Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Copper 75
seems to be quite important is their relative abundance on Earth. A second
factor is related to the fact that active centers of metalloproteins consist of
kinetically labile and thermodynamically stable units. Kinetic lability facilitates
rapid assembly/disassembly of the metal centers as well as fast association/dissociation
of substrates. Both the above criteria are fulfilled by copper,
which has been present in living organisms since the early stages of evolution,
representing a fundamental constituent of many biological systems, particularly
proteins, with the function of transporting oxygen and transferring
electrons.
A key diversity between Cu(I) and Cu(II) is the different preferential coordination
geometry. Cu(I) prefers tetrahedral four-coordinate geometries
whereas Cu(II) complexes are typically square-planar or, in some biosystems,
trigonal planar; occasionally, square planar compounds bind two additional
weakly bonded axial ligands. In metalloproteins undergoing electron
transfer processes, copper experiences a wealth of slightly different coordination
environments: a tetrahedral ligand arrangement usually stabilizes
Cu(I) over Cu(II), decreasingtheCu(II)/Cu(I) reduction potential, whereas
high reduction potentials are achieved through distortion towards trigonal
planar. In general, the thermodynamic stability of a metal center in biological
environments is determined not only by inherent preferences of the
metal for a particular oxidation state, ligand donor set, and coordination
geometry, but also by the ability of the biopolymer to control, through its
three-dimensional structure, the stereochemistry and the actual nearby ligand
available for coordination. Non-coordinating residues also contribute to
shape the local environment via hydrophilic/hydrophobic effects or steric
blocking of coordination sites [17]. The complex pattern of factors occurring
in biological systems make it possible to reach coordination geometries,
such as trigonal planar, which can hardly be reproduced via synthetic
chemistry.
Two examples of copper containing metalloproteins, namely the blue copper
site and the mixed-valence binuclear CuA center, can be illustrated to
better understand how Nature organizes metal complexed centers, with the
aim of optimizing their properties for a specific function, in this case electron
transfer [18].
In the blue copper site, which occurs in the plastocyanin that couples photosystem
I with photosystem II through electron transfer (ET) [19], the X-ray
geometrical structure of the Cu(II) center is distorted tetrahedral and not
square planar, as normally observed for cupric complexes. The coordination
environment is provided by two histidine nitrogen atoms giving 2.05 ˚A long
N-Cubonds,onethiolatesulfurofcysteinewithashortCu– Sbondof
≈ 2.1 ˚A length and one thioether methionine at a longer distance (S – Cu
≈ 2.9 ˚A), Fig. 3.
The unusual geometry and ligation are responsible for the unique spectroscopic
features of the blue copper site. In contrast to the weak d-d tran-