Hydrogen atom abstraction

About: Hydrogen atom abstraction is a research topic. Over the lifetime, 7059 publications have been published within this topic receiving 151781 citations.

Mechanistic Modeling of Polymer Degradation: A Comprehensive Study of Polystyrene

Abstract: The degradation of polystyrene was modeled at the mechanistic level by developing differential equations describing the evolution of the moments of structurally distinct polymer species. This work extends our previous modeling work1 by incorporating chain-length-dependent rate parameters, tracking branched species more explicitly, using rate parameters primarily from the literature, and comparing the model results to extensive experimental data on the degradation of polymers of different molecular weights and at different temperatures. Unique polymer groups were devised that allowed the necessary polymeric features for capturing the degradation chemistry to be tracked, while maintaining a manageable model size. The conversion among the species was described using typical free radical reaction types, including hydrogen abstraction, midchain β-scission, end-chain β-scission, 1,5-hydrogen transfer, 1,3-hydrogen transfer, radical addition, bond fission, radical recombination, and disproportionation. The model...

Pyruvate Formate-Lyase Activating Enzyme Is an Iron−Sulfur Protein

Abstract: Only a few enzymes are known to utilize stable protein-based radicals as part of their catalytic cycles.1 Among these are both the aerobic and anaerobic ribonucleotide reductases and pyruvate formate-lyase from Escherichia coli. Aerobic and anaerobic ribonucleotide reductases utilize tyrosyl and glycyl radicals, respectively, to generate a transient active site thiyl radical that effects hydrogen atom abstraction in the initial step of ribonucleotide reduction.2 Pyruvate formate-lyase (PFL) has been shown to require a stable glycyl radical to effect the rearrangement of pyruvate to formate.3 The mechanism of generation of these catalytically essential radicals is of considerable interest, but has only been elucidated in any detail for aerobic ribonucleotide reductase. For PFL, an iron-dependent activating enzyme (PFL-AE) is required for activation under reducing conditions in the presence of DTT, via an (S)-adenosylmethionine-dependent hydrogen atom abstraction, to generate a glycyl radical.4,5 However, the nature of the iron center in PFL-AE has until now remained elusive. We report here the first evidence for the presence of an iron-sulfur cluster in PFLAE. When a combination of absorption, variable temperature magnetic circular dichroism (VTMCD), EPR, and resonance Raman (RR) spectroscopies is used, anaerobically prepared PFLAE is shown to contain a mixture of diamagnetic [2Fe-2S]2+ and [4Fe-4S]2+ clusters. Since only [4Fe-4S]2+ clusters remain in dithionite-reduced samples and (S)-adenosylmethionine (SAM) is required to effect reduction to the [4Fe-4S]+ state, these results are interpreted in terms of a subunit-bridging [4Fe-4S]2+,+ cluster in active PFL-AE and suggest that this cluster is involved with generating the 5′-deoxyadenosyl radical from SAM. Purified PFL-AE6 has a distinct red-brown color and a UVvis absorption spectrum consistent with the presence of an Fe-S cluster (Figure 1a). Analysis of four distinct preparations indicated 1.5 ( 0.1 mol of iron and 1.7 ( 0.2 mol of acidlabile sulfide per mole of enzyme monomer.7 The specific activity of purified PFL-AE containing 1.5 mol of iron per mole of enzyme was 3800 U/mg, compared to 540 U/mg for enzyme with a low cluster content (<0.2 mol of iron per mole of enzyme), indicating a direct correlation between cluster content and enzyme activity.8,9 In common with most biological Fe-S centers, reduction with dithionite results in partial bleaching of the visible absorption. However, in contrast to all known types of Fe-S centers which are paramagnetic in at least one oxidation state, neither the as-purified or dithionite-reduced samples contain a paramagnetic Fe-S cluster, as evidenced by parallel and perpendicular X-band EPR and VTMCD studies over the temperature range of 4-50 K.10 The identity of the diamagnetic Fe-S clusters in these samples was revealed by RR studies in the Fe-S stretching region (Figure 1b). Although the signal-to-noise ratio is poor due to high background fluorescence, the RR spectrum of

Dioxygen: What Makes This Triplet Diradical Kinetically Persistent?

Abstract: Experimental heats of formation and enthalpies obtained from G4 calculations both find that the resonance stabilization of the two unpaired electrons in triplet O2, relative to the unpaired electrons in two hydroxyl radicals, amounts to 100 kcal/mol. The origin of this huge stabilization energy is described within the contexts of both molecular orbital (MO) and valence-bond (VB) theory. Although O2 is a triplet diradical, the thermodynamic unfavorability of both its hydrogen atom abstraction and oligomerization reactions can be attributed to its very large resonance stabilization energy. The unreactivity of O2 toward both these modes of self-destruction maintains its abundance in the ecosphere and thus its availability to support aerobic life. However, despite the resonance stabilization of the π system of triplet O2, the weakness of the O–O σ bond makes reactions of O2, which eventually lead to cleavage of this bond, very favorable thermodynamically.

Unusual metal-carbon-hydrogen angles, carbon-hydrogen bond activation, and α-hydrogen abstraction in transition-metal carbene complexes

Abstract: Alkylidene complexes of electron-deficient transition metals display an interesting structural deformation in which the carbine appears to pivot in place while the C α -H bond weakens. A molecular orbital analysis of these carbine complexes traces the deformation to an intramolecular electrophilic interaction of acceptor orbitals of the metal with the carbine lone pair. Bulky substituents on the metal and carbine protect the metal center from intermolecular reactions and control the extent of carbine pivoting. While a secondary interaction weakens the C-H α bond and attracts the a-hydrogen to the metal, full transfer of hydride to the metal is a forbidden reaction, at least for a five-coordinate, 14-electron complex. The metal-hydrogen bonding interaction guides the hydride to a neighbouring alkyl group, facilitating an α-elimination mode characteristic of the reactions of these compounds. The complexed carbene centers are unusually electron-rich, nucleophilic, by comparison with 18-electron d 6 stabilized carbene complexes. This is a consequence of an extremely effective Ta-C overlap.