Recently ( 93 ), a photochemical method for NO-separation from
an gaseous mixture was proposed, in which the FeIIsubstrate is
generated and regenerated by photoreduction:
FeIIIðÞedtaðÞH 2 Ohi
þedta^4 !hv
FeIIðÞedtaðÞH 2 Ohi 2
þedta^3 ð 18 ÞThe free edta plays here the role of sacrificial electron donor and
undergoes degradation, similarly as in the BioDeNOx technique
( 77 ). The photoreduction course was found to be almost indepen-
dent of pH. This finding is of practical meaning because
[FeII(edta)(H 2 O)]^2 oxidation by molecular oxygen (Eq. 15) is
slower at higher pH values ( 89 ) and in an excess of edta ( 94 ).
Thus, the photochemical generation of the [Fe(edta)(NO)]^2 com-
plex should be more effective under these conditions ( 93 ). The
process seems to be especially economic and environmentally
friendly as it can be driven by sunlight (16,17,95).
B.2. Ruthenium NO complexes
Complexes of other transition metals, such as Ru, Mn, Cr, Cu,
Co, are also capable of regulating the level of NO in biological
and environmental systems by the binding and chemical or pho-
tochemical release of gaseous NO. In recent years, various tran-
sition metal complexes have been synthesized to modulate NO
concentrations in cellular environments and control physiological
processes that are regulated by NO(33,46,54,75,96).
The high affinity of ruthenium for NO is well documented.
Since ruthenium complexes are in general stable, various
ruthenium nitrosyls have been isolated and studied in detail in
terms of their NO donating capacities. Ruthenium compounds
with readily available coordination sites can be used as NO
scavengers, whereas ruthenium nitrosyl complexes are
investigated as agents controlling the NO-release for medicinal
applications, in particular for the control of high blood pressure,
and as anti-tumor agents that might release cytotoxic NO within
tumor cells, thus leading to cell death.
Modulation of NO release can be induced by one-electron
reduction, which occurs at NOþto yield coordinated NO, or by
photolysis (41,46). Thus, the ruthenium complexes were studied
in search for an ideal system for the site-directed NO delivery
from thermally stable precursors which can release NO when
triggered by light. A large number of [RuNO]^6 nitrosyls release
NO upon exposure to UV light and their potential as NO donors
under the control of light has been surveyed. In general, the
310 ZOFIA STASICKA