1,2-H Atom Rearrangements in Benzyloxyl Radicals

Intramolecular H-Atom Transfers in Alkoxyl Radical Intermediates Underlie the Apparent Oxidation of Lipid Hydroperoxides by Fe(II)

The one-electron reduction of lipid hydroperoxides by low-valent iron species is believed to be a driver of cellular lipid peroxidation and associated ferroptotic cell death. We investigated reactions of cholesterol 7α-OOH, the primary cholesterol autoxidation product, with Fe2+ to find that 7-ketocholesterol (7-KC, an oxidation product) is the major product under these (reducing) conditions. Mechanistic studies reveal the intervention of a 1,2-H-atom shift upon formation of the 7-alkoxyl radical to yield a ketyl radical that can be oxidized by either Fe3+ or O2 to give 7-KC, the most abundant oxysterol in vivo. We also investigated the corresponding reduction of the isomeric cholesterol 5α-OOH and again found that an oxidation product (5-hydroxycholesten-3-one) predominates under reducing conditions. An intramolecular H-atom shift (this time 1,4-) in the initially formed 5-alkoxyl radical is suggested to yield a ketyl radical that is oxidized to give the observed product. It would appear that a 1,2-H shift also accounts for the predominance of ketones over alcohols when unsaturated fatty acid hydroperoxides are exposed to iron-based reductants, which had previously been reported with hematin and demonstrated here with Fe2+. The predominance of 7-KC over the corresponding alcohol is maintained when cholesterol 7α-OOH embedded in phospholipid liposomes is treated with Fe2+ or when ferroptosis is induced in mouse embryonic fibroblasts. Our observation that 7-KC accumulates in ferroptotic cells suggests that it may be a good biomarker for ferroptosis.

Structure and Reactivity of Alloxan Monohydrate in the Liquid Phase

Alloxan is an important toxic glucose analogue used to induce diabetes in lab test animals. Once regarded as a "problem structure," the condensed-phase structure of anhydrous alloxan has largely been settled, but literature inconsistencies remain for the structure of the typically employed reagent alloxan monohydrate. Due to the criticality of structure-function relationships, we have used ¹H/¹³C{¹H} NMR, IR spectroscopy, as well as quantum mechanical (QM) calculations to probe the liquid-phase structure and reactivity of alloxan monohydrate. In protic solvents (D₂O and acetic acid-d₄), hydration at the C5 carbonyl of alloxan monohydrate occurs quantitatively to form the C5 gem-diol (5,5'-dihydroxybarbituric acid). In the aprotic solvent dimethyl sulfoxide (DMSO)-d₆, there exists a mixture of the C5 gem-diol and planar tetraketo form of alloxan monohydrate. QM calculations explain the solvent-dependent hydration reactivity, where a solvent-assisted H-atom transfer mechanism lowers the activation energy of water addition at the C5 carbonyl by ∼16 or 27 kcal/mol in water or acetic acid, respectively, compared to the unassisted hydration reaction. Prompt recrystallization of alloxan monohydrate from boiling water does not alter the structure of the reagent. These findings probe the exact structure of alloxan monohydrate to guide future research efforts in biological sciences and in organic synthesis.

Characterization of Photodegradation Products of Bepotastine Besilate and In Silico Evaluation of Their Physicochemical, Absorption, Distribution, Metabolism, Excretion and Toxicity Properties

Bepotastine (BPT) is a H₁-receptor antagonist. It is used as a besilate salt in ophthalmic solution for allergic conjunctivitis and orally for the treatment of allergic rhinitis and urticaria/pruritus. Its systematic forced degradation study is unreported. The same was carried out in different conditions prescribed by International Conference on Harmonisation. The stressed solutions were subjected to reversed phase liquid chromatographic analysis, and BPT was observed to be labile under photobasic condition only, yielding 5 photodegradation products. The structures of the latter were elucidated from data generated by liquid chromatography-high-resolution mass spectrometry and multistage mass spectrometry. Of the 5, 4 products were further isolated and subjected to nuclear magnetic resonance spectroscopy to justify the proposed structures. Two of them, with similar accurate mass, were additionally and unambiguously characterized from their heteronuclear multiple bond correlation data, hydrogen deuterium exchange mass data, and quantum chemical analysis using density functional theory calculations. One degradation product had a structure that could only be explained by unusual rearrangement involving conversions of N-oxide into hydroxylamine, similar to Meisenheimer rearrangement. The physicochemical, as well as absorption, distribution, metabolism, excretion, and toxicity properties of BPT and its characterized photodegradation products were evaluated in silico by ADMET Predictor™ software.

Radical-Pair Formation in Hydrocarbon (Aut)Oxidation

The reaction profiles for the uni- and bimolecular decomposition of benzyl hydroperoxide have been studied in the context of initiation reactions for the (aut)oxidation of hydrocarbons. The unimolecular dissociation of benzyl hydroperoxide was found to proceed through the formation of a hydrogen-bonded radical-pair minimum located +181 kJ mol^-1 above the hydroperoxide substrate and around 15 kJ mol^-1 below the separated radical products. The reaction of toluene with benzyl hydroperoxide proceeds such that O-O bond homolysis is coupled with a C-H bond abstraction event in a single kinetic step. The enthalpic barrier of this molecule-induced radical formation (MIRF) process is significantly lower than that of the unimolecular O-O bond cleavage. The same type of reaction is also possible in the self-reaction between two benzyl hydroperoxide molecules forming benzyloxyl and hydroxyl radical pairs along with benzaldehyde and water as co-products. In the product complexes formed in these MIRF reactions, both radicals connect to a centrally placed water molecule through hydrogen-bonding interactions.