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After transport of O 2 by haemoglobin in the blood stream, the oxygen is transferred to tissue
myoglobin (Mb). As shown in Fig. 5, Mb has a higher affinity to O 2 than Hb.

2+
In the desoxy form of Hb, Fe is in its high-spin state (cf. the inset on p. 7, top) and
thus exhibits a paramagnetism corresponding to four unpaired electrons. The diameter of high-
2+
2+
spin Fe is 92 pm; the Fe ion thus is too large to fit into the space left by the four N-
2+
functions of the protoporhyrin. Actually, Fe is displaced from the plane spanned by the
porphyrin by 40 pm towards the proximal His; cf. Fig. 6; resulting in a watchglass bulge of the
porphyrin, i.e. a tensed situation. Consequently, desoxy-Hb is termed T (for tensed) form. On
uptake of oxygen, the iron spin state converts to low-spin, resulting in a reduction of its
3+
2+
diameter to 75 pm (Fe , no unpaired electrons) or 69 pm (Fe , 1 unpaired electron),
respectively. The iron ion now moves into the plane of the porphyrin (R form; R = relaxed).
Oxi-Hb is diamagnetic. If iron remains in its ferrous state, overall diamagnetism can only be
achieved in case the coordinated oxygen converts from the paramagnet triplet state (in free O 2)
to the diamagnetic singlet state (in Oxi-Hb) (cf. also box below). Alternatively, the uptake of
2+
-
3+
O 2 can occur in the sense of an oxidative addition, i.e. Fe + O 2 → Fe -O 2 . In that case, the
unpaired electron of the ferric ion and the unpaired electron of superoxide have to couple in
order to provide the overall diamagnetism. The overall situation is conveniently described in
terms of a resonance hybrid:
O Fe 2+ O Fe 3+
O
O

Tutorial: Oxygen

1
One commonly distinguishes three oxygen modifications: Singlet-O 2 ( O 2; high energy content,
3
unstable, diamagnetic), triplet-O 2 ( O 2, stable, biradical and hence paramagnetic), and ozone

(O 3; toxic; very reactive [strong oxidant]). [A high pressure modification, (O 2) 4, is also known]

OO OO O OO
O O OO
1 O 2 3 O 2 O 3 O 2 - O 2-
2
Formation of ozone in the troposphere (ozone smog): NO + O 2 → NO 2 + O; O 2 + O → O 3;
NO 2 + hν → NO + O
Stratospheric ozone: Stratospheric ozone is an effective filter for “hard“ UV (responsible for
cancers of the skin):
O 2 + hν (λ < 240 nm) → 2O; O 2 + O → O 3
O 3 + hν (λ < 315 nm) → O 2 + O

Radicals, e.g. NO, degrade ozone catalytically (“ozone whole“):

NO + O 3 → NO 2 + O 2, NO 2 + O → NO + O 2

Other radicals can do the same job, e.g. Cl atoms, which are liberated from chloro fluoro

alkanes (CFC) under stratospheric conditions

•-
2-
Reduction of O 2 produces superoxide (O 2 ) or peroxide (O 2 ), both of which are strong

oxidants and physiologically harmful (reactive oxygen species, ROS). To cope with these

oxidants, the body holds ready catalases (H 2O 2 → H 2O + O 2) and superoxidedismutases

+
-
(2O 2 + 2H → H 2O 2 + O 2).
Another ROS species is the hydroxyl radical, formed, e.g. by the Fenton reaction:
2+
3+
+
•
Fe + H 2O 2 + H → Fe + H 2O + HO