Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Copper 77
Fig. 5 Inverted intensity pattern of ligand-to-metal charge transfer absorption transitions
for the blue copper site compared to a regular Cu(II) complex. L = generic organic ligand;
S = sulfur coordinating site of a cysteine residue
In photosynthesis, plastocyanin functions as an electron transfer relay
between cytochrome f (inside cytochrome b6f complex) and P700 + .Cytochrome
b6f complex (from photosystem II) and P700 + (from photosystem
I) are both membrane-bound proteins with exposed residues on the lumenside
of the thylakoid membrane of chloroplasts. Cytochrome f acts as an
electron donor while P700+ accepts electrons from reduced plastocyanin
(Fig. 6) [18].
Fig. 6 The so-called Z-scheme of photosynthesis
Plastocyanin (Cu 2+ Pc) is reduced by cytochrome f to Cu + Pc which eventually
diffuses through the lumen until recognition/binding occurs with P700 + ,
which oxidizes Cu + Pc back to Cu 2+ Pc. The electronic structure of the blue
copper is crucial for an efficient electron transfer in which Cu(II) is reduced
to Cu(I). The tetrahedral organization of the Cu(II) site minimizes the reorganizational
energy λ increasing the rate of the process, according to Marcus
theory [22].