Quassinoids from the Root Barks of Ailanthus altissima:Isolation,Configurational Assignment,and Cytotoxic Activities

Quassinoids,a class of highly oxygenated triterpenes,have been isolated as bitter principles from the plants of Simaroubaceae family.Five new quassinoids,chouch...     

Oligomer Sensor Nanoarchitectonics for “Turn-On” Fluorescence Detection of Cholesterol at the Nanomolar Level

Sensitive and rapid monitoring of cholesterol levels in the human body are highly desirable as they are directly related to the diagnosis of cardiovascular diseases. By using the nanoarchitectonic approach, a novel fluorescent conjugated oligofluorene (OFP-CD) functionalized with β-cyclodextrin (β-CD) was assembled for “Turn-On” fluorescence sensing of cholesterol. The appended β-CD units in OFP-CD enabled the forming of host-guest complexes with dabsyl chloride moieties in water, resulting in fluorescence quenching of the oligofluorene through intermolecular energy transfer. In the presence of cholesterol molecules, a more favorable host-guest complex with stoichiometry 1 cholesterol: 2 β-CD units was formed, replacing dabsyl chloride in β-CD’s cavities. This process resulted in fluorescence recovery of OFP-CD, owing to disruption of energy transfer. The potential of this nanoarchitectonic system for “Turn-On” sensing of cholesterol was extensively studied by fluorescence spectroscopy. The high selectivity of the sensor for cholesterol was demonstrated using biologically relevant interfering compounds, such as carbohydrates, amino acids, metal ions, and anions. The detection limit (LOD value) was as low as 68 nM, affirming the high sensitivity of the current system.     

Potential interstellar molecules. Formation of neutral C(6)CO from C(6)CO(-*) in the gas phase

Computations at the RCCSD(T)/aug-cc-pVDZ//B3LYP/6-31G* level of theory indicate that neutral C(6)CO is a stable species. The ground state of this neutral is the singlet cumulene oxide :C=C=C=C=C=C=C=O. The adiabatic electron affinity and dipole moment of singlet C(6)CO are 2.47 eV and 4.13 D, respectively, at this level of theory. The anion (C(6)CO)-* should be a possible precursor to this neutral. It has been formed by an unequivocal synthesis in the ion source of a mass spectrometer by the S(N)2(Si) reaction between (CH(3))(3)Si-C(triple bond)C-C(triple bond)C-C(triple bond)C-CO-CMe(3) and F(-) to form (-)C(triple bond)C-C(triple bond)C-C(triple bond)C-CO-Me(3) which loses Me(3)C* in the source to form C(6)CO(-)*. Charge stripping of this anion by vertical Franck-Condon oxidation forms C(6)CO, characterised by the neutralisation-reionisation spectrum (-NR(+)) of C(6)CO(-*), which is stable during the timeframe of this experiment (10(-6) s).     

Semiflexible polymers in a random environment

We present using simple scaling arguments and one step replica symmetry breaking a theory for the localization of semiflexible polymers in a quenched random environment. In contrast to completely flexible polymers, localization of semiflexible polymers depends not only on the details of the disorder but also on the ease with which polymers can bend. The interplay of these two effects can lead to the delocalization of a localized polymer with an increase in either the disorder density or the stiffness. Our theory provides a general criterion for the delocalization of polymers with varying degrees of flexibility and allows us to propose a phase diagram for the highly folded (localized) states of semiflexible polymers as a function of the disorder strength and chain rigidity.     

Dereplication coupled with in vitro antioxidant assay of two flavonoid glycosides from Diospyros peregrina fruit

Diospyros peregrina is an edible seasonal fruit found in coastal West Bengal, India. The fruits have been reported to possess a significant antioxidant activity. In this study, the aim was to isolate the lead compound responsible for the above-mentioned activity. The aqueous extract of D. peregrina fruit was subjected to dereplication coupled with an in?vitro 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical scavenging assay. The n-butanol fraction of the aqueous extract of D. peregrina fruit exhibited significant antioxidant activity (IC(50), 131.10?μg?mL(-1)) as compared with the parent extract (IC(50), 285.15?μg?mL(-1)). The n-butanol fraction was subjected to silica gel column chromatographic separation coupled with a chemo-autographic study of column eluents, employing ethanolic DPPH as a spraying reagent. Two bioactive flavonoid glycosides, namely luteoline-4'-methyl-ether-7-O-glucoside and quercetin-3-O-(glucosyl)-glucoside, were identified to exhibit IC(50) values of 74.04 and 65.78?μg?mL(-1), respectively in the DPPH assay.     

Are the anions MeO(CO) (n = 1 and 2) methoxide anion donors in the gas phase? A theoretical investigation

• 1. The anions CH₃O‐^–CO and CH₃OCO‐^–CO are both methoxide anion donors. The processes CH₃O‐^–CO → CH₃O^– + CO and CH₃OCO—CO → CH₃O^– + 2CO have ΔG values of +8 and ?68 kJ mol^?1, respectively, at the CCSD(T)/6‐311++G(2d, 2p)//B3LYP/6‐311++G(2d,2p) level of theory.
• 2. The reactions CH₃OCOCO → CH₃OCO + CO (ΔG = ?22 kJ mol^?1) and CH₃COCH(O^–)CO₂CH₃ → CH₃COCH(O^–)OCH₃ + CO (ΔG = +19 kJ mol^?1) proceed directly from the precursor anions via the transition states (CH₃OCO…CO₂)^– and (CH₃COCHO…CH₃OCO)^–, respectively.
• 3. Anion CH₃COCH(O^–)CO₂CH₃ undergoes methoxide anion transfer and loss of two molecules of CO in the reaction sequence CH₃COCH(O^–)CO₂CH₃ → CH₃CH(O^–)COCO₂CH₃ → [CH₃CHO (CH₃OCO‐^–CO)] → CH₃CH(O^–)OCH₃ + 2CO (ΔG = +9 kJ mol^?1). The hydride ion transfer in the first step is a key feature of the reaction sequence.

Copyright © 2010 John Wiley & Sons, Ltd.     

Single-molecule enzyme kinetics in the presence of inhibitors

Recent studies in single-molecule enzyme kinetics reveal that the turnover statistics of a single enzyme is governed by the waiting time distribution that decays as mono-exponential at low substrate concentration and multi-exponential at high substrate concentration. The multi-exponentiality arises due to protein conformational fluctuations, which act on the time scale longer than or comparable to the catalytic reaction step, thereby inducing temporal fluctuations in the catalytic rate resulting in dynamic disorder. In this work, we study the turnover statistics of a single enzyme in the presence of inhibitors to show that the multi-exponentiality in the waiting time distribution can arise even when protein conformational fluctuations do not influence the catalytic rate. From the Michaelis-Menten mechanism of inhibited enzymes, we derive exact expressions for the waiting time distribution for competitive, uncompetitive, and mixed inhibitions to quantitatively show that the presence of inhibitors can induce dynamic disorder in all three modes of inhibitions resulting in temporal fluctuations in the reaction rate. In the presence of inhibitors, dynamic disorder arises due to transitions between active and inhibited states of enzymes, which occur on time scale longer than or comparable to the catalytic step. In this limit, the randomness parameter (dimensionless variance) is greater than unity indicating the presence of dynamic disorder in all three modes of inhibitions. In the opposite limit, when the time scale of the catalytic step is longer than the time scale of transitions between active and inhibited enzymatic states, the randomness parameter is unity, implying no dynamic disorder in the reaction pathway.     

Nonrenewal statistics in the catalytic activity of enzyme molecules at mesoscopic concentrations

Recent fluorescence spectroscopy measurements of single-enzyme kinetics have shown that enzymatic turnovers form a renewal stochastic process in which the inverse of the mean waiting time between turnovers follows the Michaelis-Menten equation. We study enzyme kinetics at physiologically relevant mesoscopic concentrations using a master equation. From the exact solution of the master equation we find that the waiting times are neither independent nor identically distributed, implying that enzymatic turnovers form a nonrenewal stochastic process. The inverse of the mean waiting time shows strong departure from the Michaelis-Menten equation. The waiting times between consecutive turnovers are anticorrelated, where short intervals are more likely to be followed by long intervals and vice versa. Correlations persist beyond consecutive turnovers indicating that multiscale fluctuations govern enzyme kinetics.     

Temperature effect on the kinetics of persulfate oxidation of p-chloroaniline

The kinetics of heat-facilitated persulfate oxidation of p-chloroaniline（PCA） in aqueous solutions was measured at five different temperature conditions and at four different oxidant concentrations.The PCA degradation was found to follow a pseudo-first-order decay model when the persulfate was excessive.The pseudo-first-order rate constants of PCA degradation by persulfate（50 mmol/ L） at pH 7.0 are 0.12×10^-4,0.28×10^-4,0.43×10^-4,0.83×10^-4,1.32×10^-4 s^-1 at 10,20,30,40 and 50℃,respectively. Under the above reaction conditions,the reaction has activation energy of 49.97 kJ/mol.The observed rate was found to be function of temperature and oxidant concentration.Raising temperature and increasing persulfate concentration can significantly accelerate the PCA degradation.     

Eu(III) complex-doped PMMA having fast radiation rate and high emission quantum efficiency

Three ternary luminescent complexes, Eu (deuterated 1,3-diphenyl-1,3-propanedione)₃(1,10-phenanthroline), Eu (deuterated 1,3-diphenyl-1,3-propanedione)₃(2,2′-bipyridine), and Eu (deuterated 1,3-diphenyl-1,3-propanedione)₃ (bathophenanthroline) were synthesized using bidental oxygen and nitrogen as ligands. Luminescent polymers were fabricated by incorporating deuterated Eu(III) complexes in a poly(methyl methacrylate) matrix. Luminescent poly(methyl methacrylate) containing Eu (deuterated 1,3-diphenyl-1,3-propanedione)₃ (bathophenanthroline) exhibited relatively higher quantum yield, faster radiation rate, sharper red emission and larger stimulated emission cross-section (quantum yield 36%, radiation rate 8.6 × 10² s^?1, full width at half maximum 3.4 nm, and stimulated emission cross-section σ _p = 1.4 × 10^?20 cm²) of the PMMA matrix. The value of σ _p was the same order as the values of Nd-glass laser for practical use. Additionally, the thermal behaviors of the Eu(III) ternary complexes were studied, and the results indicated that all of them can be long-term used in high temperature environment. Prepared luminescent polymer including Eu (deuterated 1,3-diphenyl-1,3-propanedione)₃ (bathophenanthroline) showed promising results for applications in novel organic Eu(III) devices, such as high-power laser materials and optical fibers.