Structure of an accurate ab initio model of the aqueous Na+ ion at high temperatures

The structure of an accurate ab initio model of aqueous sodium ion was calculated at two high temperature state points (573 K, 0.72 g/cm(3) and 723 K, 0.0098 g/cm(3)) by a two-step procedure. First, the structure of an approximate model (the TIP4-FQ model for water and Na-H2O interactions from Liu et al.) was calculated from a molecular dynamics simulation of the model. Then the difference between the structure of the ab initio model and the approximate model was calculated by non-Boltzmann weighting of a sample of 500 configurations taken from the approximate model simulation. Radial distribution functions, average coordination numbers, the distribution of coordination numbers, and an analysis of orientations of water in the first coordination shell are reported for both state points. The average oxygen coordination number (calculated up to the inflection point in the running coordination number) was 4.71 at 573 K and 3.48 at 723 K. Most configurations have four or five coordinated waters at 573 K and three or four at 723 K. At 723 K, waters with their dipole moments pointed at the sodium ion were most common. A wider variety of orientations was found at 573 K and higher density. The difference in structure between the approximate and quantum models was small but significant.     

Study of a new derivatizing reagent that improves the analysis of amino acids by HPLC with fluorescence detection: application to hydrolyzed rape bee pollen

A simple and sensitive method for evaluating the chemical compositions of protein amino acids, including cystine (Cys)₂ and tryptophane (Try) has been developed, based on the use of a sensitive labeling reagent 2-(11H-benzo[α]-carbazol-11-yl) ethyl chloroformate (BCEC–Cl) along with fluorescence detection. The chromophore of the 1,2-benzo-3,4-dihydrocarbazole-ethyl chloroformate (BCEOC-Cl) molecule was replaced with the 2-(11H-benzo[α]-carbazol-11-yl) ethyl functional group, yielding the sensitive fluorescence molecule BCEC–Cl. The new reagent BCEC–Cl could then be substituted for labeling reagents commonly used in amino acid derivatization. The BCEC–amino acid derivatives exhibited very high detection sensitivities, particularly in the cases of (Cys)₂ and Try, which cannot be determined using traditional labeling reagents such as 9-fluorenyl methylchloroformate (FMOC-Cl) and ortho-phthaldialdehyde (OPA). The fluorescence detection intensities for the BCEC derivatives were compared to those obtained when using FMOC-Cl and BCEOC-Cl as labeling reagents. The ratios I _BCEC/I _BCEOC = 1.17–3.57, I _BCEC/I _FMOC = 1.13–8.21, and UV_BCEC/UV_BCEOC = 1.67–4.90 (where I is the fluorescence intensity and UV is the ultraviolet absorbance). Derivative separation was optimized on a Hypersil BDS C₁₈ column. The detection limits calculated from 1.0 pmol injections, at a signal-to-noise ratio of 3, ranged from 7.2 fmol for Try to 8.4 fmol for (Cys)₂. Excellent linear responses were observed, with coefficients of >0.9994. When coupled with high-performance liquid chromatography, the method established here allowed the development of a highly sensitive and specific method for the quantitative analysis of trace levels of amino acids including (Cys)₂ and Try from bee-collected pollen (bee pollen) samples.     

On the mutual diffusion properties of ethanol-water mixtures

The structural organization, the number of hydrogen bonds (H bond), and the self- and mutual diffusion coefficients of ethanol-water mixtures were studied by molecular dynamics simulation. It was found that both the numbers of H bonds per water and per ethanol decrease as the mole fraction of ethanol increases. The composition dependences and the relationships between the self- and the mutual diffusion coefficients were further discussed. The self-diffusion coefficient of water has a large drop as the concentration of ethanol increases from 0 to 0.3 and then it nearly keeps constant, while that of ethanol has a minimum around ethanol mole fraction of 0.5. The mutual diffusion coefficient could be divided into two parts, the kinematic factor and the thermodynamic factor. Both the kinematic and thermodynamic factors for ethanol-water mixtures were calculated. It was found that the change trend of mutual diffusion coefficients with the composition is mainly dependent on the thermodynamic factors.     

Core-shell effects in the ionization of doped helium nanodroplets

Core-shell particles with water clusters as the core and surrounded by an atomic or molecular shell have been synthesized for the first time by adding water and a co-dopant sequentially to helium nanodroplets. The co-dopants chosen for investigation were Ar, O(2), N(2), CO, CO(2), NO and C(6)D(6). These co-dopants have been used to investigate the effect of an outer shell on the ionization of the core material by charge transfer in helium nanodroplets. The specific aim was to determine how the identity of the shell material affects the fragmentation of water cluster ions, i.e. whether it helps to stabilize parent ion ((H(2)O)(n)(+)) formation or increases fragmentation (to form (H(2)O)(n)H(+)). N(2), O(2), CO(2) and C(6)D(6) all show a marked softening effect, which is consistent with the formation of a protective shell around the water cluster core. For CO and NO co-dopants, the response is complicated by secondary reactions which actually favour water cluster ion fragmentation for some water cluster sizes.     

Can TMS and DSS be used as NMR references for cyclodextrin species in aqueous solution?

Tetramethylsilane (TMS) can be included by -cyclodextrin (-CD), and sodium 2,2-dimethylsilapentane-5-sulphonate (DSS) can form inclusion complexes with - and -CD. The NMR chemical shifts are changed considerably as a result of the strong interaction between CD and the guest compound in the inclusion complexes. A downfield shift of as much as 0.63 ppm shift downfield has been observed for the protons of external TMS in CD aqueous solution. In view of this, the question arises of whether TMS and DSS can be used as internal references. DSS in D₂O is suggested as an external reference for aqueous cyclodextrin solution in NMR measurements.     

Density measurement of 1-d confined water by small angle neutron scattering method: pore size and hydration level dependences

Small angle neutron scattering (SANS) is used to measure the absolute density of water contained in 1-D cylindrical pores of a silica material MCM-41-S with pore diameters of 19 and 15 A. By being able to suppress the homogeneous nucleation process inside the narrow pore, one can keep water in the liquid state down to at least 160 K. From a combined analysis of SANS data from both H(2)O and D(2)O hydrated samples, we determined the absolute value of the density of 1-D confined water. We found that the average density of water inside the fully hydrated 19 A pore is 8% higher than that of the bulk water at room temperature. The temperature derivative of the density shows a pronounced peak at T(L) = 235 K signaling the crossing of the Widom line at ambient pressure and confirming the existence of a liquid-liquid phase transition at an elevated pressure. Pore size and hydration level dependences of the density are also studied.     

Preparation of well-defined core-shell particles by Cu2+-mediated graft copolymerization of methyl methacrylate from bovine serum albumin

Small well-defined core-shell poly(methyl methacrylate)-bovine serum albumin (PMMA-BSA) particles have been prepared in a direct one-step graft copolymerization of MMA from BSA at 75 degrees C in water with a trace amount of Cu2+ (5 microM). Initially, BSA generates free radicals and acts as a multifunctional macroinitiator, which leads to the formation of an amphiphilic PMMA-BSA grafting copolymer. Such formed copolymer chains act as a polymeric stabilizer to promote further emulsion polymerization of MMA inside, resulting in surfactant-free stable core-shell particles, confirmed by a transmission electron microscopic (TEM) analysis. The PMMA-BSA copolymers as well as PMMA homopolymer inside the particles were isolated by Soxhlet extraction and characterized by Fourier transform infrared spectroscopy (FT-IR) and thermogravimetry (TG). The highest grafting efficiency was approximately 80%. Effects of the reaction temperature, the MMA/BSA ratio, and the concentrations of Cu2+ and BSA on such core-shell particle formation have been systematically studied. Due to their inert PMMA core and biocompatible BSA shell, these small polymer particles are potentially useful in biomedical applications.     

A tunable 3D optofluidic waveguide dye laser via two centrifugal Dean flow streams

This paper presents a tunable optofluidic waveguide dye laser utilizing two centrifugal Dean flows. The centrifugal Dean flow increases the light confinement of the dye laser by shaping a three-dimensional (3D) liquid waveguide from curved microchannels. The active medium with the laser dye is dissolved in the liquid core and pumped with an external pump laser to produce stimulated emission. The laser's Fabry-Pérot microcavity is formed with a pair of aligned gold-coated fiber facets to amplify the fluorescent emission. The advantage of the 3D optofluidic waveguide dye laser is its higher efficiency, thus to obtain lasing at a reduced threshold (60%) with higher output energy. The demonstrated slope efficiency is at least 3-fold higher than its traditional two-dimensional equivalent. In addition, the laser output energy can be varied on demand by tuning the flow rates of the two flows. This technique provides a versatile platform for high potential applications microfluidic biosensor and bioanalysis.     

Electron capture dissociation of peptides metalated with alkaline-earth metal ions

The possible use of divalent alkaline-earth metal ions, including Mg2+, Ca2+, Sr2+, and Ba2+, as charge carrier for electron capture dissociation of peptides was investigated. Model peptides of RGGGVGGGR and NGGGWGGGN were used to simplify the interpretation of spectral information. It was demonstrated that useful electron capture dissociation (ECD) tandem mass spectra of these metalated peptides could be generated. Interestingly, peptides metalated with different alkaline-earth metal ions generated very similar ECD tandem mass spectra. Metalated c-ions and z-ions were the predominant fragment ions. Only Mg2+-metalated peptides gave somewhat different results. Some nonmetalated c-ions were observed from ECD of [RGGGVGGGR + Mg]2+ but not from [NGGGWGGGN + Mg]2+. Together with some ab initio calculations, it was established that the bound metal ions might activate the acidity of the amide hydrogen. With the presence of high proton affinity moiety, such as N-terminal amino group and/or side chain of the arginine residues, the metalated peptide ions could exist predominantly in their zwitterion forms, in which one or two backbone amide group(s) was deprotonated and the high proton affinity functional group(s) was protonated. It was believed that electron capture leads primarily to the reduction of the mobile proton rather than the metal ions. With this zwitterion model, the formation of nonmetalated c-fragments and the generation of similar ECD spectra for peptides metalated with various alkaline-earth metal ions could readily to be explained. Another interesting observation in the ECD mass spectra of metalated peptides is related to the enhanced formation of the minor ECD products, i.e., (c - 1)(+*) and (z + 1)+ ions. Together with ab initio calculations using a truncated peptide model, various possible reaction mechanisms for the formation of these minor ECD products were evaluated. It was concluded that hydrogen transfer between the initiated formed c and z(.) species plays an important role in the formation (c - 1)(+*) and (z + 1)+ ions. Although peptides metalated with these metal ions do not have better ECD efficiency compared to the multiply-protonated peptides, it provides practical accessibility of ECD methods to analyze small peptides with no basic amino acid residues.     

Phosphorus(V) porphyrins with axial carbazole-based dendritic substituents

[structure: see text] Three phosphorus(V) porphyrins with axial carbazole-based dendritic substituents (D-A-D) have been designed and synthesized, which are nonfluorescent due to their effective electron transfer from the carbazole dendron to the excited porphyrin within the dendritic matrix. The incident photon to current conversion efficiencies (IPCE) spectra demonstrate that the molecular structure of the dendrimers can significantly affect the photovoltaic response to the visible light.