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P 680
(Asp)O Tyr
+
OH 2 P 680 - e -
H 2 O Mn O(Glu) Tyr
H 2 O
H 2 O O O III H III H IV H
Mn (Asp) Mn O Mn O Mn O
H 2 O Ca O O H
H +
Mn
(Glu)O O O III H
Mn H 2O H Mn O .
N(His) O(Glu) III O 2x
N(His) 2 Mn Mn O Mn
H
H O + 1/2O 2
2

Figure 16. Organisation of the metal cluster in the oxygen evolving centre (left), and the
-
•
presumed catalytic process of water oxidation (right). Tyr = tyrosyl radical, Tyr = tyrosinate.

Plastocyanin, at the end of the electron transfer chain between PSII and PSI belongs to
the category of ’blue copper proteins’, or type I Cu proteins. In these Cu proteins, Cu 1+/2+ is
-
coordinated in a flat trigonal-bipyramidal fashion by two Cys and one His, and – in the axial
position at a rather long distance of 2.9 Å – methionine. The intense blue colour of the
9
2+
oxidised (Cu , d ) form comes about by a ligand-to-metal charge transfer (LMCT), i.e.
transfer of electron density from non-bonding orbitals of the coordinated Cys-S into the 3d
2+
‘hole’ of Cu . While charge transfer within the d-d system of a metal ion is parity (Laport)
forbidden, and the corresponding absorption bands hence are weak in intensity (see, e.g.,
2+
[Cu(H 2O) 6] ), LMCT transitions are allowed and thus very intense.

Disgression: Systematics of copper proteins

Type I (Blue Cu-proteins): trigonal coordination geometry; ligands: 2 Cys(1-), 1 His, 1
weakly bonded Met. Strong LMCT at 600 nm; small EPR-spectroscopic hyperfine coupling
-1
-
-
-4
constant (A = 5⋅10 cm ). Function: e transfer; Example: Plastocyanin in the e transfer chain
PSII→PSI.

Type II: Tetragonal coordination geometry; ligands: His, Tyr(1-), H 2O, no Cys. “Normal”
optical behaviour (d-d transitions); normal EPR patterns (A = 18⋅10 cm ). Function: Redox
-4
-1
reactions; Examples: Galactoseoxidase (RCH 2OH → RCHO + 2H + 2e ), CuZn-
+
-
superoxidedismutase (2O 2 + 2H → O 2 + H 2O 2).
-
+

Type III: Contain 2 cooperating Cu centres; trigonal coordination geometry; ligands: 3 His
per Cu. Intensely blue in the oxidised form (O 2 →Cu LMCT); no EPR signal
2+
2-
(antiferromagnetically coupled). Function: Transport and transfer (to a substrate) of oxygen;
examples: haemocyanin, Fig. 8; tyrosinase (Tyr + ½O 2 + 2e → DOPA).
-

Ceruloplasmin, important for the absorption of iron, is a Cu protein containing 7 Cu centres
representing types I, II and III. Nitritereductase contains type II (substrate activation) and type
I Cu centres (e transfer)
-

A
B
Others: e.g. Cu und Cu in cytochrom-c oxidase; Fig. 13.