characteristic absorption bands( 25 ). Most of these compounds
are able to interact with metal ions or may serve as ligands in
biocoordination chemistry ( 10 ). Under natural conditions, these
chromophores are usually thought not to be directly exposed to
electromagnetic radiation. They are therefore considered as irrel-
evant for photobiological primary processes. Nevertheless, the
corresponding compounds frequently turn out to be light
sensitive under bothin vitroandin vivoconditions. This feature
can be directly exploited to influence biological functions or to
construct artificial photoresponsive systems.
Illustrative examples for such a possibility are found with the
cytochromes. The name of these proteins comes from the Greek
words meaning“colored substances in the cell.” Cytochromes
are intensely red-colored redox enzymes containing a heme pros-
thetic group as their dominant chromophore. Hemes are iron
complexes of protoporphyrin IX derivatives(10,26). One of the
most frequently studied metalloproteins of this family is cyto-
chromec( 27 ). The ribbon structure of a cytochromecenzyme
together with the protein-bound heme c cofactor 6 is shown
in Fig. 4.
Cytochromechas a number of vital functions inside the cell.
As part of the mitochondrial electron transport chain in respira-
tion, it shuttles redox equivalents to cytochromecoxidase ( 28 ),
which can be considered as a kind of natural fuel cell for energy
conversion. Mitochondrial dysfunction accompanied with changes
in cytochromecoxidase activity, however, seems to be intimately
related to aging, neurodegeneration, and disease ( 29 ). Effects
that can manipulate the functions of such redox enzymes
are therefore of great importance for many research fields.
Photons are able to cause such effects. Light sensitivity of cyto-
chrome containing systems has been known for a long time. For
example, when bacteria of the type Rhodomicrobium (red
microbe) were exposed to near infrared (NIR) radiation, their
cytochrome system was oxidized from the ferrous to the ferric
state ( 30 ). It is important to note that an archaic type of
anoxygenic photosynthesis based on the oxidation of Fe(II) spe-
cies has in the mean time been discovered ( 31 ), and that
Rhodomicrobiumstrains are now known to belong to such types
of photoferrotrophic microorganisms ( 32 ).
More recently, a novel mitochondrial signaling pathway
activated by red light in the visible-to-NIR spectral region has
been discovered( 33 ), which is thought to depend on a direct light
absorption by the metal-binding sites of the cytochromecoxidase
machinery. These findings may help to understand the beneficial
effects of red light therapy for accelerated wound healing and
PHOTOSENSITIZATION AND PHOTOCATALYSIS 241