Ken Kokubo

Facile Synthesis of Highly Water-Soluble Fullerenes More Than Half-Covered by Hydroxyl Groups

Using a novel hydrogen peroxide heating method, we synthesized milky white, water-soluble polyhydroxylated fullerenes (fullerenols) with 36-40 hydroxyl groups (estimated average) along with 8-9 secondary bound water molecules. The fullerenols exhibited high water solubility up to 58.9 mg/mL in a neutral (pH = 7) condition. Dynamic light scattering analysis showed a high dispersion property, to give a narrow particle size distribution within 0.7-2.0 nm.

Super-highly hydroxylated fullerene derivative protects human keratinocytes from UV-induced cell injuries together with the decreases in intracellular ROS generation and DNA damages.

Polyhydroxylated fullerenes (fullerenols: C(60)(OH)(n)) are known as the major water-soluble fullerene derivatives which possess particular significance as free radical scavengers or antioxidants in biological systems. Recently, the novel polyhydroxylated fullerene (C(60) (OH)(44)·8H(2)O: SHH-F) was successfully synthesized. In the present study, we investigated the radical-scavenging effects and cytoprotective effects of three types of fullerenols (C(60)(OH)(6-12): LH-F, C(60) (OH)(32-34)·7H(2)O: HH-F, and C(60) (OH)(44)·8H(2)O: SHH-F) on UV-irradiation-induced cell injuries. HH-F and SHH-F exerted hydroxyl-radical scavenging activities as shown by DMPO-spin trap/ESR method, more markedly than LH-F. UVA or UVB irradiation-induced injuries in human skin keratinocytes HaCaT were significantly suppressed by HH-F and SHH-F, but scarcely by LF-H. The cytoprotective effects of SHH-F had a tendency to be superior to that of HH-F. And the cytoprotective effects of SHH-F against UVB-induced injuries were more effective than those of UVA. Irradiation with UVB to HaCaT cells was shown to cause rapid increases in cell-injury-associated symptoms such as intracellular oxidative stress levels, the formation of cyclobutane pyrimidine dimers and chromatin condensation, all of which were repressed by SHH-F. Thus, UVB-induced diverse harmful effects could be prevented by SHH-F, which was suggested to exert the cytoprotective effects through intracellular reactive oxygen species-scavenging in the keratinocytes.

Antioxidant Activity of Supramolecular Water-Soluble Fullerenes Evaluated by β-Carotene Bleaching Assay

We investigated the antioxidant activity of supramolecular water-soluble fullerenes, polyvinylpyrrolidone (PVP)-entrapped C60, and γ-cyclodextrin (CD)-bicapped C60, based on comparable β-carotene bleaching assay. Antioxidant activity against reactive oxygen species (ROS) generated by three different methods, (i) autoxidation of linoleic acid, (ii) hydrogen peroxide promoter, and (iii) photoirradiation, was evaluated as percent of inhibition relative to a control experiment in view of the bleaching rate constant (k obs) as well as the persistent absorbancy of β-carotene. Water-soluble fullerenes exhibit significant inhibitory effects on the oxidative discoloration of β-carotene in any system.

Kinetic Study of the Diels–Alder Reaction of Li+@C60 with Cyclohexadiene: Greatly Increased Reaction Rate by Encapsulated Li+

We studied the kinetics of the Diels-Alder reaction of Li(+)-encapsulated [60]fullerene with 1,3-cyclohexadiene and characterized the obtained product, [Li(+)@C60(C6H8)](PF6(-)). Compared with empty C60, Li(+)@C60 reacted 2400-fold faster at 303 K, a rate enhancement that corresponds to lowering the activation energy by 24.2 kJ mol(-1). The enhanced Diels-Alder reaction rate was well explained by DFT calculation at the M06-2X/6-31G(d) level of theory considering the reactant complex with dispersion corrections. The calculated activation energies for empty C60 and Li(+)@C60 (65.2 and 43.6 kJ mol(-1), respectively) agreed fairly well with the experimentally obtained values (67.4 and 44.0 kJ mol(-1), respectively). According to the calculation, the lowering of the transition state energy by Li(+) encapsulation was associated with stabilization of the reactant complex (by 14.1 kJ mol(-1)) and the [4 + 2] product (by 5.9 kJ mol(-1)) through favorable frontier molecular orbital interactions. The encapsulated Li(+) ion catalyzed the Diels-Alder reaction by lowering the LUMO of Li(+)@C60. This is the first detailed report on the kinetics of a Diels-Alder reaction catalyzed by an encapsulated Lewis acid catalyst rather than one coordinated to a heteroatom in the dienophile.

Novel polyhydroxylated fullerene suppresses intracellular oxidative stress together with repression of intracellular lipid accumulation during the differentiation of OP9 preadipocytes into adipocytes

Abstract Along with differentiation of mouse stromal preadipocytes OP9 into adipocytes, intracellular ROS, especially superoxide anion radicals detected by NBT reduction assay, were found to appreciably increase, mainly in cytoplasmic area, parallelling with increases in intracellular lipid-droplet accumulation, whereas undifferentiated OP9 cells kept lower levels of ROS and lipid-droplets. β-Carotene bleaching assay showed that super-highly hydroxylated fullerene (SHH-F; C60 (OH)44) exerted higher antioxidant ability than highly hydroxylated fullerene (HH-F; C60 (OH)32–34) or lowly hydroxylated fullerene (LH-F; C60 (OH)6–12). Differentiation-dependent lipid-droplet accumulation was suppressed by SHH-F or HH-F more efficiently than LH-F. Furthermore, SHH-F significantly repressed intracellular ROS generation accompanied by adipocyte differentiation. Thus, lipid-droplet accumulation was shown to positively correlate with ROS upon the differentiation of OP9 preadipocytes into adipocytes and SHH-F significantly suppressed intracellular ROS together with repression of intracellular lipid accumulation.

Facile and Exclusive Formation of Aziridinofullerenes by Acid-catalyzed Denitrogenation of Triazolinofullerenes

Variously substituted [6,6]closed aziridinofullerenes were exclusively obtained from acid-catalyzed denitrogenation of triazolinofullerenes without formation of relevant [5,6]open azafulleroids, which are the major products on noncatalyzed denitrogenation. The mechanistic consideration by DFT calculations suggested a reaction sequence involving initial pre-equilibrium protonation of the triazoline N(1) atom, generation of aminofullerenyl cation by nitrogen-extrusion, and final aziridination.

Synthesis of a lithium-encapsulated fullerenol and the effect of the internal lithium cation on its aggregation behavior

AbstractA lithium-encapsulated fullerenol Li@C60(OH)18, as an example of a polar solvent-soluble endohedral fullerene derivative, has been synthesized and fully characterized by infrared spectroscopy, nuclear magnetic resonance spectroscopy, UV spectroscopy, electron spin resonance (ESR) spectroscopy, matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS), elemental analysis, thermogravimetric analysis, and inductively coupled plasma-atomic emission spectroscopy (ICP-AES), and the particle size was determined using the induced grating (IG) method, and scanning probe microscopy. The encapsulated Li+ was clearly detected by 7Li NMR at very high field in the range −15 to −19 ppm, an intermediate lithium-encapsulated fullerenol was detected by MALDI-TOF-MS, and the molar ratio of lithium-encapsulated fullerenol to empty fullerenol was quantitatively determined to be 12:88 by ICP-AES. The solid-state ESR and particle size measurements using the IG method showed the characteristic anionic behavior with no external counter cations, in what can be called a “cation-encapsulated anion nanoparticle”, revealing the drastic differences between its properties and those of empty C60(OH)16.

One‐step Synthesis of Water‐soluble Fullerenols Bearing Nitrogen‐containing Substituents

We found a facile one‐step synthetic method for water‐soluble fullerenols bearing 8–16 nitrogen‐containing substituents as well as more than 16 hydroxyl groups from commercially available C60 as a starting material. The water solubility of one of the products showed the same as the highest one reported (>200 mg/mL). Other fullerenols bearing hydrogen or diethylamino group on the same fullerene cage were also synthesized by the modified method using hydrogen peroxide as a simple hydroxylation reagent.

New efficient (thio)acetalized fullerene monoadducts for organic solar cells: characterization based on solubility, mobility balance, and dark current

We report a systematic study of organic photovoltaic cells using novel spiro-acetalized and (thio)acetalized [60]fullerene monoadducts bearing five- to seven-membered rings. One of these compounds had power-conversion efficiencies of 5.8% with PTB7 and 4.0% with poly(3-hexylthiophene); the latter is comparable to that of the commonly used [6,6]-phenyl-C61-butyric acid methyl ester. We investigated the precise factor that governs the device performance by examining the solubility, space-charge-limited current mobility balance of holes and electrons, morphology, and dark current. Single-crystal X-ray diffraction highlighted the key role of compact and flexible spiro ring folding in the rim space in fullerene packing.

Systematic evaluation and mechanistic investigation of antioxidant activity of fullerenols using β -carotene bleaching assay

Antioxidant activity of hydroxylated fullerenes, so-called fullerenols, against lipid peroxyl radical was evaluated by β-carotene bleaching assay. All samples showed moderate to high antioxidant activity (%AOA), especially for C60(OH)12 (70.1) and C60(OH)44 (66.0) as compared with 8, 24, 26, and 36 hydroxylated ones (31.7-62.8). The detection of the possible products was conducted in the model reaction of both fullerenols and C60 with methyl linoleate by MALDI-TOF-MS. These results suggested that the two possible mechanisms, such as C-addition to double bonds and H-abstraction from-OH groups, are involved in the present radical scavenging reaction.