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The electronic and structural properties of the cubane-type mixed-metal sulfido clusters with a FeIr3S4 core and possible dinitrogen binding and activation were analyzed by density functional theor...

Early reports of a severely bent CO adduct in myoglobin inspired the idea that heme proteins discriminate against CO, relative to O2, via steric hindrance imposed by a distal histidine residue. Recent results showing that the bound CO is only slightly distorted do not by themselves overthrow the steric hypothesis, because the steric energy could be stored in displacements of the protein. However, experimental data on site-directed mutants show that the main determinant of ligand affinity changes is the polarity of the binding pocket and that H-bonding by the distal histidine accounts for about 85% of the O2/CO discrimination while steric hindrance accounts for the remaining 15%.