Ultrafast Excited-State Intramolecular Proton Transfer of Aloesaponarin I

Time-resolved emission of aloesaponarin I was studied with the fluorescence up-conversion and time-correlated single-photon-counting techniques. The rates of the excited-state intramolecular proton transfer, of the solvent and molecular rearrangements, and of the decay from the excited proton-transferred species were determined and interpreted in the light of time-dependent density functional calculations. These results were discussed in conjunction with UV protection and singlet-oxygen quenching activity of aloe

Kinetic Study of the Aroxyl-Radical-Scavenging Activity of Five Fatty Acid Esters and Six Carotenoids in Toluene Solution: Structure–Activity Relationship for the Hydrogen Abstraction Reaction

A kinetic study of the reaction between an aroxyl radical (ArO^•) and fatty acid esters (LHs <b>1</b>–<b>5</b>, ethyl stearate <b>1</b>, ethyl oleate <b>2</b>, ethyl linoleate <b>3</b>, ethyl linolenate <b>4</b>, and ethyl arachidonate <b>5</b>) has been undertaken. The second-order rate constants (<i>k</i>_s) for the reaction of ArO^• with LHs <b>1</b>–<b>5</b> in toluene at 25.0 °C have been determined spectrophotometrically. The <i>k</i>_s values obtained increased in the order of LH <b>1</b> < <b>2</b> < <b>3</b> < <b>4</b> < <b>5</b>, that is, with increasing the number of double bonds included in LHs <b>1</b>–<b>5</b>. The <i>k</i>_s value for LH <b>5</b> was 2.93 × 10^–3 M^–1 s^–1. From the result, it has been clarified that the reaction of ArO^• with LHs <b>1</b>–<b>5</b> was explained by an allylic hydrogen abstraction reaction. A similar kinetic study was performed for the reaction of ArO^• with six carotenoids (Car-Hs <b>1</b>–<b>6</b>, astaxanthin <b>1</b>, β-carotene <b>2</b>, lycopene <b>3</b>, capsanthin <b>4</b>, zeaxanthin <b>5</b>, and lutein <b>6</b>). The <i>k</i>_s values obtained increased in the order of Car-H <b>1</b> < <b>2</b> < <b>3</b> < <b>4</b> < <b>5</b> < <b>6</b>. The <i>k</i>_s value for Car-H <b>6</b> was 8.4 × 10^–4 M^–1 s^–1. The <i>k</i>_s values obtained for Car-Hs <b>1</b>–<b>6</b> are in the same order as that of the values for LHs <b>1</b>–<b>5</b>. The results of detailed analyses of the <i>k</i>_s values for the above reaction indicated that the reaction was also explained by an allylic hydrogen abstraction reaction. Furthermore, the structure–activity relationship for the reaction was discussed by taking the result of density functional theory calculation reported by Martinez and Barbosa into account

Correlation among Singlet-Oxygen Quenching, Free-Radical Scavenging, and Excited-State Intramolecular-Proton-Transfer Activities in Hydroxyflavones, Anthocyanidins, and 1‑Hydroxyanthraquinones

Singlet-oxygen (¹O₂) quenching, free-radical scavenging, and excited-state intramolecular proton-transfer (ESIPT) activities of hydroxyflavones, anthocyanidins, and 1-hydroxyanthraquinones were studied by means of laser, stopped-flow, and steady-state spectroscopies. In hydroxyflavones and anthocyanidins, the ¹O₂ quenching activity positively correlates to the free-radical scavenging activity. The reason for this correlation can be understood by considering that an early step of each reaction involves electron transfer from the unfused phenyl ring (B-ring), which is singly bonded to the bicyclic chromen or chromenylium moiety (A- and C-rings). Substitution of an electron-donating OH group at B-ring enhances the electron transfer leading to activation of the ¹O₂ quenching and free-radical scavenging. In 3-hydroxyflavones, the OH substitution at B-ring reduces the activity of ESIPT within C-ring, which can be explained in terms of the nodal-plane model. As a result, the ¹O₂ quenching and free-radical scavenging activities negatively correlate to the ESIPT activity. A catechol structure at B-ring is another factor that enhances the free-radical scavenging in hydroxyflavones. In contrast to these hydroxyflavones, 1-hydroxyanthraquinones having an electron-donating OH substituent adjacent to the O–H---OC moiety susceptible to ESIPT do not show a simple correlation between their ¹O₂ quenching and ESIPT activities, because the OH substitution modulates these reactions

Correlation between Excited-State Intramolecular Proton-Transfer and Singlet-Oxygen Quenching Activities in 1‑(Acylamino)anthraquinones

Excited-state intramolecular proton-transfer (ESIPT) and singlet-oxygen (¹O₂) quenching activities of intramolecularly hydrogen-bonded 1-(acylamino)­anthraquinones have been studied by means of static and laser spectroscopies. The ESIPT shows a substituent effect, which can be explained in terms of the nodal-plane model. The ESIPT activity positively and linearly correlates with their ¹O₂ quenching activity. The reason for this correlation can be understood by considering ESIPT-induced distortion of their ground-state potential surface and their encounter complex formation with ¹O₂. Intramolecularly hydrogen-bonded hydroxyanthraquinones found in aloe also show a similar positive and linear correlation, which can be understood in the same way

Notable Effects of Metal Salts on UV–Vis Absorption Spectra of α‑, β‑, γ‑, and δ‑Tocopheroxyl Radicals in Acetonitrile Solution. The Complex Formation between Tocopheroxyls and Metal Cations

The measurements of the UV–vis absorption spectra of α-, β-, γ-, and δ-tocopheroxyl (α-, β-, γ-, and δ-Toc^•) radicals were performed by reacting aroxyl (ArO^•) radical with α-, β-, γ-, and δ-tocopherol (α-, β-, γ-, and δ-TocH), respectively, in acetonitrile solution including three kinds of alkali and alkaline earth metal salts (LiClO₄, NaClO₄, and Mg­(ClO₄)₂) (MX or MX₂), using stopped-flow spectrophotometry. The maximum wavelengths (λ_max) of the absorption spectra of the α-, β-, γ-, and δ-Toc^• located at 425–428 nm without metal salts increased with increasing concentrations of metal salts (0–0.500 M) in acetonitrile and approached some constant values, suggesting (Toc^•···M⁺ (or M²⁺)) complex formations. Similarly, the values of the apparent molar extinction coefficient (ε_max) increased drastically with increasing concentrations of metal salts in acetonitrile and approached some constant values. The result suggests that the formations of Toc^• dimers were suppressed by the metal ion complex formations of Toc^• radicals. The stability constants (<i>K</i>) were determined for Li⁺, Na⁺, and Mg²⁺ complexes of α-, β-, γ-, and δ-Toc^•. The <i>K</i> values increased in the order of NaClO₄ < LiClO₄ < Mg­(ClO₄)₂, being independent of the kinds of Toc^• radicals. Furthermore, the <i>K</i> values increased in the order of δ- < γ- < β- < α-Toc^• radicals for each metal salt. The alkali and alkaline earth metal salts having a smaller ionic radius of the cation and a larger charge of the cation gave a larger shift of the λ_max value, a larger ε_max value, and a larger <i>K</i> value. The result of the DFT molecular orbital calculations indicated that the α-, β-, γ-, and δ-Toc^• radicals were stabilized by the (1:1) complex formation with metal cations (Li⁺, Na⁺, and Mg²⁺). Stabilization energy (<i>E</i>_S) due to the complex formation increased in the order of Na⁺ < Li⁺ < Mg²⁺ complexes, being independent of the kinds of Toc^• radicals. The calculated result also indicated that the metal cations coordinate to the O atom at the sixth position of α-, β-, γ-, and δ-Toc^• radicals

Site-Specific Electron-Relaxation Caused by Si:2p Core-Level Photoionization: Comparison between F₃SiCH₂CH₂Si(CH₃)₃ and Cl₃SiCH₂CH₂Si(CH₃)₃ Vapors by Means of Photoelectron Auger Electron Coincidence Spectroscopy

Site-specific electron relaxations caused by Si:2p core-level photoionizations in F₃SiCH₂CH₂Si­(CH₃)₃ and Cl₃SiCH₂CH₂Si­(CH₃)₃ vapors have been studied by means of the photoelectron Auger electron coincidence spectroscopy. F₃SiCH₂CH₂Si­(CH₃)₃ shows almost 100% site-specificity in fragmentation caused by the Si:2p ionization. However, substitution of Cl for F of F₃SiCH₂CH₂Si­(CH₃)₃ considerably reduces the site-specificity at the Si atom bonded to three halogen atoms, with the site-specificity at the Si site bonded to three methyl groups remaining largely unchanged. The site-specificity reduction in Cl₃SiCH₂CH₂Si­(CH₃)₃ is considered to take place during the transient period between Si:L₂₃VV Auger electron emission and the subsequent fragmentation. The reason for the reduction can be explained in terms of some differences between these two molecules in the L₂₃VV Auger decay at the Si site bonded to the three halogen atoms

Site-Specific Electron-Relaxation Caused by Si:2p Core-Level Photoionization: Comparison between F₃SiCH₂CH₂Si(CH₃)₃ and Cl₃SiCH₂CH₂Si(CH₃)₃ Vapors by Means of Photoelectron Auger Electron Coincidence Spectroscopy

Site-specific electron relaxations caused by Si:2p core-level photoionizations in F₃SiCH₂CH₂Si­(CH₃)₃ and Cl₃SiCH₂CH₂Si­(CH₃)₃ vapors have been studied by means of the photoelectron Auger electron coincidence spectroscopy. F₃SiCH₂CH₂Si­(CH₃)₃ shows almost 100% site-specificity in fragmentation caused by the Si:2p ionization. However, substitution of Cl for F of F₃SiCH₂CH₂Si­(CH₃)₃ considerably reduces the site-specificity at the Si atom bonded to three halogen atoms, with the site-specificity at the Si site bonded to three methyl groups remaining largely unchanged. The site-specificity reduction in Cl₃SiCH₂CH₂Si­(CH₃)₃ is considered to take place during the transient period between Si:L₂₃VV Auger electron emission and the subsequent fragmentation. The reason for the reduction can be explained in terms of some differences between these two molecules in the L₂₃VV Auger decay at the Si site bonded to the three halogen atoms

Development of a New Free Radical Absorption Capacity Assay Method for Antioxidants: Aroxyl Radical Absorption Capacity (ARAC)

A new free radical absorption capacity assay method is proposed with use of an aroxyl radical (2,6-di-<i>tert</i>-butyl-4-(4′-methoxyphenyl)­phenoxyl radical) and stopped-flow spectroscopy and is named the aroxyl radical absorption capacity (ARAC) assay method. The free radical absorption capacity (ARAC value) of each tocopherol was determined through measurement of the radical-scavenging rate constant in ethanol. The ARAC value could also be evaluated through measurement of the half-life of the aroxyl radical during the scavenging reaction. For the estimation of the free radical absorption capacity, the aroxyl radical was more suitable than the DPPH radical, galvinoxyl, and <i>p</i>-nitrophenyl nitronyl nitroxide. The ARAC value in tocopherols showed the same tendency as the free radical absorption capacities reported previously, and the tendency was independent of an oxygen radical participating in the scavenging reaction and of a medium surrounding the tocopherol and oxygen radical. The ARAC value can be directly connected to the free radical-scavenging rate constant, and the ARAC method has the advantage of treating a stable and isolable radical (aroxyl radical) in a user-friendly organic solvent (ethanol). The ARAC method was also successfully applied to a palm oil extract. Accordingly, the ARAC method would be useful in free radical absorption capacity assay of antioxidative reagents and foods

Development of a Singlet Oxygen Absorption Capacity (SOAC) Assay Method. Measurements of the SOAC Values for Carotenoids and α‑Tocopherol in an Aqueous Triton X‑100 Micellar Solution

Recently, a new assay method for the quantification of the singlet oxygen absorption capacity (SOAC) of antioxidants (AOs) and food extracts in homogeneous organic solvents was proposed. In this study, second-order rate constants (<i>k</i>_Q) for the reaction of singlet oxygen (¹O₂) with eight different carotenoids (Cars) and α-tocopherol (α-Toc) were measured in an aqueous Triton X-100 (5.0 wt %) micellar solution (pH 7.4, 35 °C), which was used as a simple model of biomembranes. The <i>k</i>_Q and relative SOAC values were measured using ultraviolet–visible (UV–vis) spectroscopy. The UV–vis absorption spectra of Cars and α-Toc were measured in both a micellar solution and chloroform, to investigate the effect of solvent on the <i>k</i>_Q and SOAC values. Furthermore, decay rates (<i>k</i>_d) of ¹O₂ were measured in 0.0, 1.0, 3.0, and 5.0 wt % micellar solutions (pH 7.4), using time-resolved near-infrared fluorescence spectroscopy, to determine the absolute <i>k</i>_Q values of the AOs. The results obtained demonstrate that the <i>k</i>_Q values of AOs in homogeneous and heterogeneous solutions vary notably depending on (i) the polarity [dielectric constant (ε)] of the reaction field between AOs and ¹O₂, (ii) the local concentration of AOs, and (iii) the mobility of AOs in solution. In addition, the <i>k</i>_Q and relative SOAC values obtained for the Cars in a heterogeneous micellar solution differ remarkably from those in homogeneous organic solvents. Measurements of <i>k</i>_Q and SOAC values in a micellar solution may be useful for evaluating the ¹O₂ quenching activity of AOs in biological systems

Kinetic study of the quenching reaction of singlet oxygen by α-, β-, γ-, δ-tocotrienols, and palm oil and soybean extracts in solution

<div><p>Measurements of the singlet oxygen (¹O₂) quenching rates (<i>k</i>_<i>Q</i> (<i>S</i>)) and the relative singlet oxygen absorption capacity (SOAC) values were performed for 11 antioxidants (AOs) (eight vitamin E homologues (α-, β-, γ-, and δ-tocopherols and -tocotrienols (-Tocs and -Toc-3s)), two vitamin E metabolites (α- and γ-carboxyethyl-6-hydroxychroman), and trolox) in ethanol/chloroform/D₂O (50:50:1, v/v/v) and ethanol solutions at 35 °C. Similar measurements were performed for five palm oil extracts 1–5 and one soybean extract 6, which included different concentrations of Tocs, Toc-3s, and carotenoids. Furthermore, the concentrations (wt%) of Tocs, Toc-3s, and carotenoids included in extracts 1–6 were determined. From the results, it has been clarified that the ¹O₂-quenching rates (<i>k</i>_<i>Q</i> (<i>S</i>)) (that is, the relative SOAC value) obtained for extracts 1–6 may be explained as the sum of the product {Σ <i>k</i>_<i>Q</i>^AO-<i>i</i> (<i>S</i>) [AO-<i>i</i>]/100} of the rate constant (<i>k</i>_<i>Q</i>^AO-<i>i</i> (<i>S</i>)) and the concentration ([AO-<i>i</i>]/100) of AO-<i>i</i> (Tocs, Toc-3s, and carotenoid) included.</p></div