Accurate low temperature X-ray diffraction data will be collected on a selected series of iron porphyrins which model the prosthetic group of oxygen transport proteins. The data will be used to map in detail the electron density distribution of these complexes to elucidate their electronic structures. The objectives are to determine experimentally the configuration of metal d-orbital electrons and the distribution of charge in the coordinated ligands and porphyrin ring for the significant metal oxidation states, spin-states, and coordination numbers of iron porphyrins. Methods for the direct determination of the charge density distribution have been developd largely in this laboratory and have now reached the point where their application to important biological molecules is possible. Several specific questions regarding the role of the iron heme in the cooperativity mechanism for oxygen binding to hemoglobin will be analysis of the experimental electron density distribution. The size of the iron atom in both the high-spin and low-spin states will be determined. The occupancies of the iron d-orbitals will be obtained from the aspherical distribution of density about the metal atom, providing information on the iron spin-state and the nature of the bonding to coordinated ligands and to the porphyrin ring. Determination of the experimental charge distribution in the porphyrin will provide, in addition, fundamental information about this prosthetic group which occurs in many other biological systems such as several of the cytochromes.