Structural tuning intra- versus inter-molecular proton transfer reaction in the excited state

A series of 2-pyridyl-pyrazole derivatives 1-4 possessing five-membered ring hydrogen bonding configuration are synthesized, the structural flexibility of which is strategically tuned to be in the order of 1 > 2 > 3 > 4. This system then serves as an ideal chemical model to investigate the correlation between excited-state intramolecular proton transfer (ESIPT) reaction and molecular skeleton motion associated with hydrogen bonds. The resulting luminescence data reveal that the rate of ESIPT decreases upon increasing the structural constraint. At sufficiently low concentration where negligible dimerization is observed, ESIPT takes place in 1 and 2 but is prohibited in 3 and 4, for which high geometry constraint is imposed. The results imply that certain structural bending motions associated with hydrogen bonding angle/distance play a key role in ESIPT. This trend is also well supported by the DFT computational approach, in which the barrier associated with ESIPT is in the order of 1 < 2 < 3 < 4. Upon increasing the concentration in cyclohexane, except for 2, the rest of the title compounds undergo ground-state dimerization, from which the double proton transfer takes place in the excited state, resulting in a relatively blue shifted dimeric tautomer emission (cf. the monomer tautomer emission). The lack of dimerization in 2 is rationalized by substantial energy required to adjust the angle of hydrogen bond via twisting the propylene bridge prior to dimerization.     

Chemical Dealloying Mechanism of Bimetallic Pt–Co Nanoparticles and Enhancement of Catalytic Activity toward Oxygen Reduction

The chemical dealloying mechanism of bimetallic Pt–Co nanoparticles (NPs) and enhancement of their electrocatalytic activity towards the oxygen reduction reaction (ORR) have been investigated on a fundamental level by the combination of X‐ray absorption spectroscopy (XAS) and aberration‐corrected scanning transmission electron microscopy (STEM). Structural parameters, such as coordination numbers, alloy extent, and the unfilled d states of Pt atoms, are derived from the XAS spectra, together with the compositional variation analyzed by line‐scanning energy‐dispersive X‐ray spectroscopy (EDX) on an atomic scale, to gain new insights into the dealloying process of bimetallic Pt–Co NPs. The XAS results on acid‐treated Pt–Co/C NPs reveal that the Co–Co bonding in the bimetallic NPs dissolves first and the remaining morphology gradually transforms to a Pt‐skin structure. From cyclic voltammetry and mass activity measurements, Pt–Co alloy NPs with a Pt‐skin structure significantly enhance the catalytic performance towards the ORR. Further, it is observed that such an imperfect Pt‐skin surface feature will collapse due to the penetration of electrolyte into layers underneath and cause further dissolution of Co and the loss of Pt. The electrocatalytic activity decreases accordingly, if the dealloying process lasts for 4 h. The findings not only demonstrate the importance of appropriate treatment of bimetallic catalysts, but also can be referred to other Pt bimetallic alloys with transition metals.     

Ultra-performance liquid chromatography/tandem mass spectrometric determination of testosterone and its metabolites in in vitro samples

This paper describes the development and partial validation of a fast, sensitive and specific ultra-performance liquid chromatography/tandem mass spectrometric (UPLC/MS/MS) method for the determination of testosterone (T) and its four metabolites, 6beta-OH-T, 16alpha-OH-T, 16beta-OH-T and 2alpha-OH-T, in in vitro samples. The analytical method involves direct dilution of samples with acetonitrile containing an internal standard, followed by separation of testosterone and the four metabolites on an Acquity UPLCtrade mark C(18) column and detected by selected reaction monitoring (SRM) in positive ionization mode using turbo ionspray ionization. The parent compound and its metabolites investigated were well separated (Rs >1.5) with a run time of 4 min under a gradient condition. The method was partially validated. The linear concentration range was 0.01 to 5 microM for all the compounds of interest. Inter-assay mean bias and relative standard deviation (RSD) were in the range of -12% to 8% and 4.1% to 8.5%, respectively. Intra-assay mean bias and RSD were in the range of -8.0% to 5.2% and 3.4% to 9.6%, respectively. The lower limit of quantitation for this assay was 0.01 microM. The differences in LC/MS performance were investigated by conducting a comparison of UPLC with another method previously optimized for HPLC-based separation and quantification of testosterone and its metabolites.     

Extensive spectral tuning of the proton transfer emission from 550 to 675 nm via a rational derivatization of 10-hydroxybenzo[h]quinoline

Via a systematic derivatization of the excited-state intramolecular proton-transfer system, 10-hydroxybenzo[h] quinoline, the proton-transfer emission can be extensively tuned from 550 nm (1) to 675 nm (6), in which amplified spontaneous emission was readily observed for , generating a new family of proton transfer laser dyes.     

New-generation amber united-atom force field

We have developed a new-generation Amber united-atom force field for simulations involving highly demanding conformational sampling such as protein folding and protein-protein binding. In the new united-atom force field, all hydrogens on aliphatic carbons in all amino acids are united with carbons except those on Calpha. Our choice of explicit representation of all protein backbone atoms aims at minimizing perturbation to protein backbone conformational distributions and to simplify development of backbone torsion terms. Tests with dipeptides and solvated proteins show that our goal is achieved quite successfully. The new united-atom force field uses the same new RESP charging scheme based on B3LYP/cc-pVTZ//HF/6-31g** quantum mechanical calculations in the PCM continuum solvent as that in the Duan et al. force field. van der Waals parameters are empirically refitted starting from published values with respect to experimental solvation free energies of amino acid side-chain analogues. The suitability of mixing new point charges and van der Waals parameters with existing Amber covalent terms is tested on alanine dipeptide and is found to be reasonable. Parameters for all new torsion terms are refitted based on the new point charges and the van der Waals parameters. Molecular dynamics simulations of three small globular proteins in the explicit TIP3P solvent are performed to test the overall stability and accuracy of the new united-atom force field. Good agreements between the united-atom force field and the Duan et al. all-atom force field for both backbone and side-chain conformations are observed. In addition, the per-step efficiency of the new united-atom force field is demonstrated for simulations in the implicit generalized Born solvent. A speedup around two is observed over the Duan et al. all-atom force field for the three tested small proteins. Finally, the efficiency gain of the new united-atom force field in conformational sampling is further demonstrated with a well-known toy protein folding system, an 18 residue polyalanine in distance-dependent dielectric. The new united-atom force field is at least a factor of 200 more efficient than the Duan et al. all-atom force field for ab initio folding of the tested peptide.     

Two-photon absorption chromophores with a tunable [2,2′]bithiophene core

Sonogashira coupling between 3,5,3′,5′-tetrabromo-[2,2′]bithiophene and various terminal alkynes provides two-photon absorption (TPA) chromophores 1-6, which possess electron donor (D) and/or acceptor (A) alkynyl substituents at 3(3′) and 5(5′) sites of the bithiophene core. The up-converted fluorescence emission excited at 800 nm (Ti:sapphire femtosecond laser, ∼100 fs pulses) was used to determine the two-photon absorption cross-sections (σ) of these compounds. The corresponding TPA cross-section (σ) values ranging from 132 to 1120 GM (10⁻⁵⁰ cm⁴ s photon⁻¹) can be fine-tuned by the substitutents. The quadrupolar-type (A-π-D-π-A) chromophore 5 exhibits the largest σ value (1120 GM) in CH₂Cl₂ upon 800 nm excitation.     

Fabrication of microfluidic devices using dry film photoresist for microchip capillary electrophoresis

An inexpensive, disposable microfluidic device was fabricated from a dry film photoresist using a combination of photolithographic and hot roll lamination techniques. A microfluidic flow pattern was prefabricated in a dry film photoresist tape using traditional photolithographic methods. This tape became bonded to a poly(methyl methacrylate) (PMMA) sheet with prepouched holes when passed through a hot roll laminator. A copper working electrode and platinum decoupler was readily incorporated within this microchip. The integrated microchip device was then fixed in a laboratory-built Plexiglas holder prior to its use in microchip capillary electrophoresis. The performance of this device with amperometric detection for the separation of dopamine and catechol was examined. The separation was complete within 50 s at an applied potential of 200 V/cm. The relative standard deviations (RSD) of analyte migration times were less than 0.71%, and the theoretical plate numbers for dopamine and catechol were 3.2 x 10(4) and 4.1 x 10(4), respectively, based on a 65 mm separation channel.     

Impact of halides on the simultaneous separation of aromatic amines and their acidic metabolites by capillary electrophoresis with laser-induced native fluorescence detection under acidic conditions

This paper describes a simple, sensitive, efficient, and rapid method for simultaneous analysis of biologically active amines and acids by capillary electrophoresis in conjunction with laser-induced native fluorescence detection (CE-LINF) using a diode pumped solid state nanolaser at 266 nm. In order to optimize resolution of the amines that were prepared in 10.0 mM formate-Tris (FT) solutions, 10.0 mM FT solutions with and without containing halides were used to fill the capillary and reservoirs, respectively. The electrophoretic mobilities of tryptamine (TA) and serotonin (5-HT) at pH 4.0 decrease with the increase in halide concentration (0-10.0 mM). Taken together with a great effect of iodide than other halides, we suggest that the formation of ion pairs is a main contributor for altering the migration of the amines. In order to simultaneously analyze the amines and their metabolites (acids) at low pH, a high bulk EOF is required. The analysis of 10 anlytes including amines and acids was completed within 12 min by CE-LINF using a capillary treated with 0.5M NaOH and then filled with 10.0 mM FT solutions (pH 4.0) containing 10.0 mM KCl prior to analysis. The limits of detection for TA and 5-hydroxyindole-3-acetic acid (5-HIAA) are 0.12 and 6.0 nM, respectively. The present method has been further validated by analyzing urine samples, with an RSD less than 3.1% (migration times) and 3.9% (concentration).     

Electrowetting-induced capillary flow in a parallel-plate channel

This paper investigated, theoretically and experimentally, the electrowetting-induced capillary rise in a parallel-plate channel. The measured equilibrium height of the meniscus was proportional to the square of the applied potential. A model, based on the kinetic equation of capillary flow with the consideration of an electrowetting dynamic contact angle, was established to simulate the capillary rise. The effects of the electrostatic charge and the contact-line friction were linearly added to describe the electrowetting dynamic contact angle. The model was found to be able to adequately describe the experimental data under different initial heights and applied voltages. The non-Poseuille flow effect had little influence in the meniscus rising phenomenon studied in this work.