8 research outputs found
Multimetallic Cooperativity in Uranium-Mediated CO<sub>2</sub> Activation
The metal-mediated
redox transformation of CO<sub>2</sub> in mild
conditions is an area of great current interest. The role of cooperativity
between a reduced metal center and a Lewis acid center in small-molecule
activation is increasingly recognized, but has not so far been investigated
for f-elements. Here we show that the presence of potassium at a U,
K site supported by sterically demanding tris(<i>tert</i>-butoxy)siloxide ligands induces a large cooperative effect in the
reduction of CO<sub>2</sub>. Specifically, the ion pair complex [K(18c6)][U(OSi(O<sup>t</sup>Bu)<sub>3</sub>)<sub>4</sub>], <b>1</b>, promotes the
selective reductive disproportionation of CO<sub>2</sub> to yield
CO and the mononuclear uranium(IV) carbonate complex [U(OSi(O<sup>t</sup>Bu)<sub>3</sub>)<sub>4</sub>(μ-κ<sup>2</sup>:κ<sup>1</sup>-CO<sub>3</sub>)K<sub>2</sub>(18c6)], <b>4</b>. In contrast,
the heterobimetallic complex [U(OSi(O<sup>t</sup>Bu)<sub>3</sub>)<sub>4</sub>K], <b>2</b>, promotes the potassium-assisted two-electron
reductive cleavage of CO<sub>2</sub>, yielding CO and the U(V) terminal
oxo complex [UO(OSi(O<sup>t</sup>Bu)<sub>3</sub>)<sub>4</sub>K], <b>3</b>, thus providing a remarkable example of two-electron transfer
in U(III) chemistry. DFT studies support the presence of a cooperative
effect of the two metal centers in the transformation of CO<sub>2</sub>
Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their π‑Conjugated System
Polyarylamine containing <i>meta–para–para</i>-aniline units in the main chain and <i>meta–para</i>-aniline units in the pendant chains was synthesized. The polymer
can be oxidized to radical cations in chemical or electrochemical
ways. The presence of <i>meta</i>-phenylenes in the polymer
chemical structure allows for the ferromagnetic coupling of electronic
spins, which leads to the formation of high spin states. Detailed
pulsed-EPR study indicates that the <i>S</i> = 2 spin state
was reached for the best oxidation level. Quantitative magnetization
measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> =
3/2 spin states. The efficiency of the oxidation was determined to
be 74%. To the best of our knowledge, this polymer is the first example
of a linear doped polyarylamine combining such high spin states with
high doping efficiency
Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their π‑Conjugated System
Polyarylamine containing <i>meta–para–para</i>-aniline units in the main chain and <i>meta–para</i>-aniline units in the pendant chains was synthesized. The polymer
can be oxidized to radical cations in chemical or electrochemical
ways. The presence of <i>meta</i>-phenylenes in the polymer
chemical structure allows for the ferromagnetic coupling of electronic
spins, which leads to the formation of high spin states. Detailed
pulsed-EPR study indicates that the <i>S</i> = 2 spin state
was reached for the best oxidation level. Quantitative magnetization
measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> =
3/2 spin states. The efficiency of the oxidation was determined to
be 74%. To the best of our knowledge, this polymer is the first example
of a linear doped polyarylamine combining such high spin states with
high doping efficiency
Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their π‑Conjugated System
Polyarylamine containing <i>meta–para–para</i>-aniline units in the main chain and <i>meta–para</i>-aniline units in the pendant chains was synthesized. The polymer
can be oxidized to radical cations in chemical or electrochemical
ways. The presence of <i>meta</i>-phenylenes in the polymer
chemical structure allows for the ferromagnetic coupling of electronic
spins, which leads to the formation of high spin states. Detailed
pulsed-EPR study indicates that the <i>S</i> = 2 spin state
was reached for the best oxidation level. Quantitative magnetization
measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> =
3/2 spin states. The efficiency of the oxidation was determined to
be 74%. To the best of our knowledge, this polymer is the first example
of a linear doped polyarylamine combining such high spin states with
high doping efficiency
Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their π‑Conjugated System
Polyarylamine containing <i>meta–para–para</i>-aniline units in the main chain and <i>meta–para</i>-aniline units in the pendant chains was synthesized. The polymer
can be oxidized to radical cations in chemical or electrochemical
ways. The presence of <i>meta</i>-phenylenes in the polymer
chemical structure allows for the ferromagnetic coupling of electronic
spins, which leads to the formation of high spin states. Detailed
pulsed-EPR study indicates that the <i>S</i> = 2 spin state
was reached for the best oxidation level. Quantitative magnetization
measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> =
3/2 spin states. The efficiency of the oxidation was determined to
be 74%. To the best of our knowledge, this polymer is the first example
of a linear doped polyarylamine combining such high spin states with
high doping efficiency
Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their π‑Conjugated System
Polyarylamine containing <i>meta–para–para</i>-aniline units in the main chain and <i>meta–para</i>-aniline units in the pendant chains was synthesized. The polymer
can be oxidized to radical cations in chemical or electrochemical
ways. The presence of <i>meta</i>-phenylenes in the polymer
chemical structure allows for the ferromagnetic coupling of electronic
spins, which leads to the formation of high spin states. Detailed
pulsed-EPR study indicates that the <i>S</i> = 2 spin state
was reached for the best oxidation level. Quantitative magnetization
measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> =
3/2 spin states. The efficiency of the oxidation was determined to
be 74%. To the best of our knowledge, this polymer is the first example
of a linear doped polyarylamine combining such high spin states with
high doping efficiency
Mammalian Frataxin Controls Sulfur Production and Iron Entry during de Novo Fe<sub>4</sub>S<sub>4</sub> Cluster Assembly
Iron–sulfur (Fe–S) cluster-containing proteins
are
essential components of cells. In eukaryotes, Fe–S clusters
are synthesized by the mitochondrial iron–sulfur cluster (ISC)
machinery and the cytosolic iron–sulfur assembly (CIA) system.
In the mammalian ISC machinery, preassembly of the Fe–S cluster
on the scaffold protein (ISCU) involves a cysteine desulfurase complex
(NFS1/ISD11) and frataxin (FXN), the protein deficient in Friedreich’s
ataxia. Here, by comparing the biochemical and spectroscopic properties
of quaternary (ISCU/NFS1/ISD11/FXN) and ternary (ISCU/NFS1/ISD11)
complexes, we show that FXN stabilizes the quaternary complex and
controls iron entry to the complex through activation of cysteine
desulfurization. Furthermore, we show for the first time that in the
presence of iron and l-cysteine, an [Fe<sub>4</sub>S<sub>4</sub>] cluster is formed within the quaternary complex that can
be transferred to mammalian aconitase (mACO2) to generate an active
enzyme. In the absence of FXN, although the ternary complex can assemble
an Fe–S cluster, the cluster is inefficiently transferred to
ACO2. Taken together, these data help to unravel further the Fe–S
cluster assembly process and the molecular basis of Friedreich’s
ataxia
Radical <i>S</i>‑Adenosyl‑l‑Methionine Enzyme PylB: A C‑Centered Radical to Convert l‑Lysine into (3<i>R</i>)‑3-Methyl‑d‑Ornithine
PylB is a radical S-adenosyl-l-methionine
(SAM) enzyme predicted to convert l-lysine into (3R)-3-methyl-d-ornithine, a precursor in the biosynthesis
of the 22nd proteogenic amino acid pyrrolysine. This protein highly
resembles that of the radical SAM tyrosine and tryptophan lyases,
which activate their substrate by abstracting a H atom from the amino-nitrogen
position. Here, combining in vitro assays, analytical
methods, electron paramagnetic resonance spectroscopy, and theoretical
methods, we demonstrated that instead, PylB activates its substrate
by abstracting a H atom from the Cγ position of l-lysine
to afford the radical-based β-scission. Strikingly, we also
showed that PylB catalyzes the reverse reaction, converting (3R)-3-methyl-d-ornithine into l-lysine
and using catalytic amounts of the 5′-deoxyadenosyl radical.
Finally, we identified significant in vitro production
of 5′-thioadenosine, an unexpected shunt product that we propose
to result from the quenching of the 5′-deoxyadenosyl radical
species by the nearby [Fe4S4] cluster