BINDING OF TRANSITION METAL IONS IN INSULIN CRYSTALS

X-ray diffraction data are presented which suggest that cupric insulin crystals are isomorphous with the much studied zinc ones. Electron paramagnetic resonance orientation studies at room and several cryogenic temperatures were made on crystals containing two cupric ions per unit cell (hexamer). The spectra show six distinct magnetic sites. Location of the metal ions on a trigonal axis relating three insulin dimers satisfies the magnetic, X-ray and chemical evidence. The six observed cupric sites are then explained on the basis of Jahn-Teller distortions, and the two-nitrogen hyperfine pattern which is observed is seen to be a consequence of metal coordination to three nitrogens. Analysis of earlier titration data confirms this model and assigns the nitrogens to histidine residues. © 1968.