450 IRON-CONTAINING PROTEINS AND ENZYMES
latter delocalized, mixed valence (MV) state, the unpaired electron ( S =^1 / 2 ,
nuclear spinI = 3/2) gives rise to an EPR spectroscopic seven - line hyperfi ne
pattern that persists even at very low temperatures. The delocalized, MV
copper ion system — Cu II · · · Cu I ↔ Cu I · · · Cu II≡Cu1.5 · · · Cu 1.5 — has been designated,
in the inorganic literature, as a class III system.^160 As discussed in Section 7.8.2,
the two copper ions in Cu A are bridged by cysteinyl sulfur atoms, forming a
planar diamond shape with a short 2.58 - Å metal - to - metal distance. Two histi-
dine ligands form strong bonds to copper ions in the Cu A center as well. In
addition to its distinctive EPR spectrum, the Cu A center exhibits an intense
purple color with strong UV – visible bands around 480 and 530 nm. These
bands originate from S(cys) → Cu charge - transfer transitions. Resonance
Raman spectroscopy identifi es Cu 2 S 2 breathing modes, Cu – S and Cu – N stretch-
ing modes, Cu – S twisting modes, and Cu 2 S 2 accordion bending modes. The
unusual core Cu A center structure, with its extremely short Cu – Cu distance
and its thiolate bridging, is believed to be largely responsible for its rapid
electron transfer capabilities.
Over the past 10 – 15 years, several model systems that maintain the copper
ion mixed - valence, delocalized electronic state with short Cu – Cu bond dis-
tances as in the CcO enzyme — Cu II · · · Cu I ↔ Cu I · · · Cu II≡Cu1.5 · · · Cu 1.5 — have been
synthesized and characterized. One class of ligand is the so - called octaazac-
ryptands, also known as macrobicyclic polyaza ligand systems. These have
been studied by the groups of J. Nelson,^161 M. E. Barr,^162 and A. J. Thomson
and J. J. McGarvey.^163 The X - ray crystallographic structures of these complexes
reveal copper ions of trigonal bipyramidal geometry with Cu – Cu distances
between 2.36 and 2.42 Å. The EPR spectra of the complexes show the charac-
teristic seven - line hyperfi ne pattern arising from spin delocalization over two
copper ions ( I = 3/2) with a Cu – Cu bond arising through direct dz 2 orbital
overlap. The hyperfi ne couplings, A⊥ , in the model compounds are relatively
large ( A⊥ ≈ 110 – 115 G) compared to the values for CcO ’ s Cu A site ( Ax = 22.70,
Ay = 24.50, Az = 38.00). The difference indicates that the copper ions in the
model complexes do not possess the extensive spin delocalization onto the
ligands as found for the enzyme ’ s Cu A site. Intense UV – visible ( λ = 600 –
660 nm, ε = 1500 – 3000 M − 1 c m − 1 ) and near - IR ( λ = 730 – 780 nm, ε = 4500 –
5000 M − 1 c m − 1 ) bands indicate metal ion d – d and σ – σ * transitions, respectively.
In CcO ’ s Cu A site, UV – visible bands are assigned as S → Cu charge transfer
bands indicating more involvement of the thiolate ligands that probably play
an important role in the enzyme ’ s electron transfer capability. Spectroscopic
data on CcO ’ s Cu A site and these model compounds, as well as others to be
discussed in the next paragraphs, are collected in Table 14 and 15 of reference
138. The model compounds ’ structures are collected in Figure 26 of the same
publication.
The S. J. Lippard group has studied another set of Type III (class III) mixed -
valence (MV) copper complexes containing carboxylate - bridging ligands
based onm - xylenediamine bis(Kemp ’ s triacid imide), H 2 XDK, or its propyl
derivative (H 2 PXDK).^164 The preorganized, negatively charged carboxylate