Nature of O2, CO, and CN binding to hemoprotein models

Parametrization of a molecular-mechanics program to include terms specific for five- and six-coordinate transition metal complexes results in computer-simulated structures of hemo complexes. The principal new feature peculiar to five- and six-coordination is a term that measures the effect of electron-pair repulsion modified by the ligand electronegativity and takes into account the different structural possibilities. The work consists in the modification of program molecular mechanics for, penta and hexacoordination. The model system takes into account the structural differences of the fixing center in the hemoglobin subunits. The customary proximal histidine is added. The macrocycle hemo IX is wholly considered in our model. The calculations show clearly that certain conformations are much more favorable that others for fixing O-2 From the O-2 binding in hemoglobin and myoglobin and in simple Fe porphyrin models, it is concluded that the bent O-2 ligand is best viewed as bound superoxide, O-2(-). (C) 2004 Wiley Periodicals, Inc.