excited state opened the possibility of a selective photodamage of
the purine nucleobase guanine (G) under long-wavelength irradi-
ation which should also operate well under hypoxic conditions.
When 28 was irradiated in the presence of nucleobase sub-
strates, a photocatalytic oxidative degradation of guanine could
be demonstrated. A quantum yield of f¼0.03 measured for
436-nm photolysis and an initial turnover frequency of
TOF¼66 h^1 was estimated for guanine degradation. At the
same time, a maximum number of approximately 700 turnovers
for each photosensitizer molecule was observed ( 216 ).
Despite these very interesting photocatalytic properties of the
gold complex 28 , this compound still had several disadvantages
such as moderate water solubility and the existence of
atropisomeric mixtures under physiological conditions due to a
hindered rotation of the pyrenyl substituents. Therefore, we
decided to improve some crucial properties of the system and
synthesized the novel gold(III)porphyrin complex 29 ( 217 ). This
compound no longer can form isomeric mixtures and exhibits
an excellent water solubility, while keeping one functional
pyrenyl group attached. The presence of four positive charges
leads to a modification of the possible electrostatic interactions
with nucleic acids. Spectroscopic studies revealed that 29 inter-
acts with A-DNA and B-DNA and displays outside binding with
self-stacking to DNA duplexes. The modified gold(III) porphyrin
sensitizer shows light-induced guanosine and 5^0 -dGMP oxidation
under aerobic and anaerobic conditions. Light-triggered plasmid
DNA nicking and photocatalytic double-strand cleavage of oligo-
nucleotide duplex DNA are possible with this artificial
photonuclease ( 217 ). Tumor cell line tests and related studies
on other gold(III) tetrapyrrole photosensitizers including
substituted corrole complexes are currently underway.
B. LIGHT-DRIVENMODELENZYMES INCATALYSIS
One of the most challenging areas in inorganic photochemistry
is to catalyze the synthesis of valuable and energy-rich com-
pounds from abundant raw materials and sunlight. In this con-
text, the development of homogeneous photocatalysts which are
able to completely replace the function of natural enzymes for
synthetic applications is a highly desirable goal(5,18). As one
of the rare examples of such a synthetic model photoenzyme sys-
tem controlled and driven by visible light, we have chosen an
artificial oxidoreductase catalyst investigated in our own group
which has already been described in detail elsewhere (3,6).
278 GÜNTHER KNÖR AND UWE MONKOWIUS