In situ observations of water production and distribution in an operating H2/O2 PEM fuel cell assembly using 1H NMR microscopy

Proton NMR imaging was used to investigate in situ the distribution of water in a polymer electrolyte membrane fuel cell operating on H2 and O2. In a single experiment, water was monitored in the gas flow channels, the membrane electrode assembly, and in the membrane surrounding the catalysts. Radial gradient diffusion removes water from the catalysts into the surrounding membrane. This research demonstrates the strength of 1H NMR microscopy as an aid for designing fuel cells to optimize water management.     

Studies of photoinitiation and phototermination of free radical polymerization using dual pulsed laser techniques

Studies of free radical polymerization are described in which the initiation and termination events are independently controlled using two laser pulses of different wavelength. Flash photolysis of azobisisobutyronitrile (AIBN) was used to initiate the polymerization of styrene. After a predefined period the polymerization was terminated by laser photolysis of 2-naphthylmethyl 1-naphthylacetate (NMNA). The molecular weights of the polymer were controlled by the time interval between laser pulses. Irradiation with high intensity laser pulses in the presence of preformed polystyrene yields a fluorescent-labelled polymer. A two-photon process is proposed to explain the mechanism of photoaddition of naphthylmethyl groups to the polymer chain.     

A graph-theoretic approach to quantitative structure—activity/reactivity studies

Characterization of molecular species based on the use of suitable graph invariants (graph paths, in particular) can provide a quantitative means of encoding structure; the technique is complementary to commoner approaches to studies of quantitative structure— activity relationships. Graph path encoding is here applied to quantitative studies of relationships between molecular structures and biological activity; the examples are the rates of various substrate reactions with hexoldnase, and the potential opiate-like activity of enkephalin analogs.     

Use of pi-allyltricarbonyliron lactone complexes in the synthesis of taurospongin A: a potent inhibitor of DNA polymerase beta and HIV revers transcriptase

The total synthesis of taurospongin A by two new approaches has been achieved where pi-allyltricarbonyliron lactone complexes have been used to control highly stereoselective additions of the nucleophiles to a carbonyl unit located in the side chain of these complexes.     

The role of C-centered radicals on the mechanism of action of artemisinin

Artemisinin is a sesquiterpene lactone with an endoperoxide function that is essential for its antimalarial activity. Endoperoxides are supposed to act on heme leading to the reduction of the peroxide bond and production of radicals, which can be responsible for killing the parasite. The geometries of artemisinin, radical anions and neutral species generated by rearrangement after reduction of the peroxide bond were fully optimized with the AM1 and PM3 semi-empirical methods, in order to characterize the intermediates formed during the process. Among the radicals calculated along the pathway for reductive decomposition of artemisinin, the secondary radical centered on carbon C₄ has the highest stability when compared to other radicals that can be achieved either by hydrogen migration to the original O-centered radical or by homolytic break of C–C bond. This suggests that the C₄-centered radical may be the species responsible for killing the parasite, as has been indicated previously in experimental studies.     

Analysis of partially resolved liquid chromatographic peaks using dynamic modeling with the kalman filter

Establishing a calibration model is an important part of any mathematical method for multi-component determination. Use of a calibration model based on single spectra is subject to error, because the model spectrum chosen may not be representative of the response over the full range of the calibration. Alternative calibration models require more time to establish calibration, an these may not be convenient for real-time determinations. A novel calibration method is reported for use with Kalman filters. The method, dynamic modeling, is based on the use of libraries of calibration spectra. The set of used to describe the model at any time is based on component concentrations, estimated for the multi-component mixture, as determined from the Kalman filter, so that several spectra can be used to best describe a varying response. Through application of the dynamic modelingt to simulated and real chromatograms, it is demonstrated that use of the method decreases estimation errors cause by model data mismatches, and that full benefit can be obtained from relatively small libraries.     

Rapid quantitative capillary zone electrophoresis method for monitoring the micro-heterogeneity of an intact recombinant glycoprotein

A simple high-resolution capillary zone electrophoresis (CZE) method capable of rapidly assessing the micro-heterogeneity of a 24 kDa molecular weight glycoprotein, has been developed. Separation is carried out using a bare silica capillary at a pH of 2.5 in a commercially available electrophoresis buffer system composed of triethanolamine and phosphoric acid. Over 30 peaks were detected within a run time of 15 min using a 27 cm capillary and approximately 60 peaks were detected using a 77 cm capillary. Although most of the peaks arise from differences in the oligosaccharide structures present on the one glycosylation site on this molecule, other forms of micro-heterogeneity due to the presence of the nonglycosylated form of this glycoprotein and various types of chemical degradation, e.g., deamidation, are also responsible for the multitude of peaks observed. Although the exact chemical identity of each peak in the resulting electropherogram of this glycoprotein is not known, useful information can be obtained for assessing comparability, stability, and batch consistency. Factors impacting the resolution, precision, accuracy, and robustness of the assay are also discussed along with inherent advantages and limitations associated with measuring the micro-heterogeneity of intact glycoproteins.     

Automated assignment of charge states from resolved isotopic peaks for multiply charged ions

The recent proliferation of electrospray as an ionization method has greatly increased the ability to perform analyses of large biomolecules by using mass spectrometry. The major advantage of electrospray is the ability to produce multiply charged ions, which brings large molecules down to a mass-to-charge ratio range amenable to most instruments. Multiple charging is also a disadvantage because mass (m) becomes ambiguous unless charge (z) can be assigned. This is typically performed with simple algorithms that use multiple peaks of the same m and different z, but these methods are difficult to apply to complex mixtures and not applicable when only one z appears for each m. The use of mass analyzers with higher resolving powers, like the Fourier transform mass spectrometer, allows resolution of isotopic peaks, providing an internal 1-Da mass scale that can be used for unambiguous charge assignment. Manual assignment of charge state from the isotopic peaks is time consuming and becomes inaccurate when either the signal level or resolving power are low. For these cases, computer algorithms based on pattern recognition techniques have been developed to assist in assignment of charge states to isotopic clusters. These routines provide for more rapid analysis with higher accuracy than available manually.     

Efficient transesterification/acylation reactions mediated by N-heterocyclic carbene catalysts

Imidazol-2-ylidenes, a family of N-heterocyclic carbenes (NHC), are efficient catalysts in the transesterification involving numerous esters and alcohols. Low catalyst loadings of aryl- or alkyl-substituted NHC catalysts mediate the acylation of alcohols with enol acetates in short reaction times at room temperature. Commercially available and more difficult to cleave methyl esters react with primary alcohols in the presence of alkyl-substituted NHC to efficiently form the corresponding esters. While primary alcohols are selectively acylated over secondary alcohols with use of enol esters as acylating agents, methyl and ethyl esters can be employed as protective agents for secondary alcohols in the presence of the more active alkyl-substituted NHC catalysts. The NHC-catalyzed transesterification protocol was simplified by generating the imidazol-2-ylidene catalysts in situ.