ELECTRON DENSITY MAPPING OF HEMOGLOBIN MODEL COMPOUNDS

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Accurate crystallographic methods are being used to map the electron distribution of hemoglobin model compounds as a function of spin state, valency and coordination number. The research is to be focussed on small molecule (NO, CO, O2) adducts of porphyrins and cobalt Schiff bases to elucidate the electron nature of metal-adduct bonding which is essential for our understanding of the oxygen uptake mechanism. Specifically we will derive the amount of charge transfer from metal to ligand, the covalency of the metal ligand interaction and the asphericity of the metal atom charge distribution. This information is expected to fill important gaps in our present knowledge and answer a number of specific questions related to the extent to which changes in electronic structure of the heme contribute to the cooperativity mechanism for loading and unloading of oxygen. The results will aid in our understanding of the large number of biological processes involving heme proteins. Experimental data will be collected on conventional X-ray as well as synchrotron sources. In particular, the wiggler beamline at CHESS, the Cornell High Energy Synchrotron Source, will be used to obtain data at liquid helium temperature. The increased resolution available at low wavelenghts combined with very low temperatures constitute a major improvement in experimental methods.