You-Cheng Liu

REMARKABLE ENHANCEMENT OF ANTIOXIDANT ACTIVITY OF VITAMIN C IN AN ARTIFICIAL BILAYER BY MAKING IT LIPO-SOLUBLE

The reaction kinetics between 4-palmitoyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO-16) and vitamin C (VC) and its lipophilic derivative ascorbyl-6-palmitate (VC-16) was studied by stopped-flow ESR spectroscopy in a synthetic surfactant vesicle sodium 1-pentadecyl hexadecyl sulfate (SPHS). TEMPO-16 reacted with VC very slowly and showed a biphasic first-order kinetics with rate constants of 9.6 x 10(-4) and 2.5 x 10(-4) s-1, corresponding to diffusion of TEMPO-16 from the external monolayer of SPHS to the bulk water and flip-flop of TEMPO-16 from the internal to external monolayer of the vesicle, respectively. On the other hand, the reaction of TEMPO-16 with VC-16 was second-order and over three orders of magnitude faster than that with VC, presumably due to VC-16 induced fusion of the vesicle.

Stopped-flow ESR study on the reactivity of vitamin E, vitamin C and its lipophilic derivatives towards Fremy's salt in micellar systems

The reaction between Fremys salt and alpha-tocopherol (VE), ascorbic acid (VC) and its lipophilic derivatives ascorbyl-6-caprylate (VC-8), 6-laurate (VC-12) and 6-palmitate (VC-16) were studied by stopped-flow ESR spectroscopy in cetyl trimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) micelles, as a model reaction of these antioxidants with alkyl peroxy radicals in biological systems. The second order rate constants for the reaction of Fremys salt with VE in CTAB and SDS micelles were found to be 7.9 x 10(3) and 2.2 M-1 s-1, respectively, with as high as a 3600-fold variation. Rate constants for VC, VC-8, VC-12 and VC-16 are 4.3, 35, 53 and 56 x 10(3) M-1 s-1 and 3.3, 2.7, 1.2 and 0.86 x 10(3) M-1 s-1 in CTAB and SDS micelles, respectively. The results demonstrate remarkable effects of the charge type of the micelles and the side-chain of the antioxidants on the antioxidation reactivity in the micelles. It reveals that the inter-micellar diffusion may be the rate-limiting step for antioxidation carried out in micelles.

BNAH-mediated free radical addition to aromatic imines

Benzyl radical is added regioselectively to the carbon–nitrogen double bond of aromatic imines by UV irradiation (λ>300 nm) of a ternary mixture of benzyl bromide/aromatic imines/1-benzyl-1,4-dihydronicotinamide (BNAH) in acetonitrile via a BNAH-mediated photoinduced electron transfer mechanism.

Novel photoinduced aromatization of Hantzsch 1,4-dihydropyridines

4-Alkyl- and/or aryl-1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylates (Hantzsch 1,4-dihydropyridines) are quantitatively oxidized to the corresponding pyridine derivatives by irradiation in CCl4via a photoinduced electron transfer mechanism.

Kinetic studies on the single electron transfer reaction between 2,2,6,6-tetramethylpiperidine oxoammonium ions and phenothiazines: the application of Marcus theory

Electron transfer reactions take place readily between 2, 2, 6, 6-tetramethylpiperidine oxoammonium ions (la,lb) and phenothiazines (2a—2g), giving corresponding nitroxides (3a,3b) and phenothiazine radical cations (4a—4g). The rate constants for the electron self-exchange reactions between1 and3, as well as between2 and4, are determined by EPR and1HNMR line-broadening effect in acetonitrile. By application of the Marcus theory, the kinetics of the cross exchange reactions between1 and2 is studied.

The competition between δ-hydrogen abstraction and cage effects in the photochemistry of o-methyl dibenzyl ketone in various environments

Abstract The photolysis of o -methyl dibenzyl ketone ( o -MeDBK) proceeds by two primary photochemical processes: α-hydrogen abstraction leading to a 1,5-biradical which cyclizes to an indanol and α-cleavage leading to a radical pair which, after decarbonylation, forms diphenylethane coupling products. The product distribution (in the earths field and in a magnetic field of 2 kG) in the photolysis of o -MeDBK depends on the environmental conditions of the photolysis (homogeneous solutions, micellar solutions, cyclodextrin solutions and cyclodextrim solid complexes). In the presence of an external magnetic field, the recombination efficiency of the radical pair formed by α-cleavage is suppressed in micelles relative to δ-hydrogen abstraction and cyclization of the 1,5-biradical. In homogeneous solvents, aqueous solutions of cyclodextrin complexes or solid cyclodextrin complexes, there is no significant magnetic field effect on the product ratios. These results are interpreted in terms of a mechanism in which δ-hydrogen abstraction occurs from the singlet state of the ketone and type I α-cleavage occurs from the triplet state of the ketone, and by conformational factors which favor or disfavor the relative competition between the δ-hydrogen abstraction and α-cleavage.

14N/15N ISOTOPE EFFECT ON THE ELECTRON TRANSFER PROCESS BETWEEN PHENOTHIAZINE AND ITS RADICAL CATION

An appreciable equilibrium isotope effect has been observed for electron transfer from phenothiazine (PT) to the radical cation of its N-15 substituted analogue ([N-15]PT+.), i,e, PT+[N-15]PT+.K reversible arrow PT+.+[N-15]PT via electron paramagnetic resonance analysis of the mixed radical cations formed from mixing the [N-15]phenothiazlne radical cation hexachloroantimonate and phenothiazine in acetonitrile (K=0.77+/-0.10 at 25 degrees C), and by physical separation of the neutral phenothiazines from the radical cation salts in the equilibrium mixture (K=0.83+/-0.10 at 25 degrees C), Infrared and Raman spectra of [N-14]- and [N-15]phenothiazines and their radical cations were measured to assign the vibrational frequency shifts caused by the heavy-atom substitution and radical cation formation, which gave an estimate of the enthalpy change of 441.7 J mol(-1) for the electron transfer process. These results reveal that N-15 substitution of phenothiazine decreases appreciably the ionization potential of the molecule, making it easier to lose an electron to form the corresponding radical cation in solution

Photoinduced reductions of chalcone derivatives in the presence of amines

Abstract Photoinduced electron transfer reactions of chalcone (CH) derivatives (1a–1e) with triethylamine (TEA) gave cis- and trans-2-benzoyl-1,3,4-triphenylcyclopentanols (2), meso- and (±)-1,3,4,6-tetraphenyl-1,6-hexanediones (3, 4), as well as 5-benzoyl-1,3,4-triphenyl-1-penten-3-ol (5). All these products are derived from radical addition to a neutral CH molecule by CH anion radical (7) and CH ketyl radical formed in sequential electron transfer (from TEA to excited CH)-proton transfer (from TEA1+ to CH1−). Photoinduced reactions of 1a–1e with N,N-dimethylaniline (DMA) afforded, in addition to the hydrodimerization products 2–5, a CH-DMA addition product 16 formed by radical combination of CH ketyl-N-methyl-N-phenylaminomethyl radical pairs. The yield of 16 and the product ratio (addition-to-hydrodimerization ratio A/H) are affected by amine structures and reaction conditions. Therefore an increase in solvent polarity in the order benzene-acetonitrile-methanol and the presence of anhydrous magnesium perchlorate (special salt effect) result in decreases in the yield of 16 and in the ratio A/H by inhibiting in-cage proton transfer and promoting ion pair dissociation. An electron-withdrawing substituent at the benzene ring of DMA increases the yield of 16 and the A/H ratio by enhancing proton transfer from DMA1+ to CH1−, while a donor substituent on DMA has the reverse effect.

Triplex promoted intersystem crossing of ion–radical pairs in the photosensitized valence isomerization of quadricyclane: chemically induced dynamic nuclear polarization (CIDNP) evidence

Photosensitized valence isomerization of quadricyclane to norbornadiene by dibenzoylmethanatoboron difluoride in the presence of durene shows a CIDNP effect which is opposite in direction to that occurring in the absence of durene, demonstrating possible participation of triplexes in the durene co-sensitized reaction.

Photoinduced electron transfer via benzoquinone as evidenced by chemically induced dynamic nuclear polarization

Abstract A strong emissive chemically induced dynamic nuclear polarization signal of 1,4-benzoquinone (Q) was observed when a mixture of Q, 1,2,3,4-tetrahydro-1:4-methano-9,10-dimethoxyanthracene (D) and cyclohexylidenepropanedinitrile (A) was UV irradiated in situ in acetonitriled 3 . Also observed was the NMR line broadening of D. The results are explained by the quinone-mediated photoelectron transfer from D to A.