possible thermal- and photoinitiated precursors of
nitrovasodilators and anticancer agents. Experiments in various
biological systems showed that light strongly enhances their
vasodilatory and cytotoxic properties, and the biological activity
of the clusters strongly correlates with the generation of
NO (41,42).
Special attention was also paid to understanding the basic pro-
perties and reactivity patterns of the dinitrosyl iron complexes
and Roussin's red salt esters, and to resolve the mechanism of
their biological functions and NO emission( 43 – 47 ). The quantum
yield of NO release from [Fe 2 (m 2 -SR) 2 (NO) 4 ] under visible light
irradiation can be considerably enhanced by incorporating into
the ester a proper chromophore; then single- or two-photon pro-
cesses were observed (46,48,49).
Experimental work in model systems supplemented with theo-
retical investigations proved that the main photoreaction mode
of Roussin's black salt is a photoredox reaction leading to gener-
ation of NO and a Fe(II) compound. Thep*NO–d transitions
were assumed to be responsible for the photochemical reactivity
of these compounds, which result in photodissociation of the
NO group(33,36,50– 52 ). It has been found, however, that irradi-
ation of [Fe 4 (m 3 -S) 3 (NO) 7 ]in the presence of S^2 , thiolate anions
or other S-nucleophiles produces Fe(III) species and N 2 O,
instead of NO ( 53 ).
From biological and medical aspects, the primary targets for
NO in bioregulatory functions are metal centers, chiefly heme-
and nonheme iron proteins(33,41,54– 57 ). A fast reaction with
its biological targets would be necessary for NO to serve as an
effective regulatory agent at the submicromolar concentrations
foundin vivo. This is indeed the case for the reaction of NO with
the ferroheme enzyme soluble guanylyl cyclase for which
kon¼1.4 108 M^1 s^1 (277 K) ( 41 ). The rates tend to be very
fast when the NO ligand enters a previously unoccupied coordi-
nation site or substitutes a very labile leaving group like the
H 2 O ligand.
Detailed kinetic and mechanistic studies on the reaction
between NO and metmyglobin (Mb(III)) using laser flash photol-
ysis and stopped-flow techniques, demonstrated the reversible
binding of NO:
½metMb HðÞ 2 O þNO^ !½metMb NOðÞþH 2 O ð 12 ÞFrom a mechanistic perspective, the nitrosyl complex of
metmyoglobin is of particular interest because both association
and dissociation of NO are conveniently observable under
306 ZOFIA STASICKA