CYTOCHROME c OX IDASE 447
erence 138 authors collect ligand and crystallographic data for the CcO enzyme
heme a 3 – Cu B binuclear centers in their Table 5. Table 6 of this reference details
bond distance and angle data for numerous Fe III – (X) – Cu II model complexes.
Some of these data are collected in Table 7.10.
Chemists have studied a wide variety of Fe III – X – Cu II synthetic model com-
pounds, where X may be an exogenous ligand such as fl uoride (F – ), cyanide
(CN – ), and formate (HCOO – ) or naturally occurring X groups such as hydrox-
ide (OH − ), oxide (O 2 − ), carboxylate (RCOO − ), chloride (Cl − ), sulfi de (S 2 − ),
cysteinato (S - ligand atom of cysteine), or imidazolate (side chain N - ligand
atom of histidine) groups. One goal of the synthetic model research has been
to understand the magnetic properties of the CcO enzyme: (1) a strong antifer-
romagnetic coupling model for the high - spin S = 5/2 ferric heme a 3 with the S
= 1/2 Cu B center to produce the S = 2 ground state for the heme a 3 – Cu B center
with J values exceeding 100 cm − 1 ; (2) evidence in similar systems for a weak
coupling model withJ < 4 cm − 1 ; and (3) observation of an active EPR spectrum
for some CcO systems.
Space considerations limit the number of Fe III – X – Cu II synthetic model
compounds that we can discuss here. We will concentrate on model com-
pounds featuring oxide (O 2 − ) or hydroxide (OH − ) bridges. First, the heterobi-
nuclear peroxo complex [( 82 ) III−−(^2 −+) II( )] F TPP Fe, prepared O Cu TMPA
according to the self - assembly strategy as described above, will lose one - half
mole of dioxygen in a slow disproportionation reaction and convert to the μ -
oxo complex [(F 8 TPP)Fe III – (O 2 − ) – Cu II (TMPA)] + when heated to room tem-
perature (see Figure 7.44 ).^151 Subsequent addition of one equivalent of acid
yields the μ - hydroxo complex [(F 8 TPP)Fe III – (OH − ) – Cu II (TMPA)] 2+. Karlin
and co - workers have used heterobinucleating ligands (ligands for two metal
centers connected by a tether) in formation of μ - oxo and - hydroxo com-
plexes.^159 A portion of one such reaction scheme is shown in Figure 7.47. The
(^6) L (connected to the 6 - position of a TMPA pyridine arm) and (^5) L (connected
to the 5 - position) ligands each have a tetradentate TMPA ligating moiety
covalently attached to the edge of a porphyrin. Starting with the^6 L ligand, one
can form the di - iron product μ - oxo product [(^6 L)Fe III – O – Fe III – Cl] + , selectively
remove one iron to form [(^6 L)Fe III – OH], and then add copper ion to form
[(^6 L)Fe III – O – Cu II ] +. The same procedure can be repeated with the^5 L ligand.
The two heterobinuclear products behave differently when acidifi ed. The
[(^6 L)Fe III – O – Cu II ] + reaction with H + produces [(^6 L)Fe III – OH – Cu II ] 2+ , whereas
acidifi cation of [(^5 L)Fe III – O – Cu II ] + results in a broken bridge. The reference
159 researchers ascribe the differing reactivity to the Fe – O – Cu bond angles:
approximately linear for [(^6 L)Fe III – O – Cu II ] + (and for [(F 8 TPP)Fe III – (O 2 − ) –
Cu II (TMPA)] + as well) and substantially bent — 141 ± 6 ° by EXAFS
determination — for [(^5 L)Fe III – O – Cu II ] +. The linear nature of the Fe – O – Cu
connection for [(^6 L)Fe III – O – Cu II ] + has been confi rmed by an X - ray crystallo-
graphic structure determination.^158 The researchers point out that small changes
in the structures of [(^6 L)Fe III – O – Cu II ] + versus [(^5 L)Fe III – O – Cu II ] + change the
acid – base behavior of these model complexes. The differing behavior of the