inorganic chemistry

metal (M) or at the ligands (L). In such a case, three funda-
mental types of electronic transitions can be distinguished.
Transitions localized at the metals (metal-centered, MC), ligands
(ligand-centered, LC or intraligand, IL), or charge transfer (CT)
transitions with a spatial redistribution of electron density occur-
ring between reducing (donor) and oxidizing (acceptor) subunits of
the system(56,57).
A schematic representation of the typical classes of electronic
transitions (MC, LC, CT) observed in metal complexes is shown
in Fig. 6. In bioinorganic chromophores, the MC transitions usu-
ally involve metal ions with a partially filled d-electron shell,
which are then characterized by the presence of low-energy
ligand field (d!d) electronic transitions or excitations involving
metal–metals-bonds. The corresponding absorptions are some-
times covered by other bands and therefore hard to detect.
It has been pointed out that whenever metallobiomolecules are
described as being highly colored and having unique spectral
features, this may indicate the presence of intense low-energy
CT transitions ( 57 ). The typical absorption characteristics and
the conspicuous colors related to the presence of such inorganic
chromophores in biological samples have frequently been used
to classify complete enzyme families, to coin names for certain

TABLE II

EXAMPLES OFMETALLOPROTEINSINFLUENCED BYLIGHTABSORPTION

Protein Metals
involved

Effects observed Reference

Catalase Fe Inactivation ( 41 )
Horseradish peroxidase Fe Inactivation ( 42 )
[Fe]-Hydrogenase Fe (?) Inactivation ( 43 )
Methionine synthase Co Inactivation ( 44 )
Ferredoxin nitrate
reductase

Mo Inactivation ( 45 )

Cytochrome P450 Fe Activation ( 46 )
Cytochromecoxidase Fe,Cu (?) Activation ( 47 )
Nitrile hydratase Fe Activation ( 48 )
Ethanolamine
ammonia-lyase

Co Activation (?) ( 49 )

CH 3 SCoM reductase Ni (?) Activation ( 50 )
Tyrosinase Cu (?) Activation ( 51 )
Fd-thioredoxin reductase Fe Regulation ( 52 )
Ascorbate oxidase Cu Regulation ( 53 )
Xanthine oxidase Mo, Fe (?) Regulation ( 54 )
Methylamine
dehydrogenase

Cu (?) Switch of

function

( 55 )

244 GÜNTHER KNÖR AND UWE MONKOWIUS