Abstract
Parametrization of a molecular-mechanics program to include terms specific for 5- and 6-coordinate transition metal complexes results in computer-simulated structures of hemo complexes. The principal new feature peculiar to 5- and 6-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 5- and 6-coordination. The model system takes into account the structural differences of the fixing centre in the haemoglobin (Hb) 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 favourable that others for fixing O 2. From the O 2 binding in haemoglobin 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 −.
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