posted on 2018-12-21, 20:19authored byChristine
E. Schiewer, Christina S. Müller, Sebastian Dechert, Marie Bergner, Juliusz A. Wolny, Volker Schünemann, Franc Meyer
The nitrosylation of biological Fe/S
clusters to give protein-bound
dinitrosyl iron complexes (DNICs) is physiologically important. Biomimetic
studies on the reaction of synthetic [2Fe–2S] clusters with
NO have so far been limited to diferric model complexes. This work
now compares the nitrosylation of [2Fe–2S] clusters with SN-
or NN-chelating benzimidazolate/thiophenolate or bis(benzimidazolate)
capping ligands in their diferric (12– and 22–) and mixed-valent
(FeIIFeIII, 13–, and 23–) forms.
Furthermore, the effect of protonation of the imidazole part of the
SN ligand has been probed on both the nitrosylation reaction and properties
of the resulting DNIC. The reaction of 12– and 22– with 4
equiv NO yields the new anionic {Fe(NO)2}9 DNICs 3– and 4–, respectively, which have been comprehensively
characterized, including X-ray crystallography of their PPN+ salts. Nitrosylation of mixed-valent [2Fe–2S] clusters 13– and 23– first leads to slow oxidation to the
corresponding diferric congeners, followed by core degradation and
DNIC formation. In the case of 23–, a second diferric intermediate very similar to 22– is detected by UV–vis spectroscopy,
but could not be further identified. Nitrosylation of 1H2 gives the neutral, N-protonated DNIC 3H, and acid/base titrations show that interconversion between 3– and 3H is reversible.
Peripheral ligand protonation leads to a blue shift of the NO stretching
vibrations by about 23 cm–1 and a significant shift
of the reduction potential to less negative values (ΔE1/2 = 0.26 V), but no effect on 57Fe Mössbauer parameters is observed. Density functional theory
calculations based on the structure of 3– indicate that the electronic ground-state properties of 3– and 3H are
similar, although the NO(π*) → Fe 3d π-donation
is slightly increased and π-backbonding is slightly decreased
upon protonation. As a result, protonation has a significant effect
on the NO stretching frequencies, but only minor effects on the Fe–(NO)2 modes. This is confirmed by nuclear inelastic scattering
of 3– and 3H, which shows no clear influence of protonation on the energy of
the Fe–(NO)2 bending and stretching modes occurring
in the range 400–600 cm–1, but characteristic
changes below 350 cm–1 that reflect perturbation
of free rotary motion of the thiophenolate and benzimidazole ring
systems of the capping ligand after N-protonation. These findings
add to the understanding of [2Fe–2S] cluster nitrosylation
and will help to identify DNICs resulting from the reaction of NO
with Fe/S cofactors featuring alternative, proton-responsive histidine
ligands such as the Rieske and mitoNEET [2Fe–2S] clusters.