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Oxidation of 4-Substituted TEMPO Derivatives Reveals Modifications at the 1- and 4-Positions

Abstract (Expand)

Potenital pathways for the deactivation of hindered amine light stabilisers (HALS) have been investigated by observing reactions of model compounds—based on 4-substituted derivatives of … 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)—with hydroxyl radicals. In these reactions, dilute aqueous suspensions of photocatalytic nanoparticulate titanium dioxide were irradiated with UV light in the presence of water-soluble TEMPO derivatives. Electron spin resonance (ESR) and electrospray ionisation mass-spectrometry (ESI-MS) data were acquired to provide complementary structural elucidation of the odd- and even-electron products of these reactions and both techniques show evidence for the formation of 4-oxo-TEMPO (TEMPONE). TEMPONE formation from the 4-substituted TEMPO compounds is proposed to be initiated by hydrogen abstraction at the 4-position by hydroxyl radical. High-level ab initio calculations reveal a thermodynamic preference for abstraction of this hydrogen but computed activation barriers indicate that, although viable, it is less favoured than hydrogen abstraction from elsewhere on the TEMPO scaffold. If a radical is formed at the 4-position however, calculations elucidate two reaction pathways leading to TEMPONE following combination with either a second hydroxyl radical or dioxygen. An alternate mechanism for conversion of TEMPOL to TEMPONEvia an alkoxyl radical intermediate is also considered and found to be competitive with the other pathways. ESI-MS analysis also shows an increased abundance of analogous 4-substituted piperidines during the course of irradiation, suggesting competitive modification at the 1-position to produce a secondary amine. This modification is confirmed by characteristic fragmentation patterns of the ionised piperidines obtained by tandem mass spectrometry. The conclusions describe how reaction at the 4-position could be responsible for the gradual depletion of HALS in pigmented surface coatings and secondly, that modification at nitrogen to form the corresponding secondary amine species may play a greater role in the stabilisation mechanisms of HALS than previously considered.

Authors: David L. Marshall, Meganne L. Christian, Ganna Gryn'ova, Michelle L. Coote, Philip J. Barker, Stephen J. Blanksby

Date Published: 2011

Publication Type: Journal

Diffusion and association processes in biological systems: theory, computation and experiment

Abstract

Not specified

Authors: Paolo Mereghetti, Daria Kokh, J Andrew McCammon, Rebecca C Wade

Date Published: 2011

Publication Type: Journal

Sequence, Structure and Stability of Collagen and its Interaction with Collagenase.

Abstract

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Author: Sundar Raman Subramaniam

Date Published: 2011

Publication Type: Doctoral Thesis

Computational and NMR Studies of Thymidylate Synthase and its Regulation

Abstract

Not specified

Author: Divita Garg

Date Published: 2011

Publication Type: Doctoral Thesis

Structural Comparison of Pyruvate Kinases.

Abstract

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Author: Nadine Veith

Date Published: 2011

Publication Type: Master's Thesis

System Approach to the Identification of Folate-related Enzyme Inhibitors with Anti-infectious Action

Abstract

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Author: Erika Nerini

Date Published: 2011

Publication Type: Doctoral Thesis

Milnor-Wood type inequalities for Higgs bundles

Abstract (Expand)

We explain how the generalized Milnor–Wood inequality of Burger and Iozzi for reductive representations of a cocompact complex-hyperbolic lattice into a Hermitian Lie group translates under the non-abelian … Hodge correspondence into an inequality for topological invariants of the corresponding Higgs bundles. In this manner, we obtain in a uniform way a universal Milnor–Wood inequality for Higgs bundles over complex-hyperbolic manifolds of arbitrary dimensions and with arbitrary Hermitian structure group. This complements results of Biquard, Bradlow, García-Prada, Gothen, Mundet, Rubio and Chaput, Koziarz, Maubon.

Authors: Tobias Hartnick, Andreas Ott

Date Published: 2011

Publication Type: Misc

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