physiological temperatures and pH(54,58). In the reaction of NO
with oxymyoglobin, an iron(III)–nitrato complex was identified
as the millisecond intermediate ( 59 ).
NO reacts with both ferric and ferrous centers in hemoproteins
to form the respective iron(II) and iron(III) nitrosyl adducts,
whose structural features are similar to those observed for iron
(II) and iron(III) porphyrin nitrosyls. These analogies are also
reflected in similar chemical reactivity observed for nitrosylated
ferri- and ferroproteins and their respective porphyrin models.
For example, NO-adducts of Fe(III) undergo reductive
nitrosylation in the presence of an excess of NO, and a similar
process is commonly observed for synthetic Fe(III) porphyrins.
The first step of this reaction involves nucleophilic attack of
OH on the nitrosyl ligand coordinated to the iron center, as
presented in reaction(13) (33,60):
½ðÞporphFeN¼OþOH !½ðÞporphFeN¼O 2 H!
½ðÞporphFeþHONOð 13 ÞThe reversible reaction of NO with water-soluble iron(III)
porphyrins proceeds via a dissociative mechanism (41,61).
Kinetic and mechanistic studies on the reductive nitrosylation
as a function of pH, revealed a significant decrease in the rate
of NO binding to [(porph)FeIII(OH)] as compared to that of
(porph)FeIII(H 2 O) 2 . On the other hand, trace impurities
can also interact with a catalytically active species and therefore
accelerate or decelerate the reaction, or even change its mecha-
nism ( 64 ). Electron-donating substituents on the porphyrin slow
down the reductive nitrosylation process, whereas electron-
withdrawing substituents accelerate the reaction, suggesting
that they induce the formation of [(porph)FeII(H 2 O)(NOþ)] and
increase the electrophilicity of coordinated NOþ( 65 ). The nature
and charge of substituents in the porphyrin periphery affect the
dynamics of both the binding and release of NO ( 66 ). Ferric
porphyrins are able to form nitrosyl complexes by rapid trapping
of nitroxyl anions (NO)( 67 ).
The ferriheme protein metmyoglobin (metMb) at the physiolog-
ical pH 7.4 was reported to bind the NO molecule reversibly
yielding the nitrosyl adduct [metMb(NO)]; the kinetics of the
association and dissociation processes were investigated and a
limiting dissociation mechanism was proposed(58,68). 2-His-1-
Glu nonheme iron center engineered into myoglobin was
reported capable to bind Fe(II) and reduce NO to N 2 O( 69 ).
The reaction of NO with the ferric(met) nitrite derivative of
human adult hemoglobin Hb was investigated using thin films
METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 307