1,4-Addition of benzene to a dihydrocyclopent[a]indene diradical: synthesis and DFT study

Photochemical cyclization of compound 1, a homoenediyne (-CCC=CCH2CC-) bearing two ethynylanthracene chromophores, yields two isomeric dihydrocyclopent[a]indene ring systems, spiro-fused to the 9-position of a 9,10-dihydroanthracene moiety. Evidence of a photochemically initiated diradical cyclization pathway is proposed on the basis of (i) hydrogen abstraction from reaction with 1,4-cyclohexadiene (1,4-CHD) and (ii) the observation of 1,4-addition of benzene (solvent). The reaction was further analyzed by a complete density functional theory (DFT) study, using an unrestricted approach (UBLYP) with a 6-31G* basis set for the open-shell triplet states of the reactants, products, and diradical intermediates to model the photochemical nature of observed transformation. A mechanism detailing the observed cyclization/addition reaction is proposed.     

Loniceroside C,an antiinflammatory saponin from Lonicera japonica

A new triterpenoid saponin, loniceroside C was isolated from the aerial parts of Lonicera japonica. Its structure was established to be 3-O-beta-D-glucopyranosyl hederagenin 28-O-alpha-L-rhamnopyranosyl (1-->2)-[beta-D-xylopyranosyl(1-->6)]-beta-D-glucopyranosyl ester by spectroscopic techniques and chemical transformations. Loniceroside C showed in vivo antiinflammatory activity against mouse ear edema provoked by croton oil.     

Fabrication of nanocapsules with Au particles trapped inside carbon and silica nanoporous shells

Carbon capsules with hollow cores and mesoporous shells (HCMS) containing entrapped Au particles were prepared by template replication from solid core/mesoporous shell silica spheres with encapsulated Au particles. The resulting HCMS carbon capsules were then nanocast one step further to generate Au-trapping hollow core silica capsules with nanostructured shells.     

An electrochemically controllable nanomechanical molecular system utilizing edge-to-face and face-to-face aromatic interactions

[formula: see text] A new molecular system, 2,11-dithio[4,4]metametaquinocyclophane containing a quinone moiety, was designed and synthesized. As the quinone moiety can readily be converted into an aromatic pi-system (hydroquinone) upon reduction, the nanomechanical molecular cyclophane system exhibits a large flapping motion like a molecular flipper from the electrochemical redox process. The conformational changes upon reduction and oxidation are caused by changes of nonbonding interaction forces (devoid of bond formation/breaking) from the edge-to-face to face-to-face aromatic interactions and vice versa, respectively.     

Fluoroquinolone Antimicrobials: Singlet Oxygen, Superoxide and Phototoxicity

The fluoroquinolone antibacterial agents possess photo-sensitizing properties that lead to phototoxic responses in both human and animal subjects. The phototoxicity order reported in humans is: fleroxacin > lomefloxacin, pefloxacin > ciprofloxacin ? enoxacin, norfloxacin and ofloxacin. Studies both in vivo and in vitro have related this phototoxicity to the generation of reactive oxygen species including hydrogen peroxide and the hydroxyl radical. We determined the quantum yields of singlet oxygen generation (φ_Δ,) by detection of the singlet oxygen (¹O₂) luminescence at 1270 tun for several fluoroquinolones, naphthyridines and other structurally related compounds. All the fluoroquinolones examined have low φ_Δ values ranging from 0.06 to 0.09 in phosphate buffer at pD 7.5. We also determined the ¹O₂ quenching constants for these compounds and their values were on the order of 10⁶M^?1 s¹, except for lomefloxacin whose rate constant was 1.8 × 10⁷M^?1 s^?1. The φ_Δ values were significantly decreased in a solvent of lower polarity such as methanol (0.007 ≤φ_Δ≤ 0.02). The production of ¹O₂ by these antibiotics did not correlate with the order reported for their phototoxicity. We also measured the photogeneration (λ > 300 nm) of superoxide by these antibacterials in dimethylsulfoxide using electron paramagnetic resonance and the spin trap 5,5-dimethyl-l-pyrroiine N-oxide. Although there is not a one-to-one correspondence between the relative rates of superoxide generation and the phototoxicity ranking of the fluoroquinolones, the more phototoxic compounds tended to produce superoxide at a faster rate. Nevertheless, the magnitudes of the observed differences do not appear sufficient to explain the range of fluoroquinolone phototoxicity potencies in human and animal subjects in general and the high activity of fleroxacin and lomefloxacin in particular. For these latter drugs the photoinduced loss of the F₈ atom as fluoride and the concomitant generation of a highly reactive carbene at C-8 provide a more plausible mechanism for their potent phototoxic and photocarcinogenic properties.     

A SPIN TRAPPING STUDY OF THE PHOTOCHEMISTRY OF 5,5-DIMETHYL-1-PYRROLINE N-OXIDE (DMPO)

The photochemistry of 5,5-dimethyl-l-pyrroline N -oxide (DMPO) has been studied in benzene, cyclohexane and aqueous buffer solutions (pH 7.4) by means of electron paramagnetic resonance (EPR) and the spin trapping technique. Ultraviolet irradiation of DMPO in aqueous buffer with unfiltered UV radiation from a Xe arc lamp results in photoionization of the spin trap and the generation of the DMPO cation radical, DMPO⁺. The aqueous electron, e_aq⁻, was trapped by DMPO and detected as the DMPO/H adduct. The DMPO⁺- reacted with the water to yield the DMPO/OH adduct. Ultraviolet irradiation of DMPO in nitrogen-saturated benzene gave an unidentified carbon-centered DMPO adduct that was replaced by hydroperoxyl and alkoxyl adducts of DMPO when oxygen was present. Experiments employing ¹⁷O₂ gas indicated that the oxygen in the DMPO alkoxyl adduct was derived from molecular oxygen. However, UV irradiation of DMPO in cyclohexane yielded the cyclohexyl and cyclohexyloxyl adducts of DMPO in nitrogen-saturated and air-saturated solutions, respectively. These observations suggest that in aprotic solvents UV irradiation of DMPO generates a carbon-centered radical (R⁻), derived from the trap itself, which in benzene reacts with oxygen to yield an alkoxyl radical (RO⁻), possibly via a peroxyl radical (ROO⁻) intermediate. In cyclohexane R⁻ abstracts a hydrogen atom from the solvent to yield the cyclohexyl radical in the absence of oxygen and the cyclohexyloxyl radical in the presence of oxygen. These findings indicate that when DMPO is used as a spin trap in studies employing short-wavelength UV radiation (λ < 300 nm) the photochemistry of DMPO cannot be ignored.     

Structural basis for inhibition of protein tyrosine phosphatases by Keggin compounds phosphomolybdate and phosphotungstate

Protein-tyrosine phosphatases (PTPs) constitute a family of receptor-like, and cytoplasmic enzymes, which catalyze the dephosphorylation of phosphotyrosine residues in a variety of receptors and signaling molecules. Together with protein tyrosine kinases (PTKs), PTPs are critically involved in regulating many cellular signaling processes. In this study, diverse compounds were screened for PTP inhibition and selectively screened for inhibitors with the end product inhibition properties. Among phosphate analogues and their derivatives for PTP inhibition, Keggin compounds phosphomolybdate (PM) and phosphotungstate (PT) strongly inhibited both PTP-1B and SHP-1, with K(i) values of 0.06-1.2 micromM in the presence of EDTA. Unlike the vanadium compounds, inhibition potencies of PM and PT were not significantly affected by EDTA. PM and PT were potent, competitive inhibitors for PTPs, but relatively poor inhibitors of Ser/Thr phosphatase. Interestingly, PM and PT did not inhibit alkaline phosphatase at all. The crystal structure of PTP-1B in complex with PM, at 2.0 A resolution, reveals that MoO(3), derived from PM by hydrolysis, binds at the active site. The molybdenium atom of the inhibitor is coordinated with six ligands: three oxo-ligands, two apical water molecules and a S atom of the catalytic cysteine residue. In support of the crystallographic finding, we observed that molybdenium oxides (MoO(3), MoO(2), and MoO(2)Cl(2)) inhibited PTP-1B with IC(50) in the range 5-15 micromM.     

Aqua dissociation nature of cesium hydroxide

To understand the mechanism of aqueous base dissociation chemistry, the ionic dissociation of cesium-hydroxide in water clusters is examined using density functional theory and ab initio calculations. In this study, we report hydrated structures, stabilities, thermodynamic quantities, dissociation energies, infrared spectra, and electronic properties of CsOH(H(2)O)(n=0-4). With the addition of water molecules, the Cs-OH bond lengthened significantly from 2.46 A for n=1 to 3.08 A for n=4, which causes redshift in Cs-O stretching frequency. It is found that three water molecules are needed for the dissociation of Cs-OH, in contrast to the case of strong acid dissociation which requires at least four water molecules. However, the dissociation for n=3 could be considered as incomplete because a very weak CS em leader OH stretch mode is still present, while that for n=4 is complete since the Cs em leader OH mode no longer exists. This study can be related with hydration chemistry of cations and anions, and extended into the intra- and intercharge-transfer phenomena.     

XPS analysis of the effect of Pt addition to Pd catalysts for complete benzene oxidation

Pt-Pd bimetallic catalysts were prepared in order to develop and investigate catalysts having excellent activity and stability for benzene destruction. The effect of Pt addition to Pd catalysts is studied by XPS analysis. This revised version was published online in August 2006 with corrections to the Cover Date.