compounds, or to identify the involvement of characteristic inter-
mediates or cofactors in biochemical processes. Illustrative
examples in this context are metalloproteins with blue copper
sites, purple acid phosphatases, or the red iron–sulfur proteins
of the rubredoxin type (Fig. 7).
Many of the biological ligands present in metalloproteins
(oxide, sulfide, phenolate, thiolate, and peroxide) exhibit low-
energy ligand-to-metal charge transfer (LMCT) transitions. Due
to the strong donor character of the coordinated groups involved
in such systems, this may also reflect the presence of highly cova-
lent ligand–metal bonds, which considerably contribute to the
observed reactivity of these active sites in biology. Low-energy
metal-to-ligand charge transfer (MLCT) transitions are less fre-
quently assigned in bioinorganic systems. They require the pres-
ence of a reducing metal donor site and ligands with sufficiently
low-lying acceptor orbitals. This situation is frequently observed
in bio-organometallic systems and in the presence of tetrapyrrole
macrocycles or redox cofactors.
Besides the CT transitions shown inFig. 6, several other com-
binations of donor and acceptor moieties in metalloproteins can
MC
(n-1)dnsnpLCMLCTM ML 6 LLMCTσM∗ππ*πL∗σM∗ΔOπMπLσLσFIG. 6. Molecular orbital diagram for an octahedral transition metal
complex ML 6 illustrating different types of electronic transitions based
on localized orbital configurations (MC, metal-centered; LC, ligand-
centered; MLCT, metal-to-ligand charge transfer; LMCT, ligand-to-metal
charge transfer). Adapted from Ref. ( 1 ).
PHOTOSENSITIZATION AND PHOTOCATALYSIS 245