MYOGLOBIN AND HEMOGLOBIN 349
The triplet ground state (0 kJ/mol) for the O 2 molecule is represented by
the term symbol^3 Σg+ and has two unpaired electrons, one each in the π x or
theπ y level. The fi rst excited state (92 kJ/mol above the ground state) is a^1 Δg
singlet (electrons spin paired with both electrons in either theπ x or the π y
level). The singlet^1 Σg+ state with paired spin electrons, one each in the π x and
π y levels, is the next excited level 155 kJ/mol above the ground state. Reduction
of O 2 by one electron yields the superoxide ion ( O 2 −) a radical anion. Reduc-
tion by two electrons yields the peroxide ion, ( O 22 −). As noted in equation 7.1 ,
the reduction potential for O 2 in the presence of protons is thermodynamically
favorable. Therefore reversible binding of O 2 to a metal can only be achieved
if competition with protons and further reduction to superoxide and peroxide
are both controlled.^9
7.2.4 Structure of the Active Site in Myoglobin and Hemoglobin:
Comparison to Model Compounds
X - ray crystallographic structures of myoglobin and hemoglobin were fi rst
completed in 1966^3 and 1975,^4 respectively. Subsequently, many other X - ray
crystallographic studies of deoxy - and oxy - as well as met - myoglobin and
hemoglobin have been carried out. Additionally, researchers have studied the
carbon monoxide - bound moieties MbCO and HbCO as well as MbNO. Site -
directed mutagenesis of residues near the active sites of Mb and Hb have
yielded information on the exact nature of O 2 , CO, and NO binding and the
small molecule ’ s orientation at the heme site. With information from these
Figure 7.6 Ground - state electron confi guration for the O 2 molecule.2p πu 2pπgσuπxπ*yπyπ*xσg(2pz)σu(2s)σg(2s)2s 2sO O 2 O