From Metal–Organic Framework to Intrinsically Fluorescent Carbon Nanodots

Highly photoluminescent carbon nanodots (CNDs) were synthesized for the first time from metal–organic framework (MOF, ZIF‐8) nanoparticles. Coupled with fluorescence and non‐toxic characteristics, these carbon nanodots could potentially be used in biosafe color patterning.     

Synthesis, photophysical properties and sugar-dependent in vitro photocytotoxicity of pyrrolidine-fused chlorins bearing S-glycosides

Eight S-glycosylated 5,10,15,20-tetrakis(tetrafluorophenyl)porphyrins (1a′, 1b′, 1a and 1b (a: S-glucosylated, b: S-galactosylated)) and their 1,3-dipolar cycloadducts, i.e. chlorins 2a′, 2b′, 2a and 2b were prepared by nucleophilic substitution of the pentafluorophenyl groups with S-glycoside. These photosensitizers were characterized by ¹H, ¹³C and ¹⁹F NMR spectroscopies and elemental analysis. The photocytotoxicity of the S-glycosylated photosensitizers and the parent porphyrin (1) and chlorin (2) was examined in HeLa cells. Photosensitizers 1, 2, 1a′, 1b′, 2a′ and 2b′ showed no significant photocytotoxicity at the concentration of 0.5 μM, while the deprotected photosensitizers 1a, 1b, 2a and 2b were photocytotoxic. The strong inhibition by sodium azide of the photocytotoxicity of these photosensitizers suggested that ¹O₂ is the main mediator. The S-glucosylated photosensitizers 1a and 2a showed higher photocytotoxicity than S-galactosylated 1b and 2b, respectively. The cellular uptake of 1a and 2a increased up to 24 h, while that of 1b and 2b was saturated by 12 h.     

Specific crystal structure of poly(3-hydroxybutyrate) thin films studied by infrared reflection–absorption spectroscopy

Infrared reflection–absorption (IR-RAS) and transmission spectra were measured for poly(3-hydroxybutyrate) (PHB) thin films to explore its specific crystal structure in the surface region. As IR-RAS is sensitive to the vibration mode of perpendicular orientation of the surface, differences between IR-RAS and transmission spectra indicate an orientation of the lamella structure in the surface of PHB thin films. The relative intensity of the crystalline CO stretching band in the IR-RAS spectrum is significantly weaker than that in the transmission spectrum. It may be concluded that the transient dipole moment of the CO stretching mode of the crystalline state is not oriented perpendicular but nearly parallel to the substrate surface. On the other hand, the relative intensity of the band at 3009 cm⁻¹ due to the C–H stretching mode of the C–HOC hydrogen bonding is similar between the IR-RAS and transmission spectra, suggesting that the C–H bond is oriented neither perpendicular nor parallel to the substrate surface but in an intermediate direction. Since the CO group of the C–HOC hydrogen bonding is oriented nearly parallel to the surface and its C–H group is in the intermediate direction, it is very likely that the C–HOC hydrogen bonding has a somewhat bent structure. These results are in good agreement with our previous conclusion that the C–HOC hydrogen bonding of PHB exists along the a-axis (not the b-axis) between the CH₃ group of one helix and the CO group of another helix.     

Adsorption and photochemical behaviors of the novel cationic xanthene derivative on the clay surface

Novel tetra-cationic xanthene derivative (Flu) was synthesized. Its adsorption and photochemical behaviors on the clay surface were investigated. Fluorescence quantum yield (?_f) and fluorescence lifetime were 0.50 and 2.9 ns for Flu/clay complex. ?_f of Flu was enough high (>0.1) even at high density conditions (0.080 molecules nm⁻²). It is supposed that the strong interaction between clay and Flu by the ‘Size-Matching Effect’ realizes the highly emissive clay complexes at high density adsorption condition by a suppression of a molecular aggregation, which tends to decrease the photochemical activity.     

Synthetic Studies on Sialoglycoconjugates 40: Stereocontrolled Synthesis of Sialyl Lewis X Epitope and Its Ceramide Derivative

Abstract

Stereocontrolled synthesis of sialyl Le^x epitope and its ceramide derivative with regard to the introduction of galactose or β-D-galactosyl ceramide into the terminal N-acetylglucosamine residue of sialyl Le^x determinant is described. Königs-Knorr condensation of 2-(trimethylsilyl)ethyl 2, 4, 6-tri-O-benzyl-β-D-galactopyranoside (4) with 3, 4, 6-tri-O-acetyl-2-deoxy-2-phthalimido-D-glucopyranosyl bromide (5) gave the desired β-glycoside 6, which was converted into 2-(trimethylsilyl)ethyl O-(2-acetamido-4, 6-O-benzylidene-2-deoxy-β-D-glucopyranosyl)-(l→3)-2, 4, 6-tri-O-benzyl-β-D-galactopyranoside (8) via removal of the phthaloyl and O-acetyl groups, followed by N-acetylation and 4, 6-O-benzylidenation. Glycosylation of 8 with methyl 2, 3, 4-tri-O-benzyl-1-thio-β-L-fucopyranoside (9) gave the α-glycoside (10), which was transformed by reductive ring-opening of the benzyliderie acetal into the acceptor (11). Dimethyl(methylthio)sulfonium triflate (DMTST)-promoted coupling of 11 with methyl O-(methyl 5-acetamido-4, 7, 8, 9-tetra-O-acetyl-3, 5-dideoxy-D-glycero-α-D-galacto-2-nonulopyranosylonate)-(2→3)-2, 4, 6-tri-O-benzoyl-l-thio-β-D-galactopyra-noside (12) afforded the desired pentasaccharide (13), which was converted into the α-trichloroacetimidate 16 via reductive removal of the benzyl groups, then O-acetylation, removal of the 2-(trimethyIsilyl)ethyl group and treatment with trichloroacetonitrile. Condensation of 16 with (2S, 3R, 4E)-2-azido-3-O-benzoyl-4-octadecene-l, 3-diol (18) gave the β-glycoside 19, which was transformed into the title compound 21, via reduction of the azido group, coupling with octadecanoic acid, O-deacylation and hydrolysis of the methyl ester group. On the other hand, O-deacylation of 13 and subsequent hydrolysis of the methyl ester group gave the pentasaccharide epitope 17.     

Probing the Catalytic Activity of Reduced Graphene Oxide Decorated with Au Nanoparticles Triggered by Visible Light

Hybrid materials in which reduced graphene oxide (rGO) is decorated with Au nanoparticles (rGO–Au NPs) were obtained by the in situ reduction of GO and AuCl₄^?_(aq) by ascorbic acid. On laser excitation, rGO could be oxidized as a result of the surface plasmon resonance (SPR) excitation in the Au NPs, which generates activated O₂ through the transfer of SPR‐excited hot electrons to O₂ molecules adsorbed from air. The SPR‐mediated catalytic oxidation of p‐aminothiophenol (PATP) to p,p′‐dimercaptoazobenzene (DMAB) was then employed as a model reaction to probe the effect of rGO as a support for Au NPs on their SPR‐mediated catalytic activities. The increased conversion of PATP to DMAB relative to individual Au NPs indicated that charge‐transfer processes from rGO to Au took place and contributed to improved SPR‐mediated activity. Since the transfer of electrons from Au to adsorbed O₂ molecules is the crucial step for PATP oxidation, in addition to the SPR‐excited hot electrons of Au NPs, the transfer of electrons from rGO to Au contributed to increasing the electron density of Au above the Fermi level and thus the Au‐to‐O₂ charge‐transfer process.     

Diastereoselective syntheses and oxygenation of silyl fulleroids

The addition of silyl diazomethane (1a-d) to fullerene C₆₀ at room temperature provided the mono-adducts, the bis- and tris-adducts of silyl fulleroid (3a-d) in moderate yields. The structures of the silyl fulleroids were characterized by mass spectroscopy, as well as ¹H and ¹³C NMR. The gated ¹H NMR and ¹³C-¹H COLOC analyses of 3a-d showed a correlation between the methine proton resonances and three fullerene carbons. These observations, as well as the ¹H NMR chemical shifts of the methine protons, suggest a remarkable diastereoselectivity, with the silyl groups located above a five-membered ring. Two transition states of the thermal nitrogen-extrusion of pyrazoline intermediate (2a) were theoretically obtained, the structures of which disclosed that the diastereoselectivity is a consequence of minimization of the repulsive interaction between the silyl groups and the N₂ moiety. The bridgehead CC double bond of the silyl fulleroid is thought to be reactive by POAV analyses. The silyl fulleroids (3a,b) were found to react with singlet oxygen to afford the silyl enol ether (9a,b) via 1,3-silyl migration of a diketone (8a,b). This is the first example of ¹O₂ oxygenation of fulleroids.     

Effect of cholesterol or phospholipid incorporation on the chemical stability of the muramyldpeptide derivative B30-MDP in mixed vesicles

Application of the muramyldipeptide derivative B30-MDP to liposomal vaccines will aid in the development of improved high immunogenicity vaccines. To give full play to the effectiveness of B30-MDP as a liposomal vaccine, it is important to evaluate the effect of cholesterol, dimyristoylphosphatidylcholine (DMPC) or distearoylphosphatidylcholine (DSPC) incorporation on the chemical stability of B30-MDP and physicochemical properties of B30-MDP/lipid mixed vesicles from the view point of pharmaceutics.The observed degradation rate constants of B30-MDP by hydrolysis in B30-MDP/cholesterol mixed vesicles were increased with increasing concentration of cholesterol, however, those in B30-MDP/DMPC and B30-MDP/DSPC mixed vesicles were unchanged with increasing concentration of DMPC and DSPC. The degradation behavior of B30-MDP was then compared with physicochemical properties of B30-MDP/lipid mixed vesicles, such as membrane fluidity and particle size. It was apparent that the degradation of B30-MDP in B30-MDP/cholesterol mixed vesicles was influenced by the particle size, but not by the fluidity of the membranes. In the case of B30-MDP/phospholipid mixed vesicles, MDP/phospholipid mixed vesicles, the degradation of B30-MDP was not influenced by either the membranes' fluidity or the particle size of the mixed vesicles.It is considered that the degradation of B30-MDP in the mixed vesicles is dependent on the membrane state, and the addition of cholesterol to B30-MDP vesicle inhibits the mutual interaction of MDP regions, whereas the addition of phospholipids hardly influences the mutual interaction of MDP regions, possibly owing to phase separation between B30-MDP and phospholipids.