been published by Blake [11]. Of particular interest was the fact that Iridium and Osmium
both formed isomorphous derivatives when introduced as 
and 
ions
into tetragonal (
) crystals of lysozyme
(cell dimensions 
Å
).
In these studies it was reported that the Iridium and Osmium complexes were taken up in the same
position as that occupied in the mercuriiodide derivative. The site is on a two-fold axis running
along the (
) direction in the lattice and the included ion was bound by two Arg13 residues from
symmetry related protein molecules.