Chemistry Reference
In-Depth Information
intermediate, and the proposed structure consists of a sulfur donor bound to
the iron center of an N4Py complex. Reaction (6.100) followed a second-order
process. The final step was the second-order decay of the green intermediate,
homolysis of the Fe
III
-SG bond, and dimerization of the two GS
•
radicals to
result in glutathione disulfide (GSSG) (Eq. 6.101):
[
Fe O N Py
IV
(
)(
4
)]
2
+
+
GSH
→
[
Fe OH N Py
III
(
)(
4
)]
2
+
+
1 2
/ GSSG
(6.99)
III
2
+
III
2
+
[
Fe OH N Py
(
)(
4
)]
+
GSH
→
[
Fe
(
SG N Py
)(
4
)]
+
H O
k
=
14 3
.
/s
(6.100)
/M
2
110
2
+ → + =
k
8/M/s.
(6.101)
Recently, the ferryl-peptide conjugate was synthesized using a ligand-
dipeptide [351]. This conjugate was stable for more than 1 hour at room tem-
perature. The decay of the ferryl-peptide conjugate, at a later time, was first
order. The role of the functional group in the conjugated ferryl-peptide was
also determined by the synthesis of several ester derivatives of the ferryl-
peptide conjugate. Ester derivatives decayed at different rates, suggesting the
role of a remote ester group to control the stability of the ferryl species. The
mechanism of decomposition followed a hydrogen atom transfer pathway,
which was supported by the KIE value of 4.5 and a slope (ρ) of −1.3 in the
Hammett plot [351]. More recently, the study of oxidative inactivation of
serine proteases trypsin and chymotrypsin by [Fe
IV
(O)(N4Py)]
2+
and [Fe
IV
(O)
(3CG-N4Py)]
2+
(3CG-3-carbonguanidinium) has been performed [352]. The
side chains residues those were involved in oxidation were Cys, Tyr, and Trp.
The [Fe
IV
(O)(3CG-N4Py)]
2+
was more effective in inactivating chymotrypsin
than [Fe
IV
(O)(N4Py)]
2+
. A separate inactivating experiment using [Fe
II
(OH
2
)
(N4Py)]
2+
or [Fe
II
(Cl)(3CG-N4Py)]
2+
showed a role of ferryl species in inactia-
vting enzymes [352].
Recently, the role of metal ions in the oxidation carried out by a nonheme
oxoiron(IV) complex, [(TMC)Fe
IV
(O)]
2+
(TMC = 1,4,8,11-tetramethyl-1,4,8,11-
tetraazacyclotetradecane), has been studied [353, 354]. The X-ray crystal struc-
ture characterization on the binding of the redox-inactive metal ion, Sc
3+
to
[(TMC)Fe
IV
(O)]
2+
showed a structural distortion of the oxoiron(IV) moiety
due to an oxo-Sc
3+
interaction. This Lewis metal ion binding to the oxo
atom can significantly alter the electron transfer behavior of a nonheme
oxoiron(IV) complex [355]. For example, ferrocene reduces [(TMC)Fe
IV
(O)]
2+
via a two-electron process in the presence of metal ions. Comparatively,
only a one-electron reduction of [(TMC)Fe
IV
(O)]
2+
by ferrocene occurs without
the metal ions [206]. Thus, the binding of a positively charged metal ion to the
oxo group of the nonheme oxoiron(IV) moiety facilitates further reduction.
The results imply a role of such interactions in the oxygen-evolving center
exists in the protein complex PSII [355]. The role of Sc
3+
has also been studied
in the sulfoxidation of thioanisoles by [Fe
IV
(O)(N4Py)]
2+
[14]. The addition to
Sc
3+
enhanced the rate of sulfoxidation up to 10
2
-fold and also switched the
[
Fe
III
(
SG N Py
)(
4
)]
2
+
H O
[
Fe OH N Py
III
(
)(
4
)]
2
+
GSSG
1 6
.
2
2
111
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