The McClure and Weiss models of Fe-O2 bonding for oxyhemes, and the HbO2 + NO reaction.

For the Fe-O2(S = 0) linkages of oxyhemes, valence bond (VB) structures are re-presented for the McClure [Fe(II)(S = 1) + O2(S = 1)], Pauling-Coryell [Fe(II)(S = 0) + O2*(S = 0)], and Weiss [Fe(III)(S = ½) + O2 (-)(S = ½)] models of bonding. The VB structures for the McClure and Weiss models are of the increased-valence type, with more electrons participating in bonding than occur in their component Lewis structures. The Fe-O bond number and O-O bond order for the McClure structure are correlated with measured Fe-O and O-O bond lengths for oxymyoglobin. Back-bonding from O2(-) to Fe(III) of the Weiss structure gives a restricted form of the McClure structure. The McClure and Weiss increased-valence structures are used to provide VB formulations of mechanisms for the oxyhemoglobin + NO reaction. The products of these two formulations are Hb(+) and NO3(-) (where Hb is hemoglobin) and Hb(+) and OONO(-), respectively. Because Hb(+) and NO3(-) are the observed products, they provide an experimental procedure for distinguishing the McClure and Weiss models. It is also shown that the same type of agreement between McClure-type theory and experiment occurs for oxycoboglobin + NO, cytochrome P450 monooxygenases, and related hydrogen atom transfer reactions. In the appendices, the results of density functional theory and multireference molecular orbital calculations for oxyhemes are related to one formulation of the increased-valence wavefunction for the McClure model, and theory is presented for the calculation of approximate weights for the Lewis structures that are components of the McClure increased-valence structure.