Make-up Solvent Research Articles

Reaction product analysis by high-performance liquid chromatography-solid-phase extraction-nuclear magnetic resonance: Application to the absolute configuration determination of naturally occurring polyyne alcohols

The absolute configuration of secondary hydroxy functions of seven natural occurring polyyne derivatives has been elucidated by the application of Mosher method of diastereomeric methoxy-2-trifluoromethyl-phenylacetyl (MTPA) ester formation. High-performance liquid chromatography with diode array detection (HPLC-DAD) of the reaction mixture using a water/acetonitrile gradient allowed monitoring the reaction progress. Coupling of high-performance liquid chromatography to solid-phase extraction combined with nuclear magnetic resonance (HPLC-SPE-NMR) was utilized to generate highly reproducible 1H and 19F NMR data needed as input for the absolute configuration determination based on the analysis of relative shift differences. Chromatographic peaks of reaction substrates and reaction products bearing less 10 μg analyte were trapped on SPE cartridges with the aid of water as makeup solvent. Deuterated chloroform was used to elute and transfer the peak content from the SPE to the 60 μl flow cell of a 500 MHz NMR spectrometer. For each analyte 1H NMR spectra were obtained within 15 min. Additionally 19F NMR spectra were recorded for selected analytes in the same timeframe. Based on the obtained NMR data, the absolute configuration of all polyynes under investigation was successfully designated.

Mechanistic studies on enzymatic reactions by electrospray ionization MS using a capillary mixer with adjustable reaction chamber volume for time-resolved measurements.

Mass spectrometry (MS)-based techniques have enormous potential for kinetic studies on enzyme-catalyzed processes. In particular, the use of electrospray ionization (ESI) MS for steady-state measurements is well established. However, there are very few reports of MS-based studies in the pre-steady-state regime, because it is difficult to achieve the time resolution required for this type of experiment. We have recently developed a capillary mixer with adjustable reaction chamber volume for kinetic studies by ESI-MS with millisecond time resolution (Wilson, D. J.; Konermann, L. Anal. Chem. 2003, 75, 6408-6414). Data can be acquired in kinetic mode, where the concentrations of selected reactive species are monitored as a function of time, or in spectral mode, where entire mass spectra are obtained for selected reaction times. Here, we describe the application of this technique to study the kinetics of enzyme reactions. The hydrolysis of p-nitrophenyl acetate by chymotrypsin was chosen as a simple chromophoric model system. On-line addition of a "makeup solvent" immediately prior to ionization allowed the pre-steady-state accumulation of acetylated chymotrypsin to be monitored. The rate constant for acetylation, as well as the dissociation constant of the enzyme-substrate complex obtained from these data, is in excellent agreement with results obtained by conventional stopped-flow methods. Bradykinin was chosen to illustrate the performance of the ESI-MS-based method with a nonchromophoric substrate. In this case, the unfavorable rate constant ratio for acylation and deacylation of the enzyme precluded measurements in the pre-steady-state regime. Steady-state experiments were carried out to determine the turnover number and the Michaelis constant for bradykinin. The methodologies used in this work open a wide range of possibilities for future ESI-MS-based kinetic assays in enzymology.

Laser Vaporization/Ionization Interface for Capillary Electrophoresis−Time-of-Flight Mass Spectrometry

Mass spectrometry (MS) is usually coupled on-line with capillary electrophoresis (CE) to analyze biomolecules by using electrospray ionization or continuous-flow fast-atom bombardment. We present a new design for laser vaporization/ionization time-of-flight mass spectrometry. CE, with its low flow rate (<1 μL/min), is highly compatible with MS, even if the total column effluent is introduced directly. A UV laser is used to vaporize and ionize the solution eluting from the column. There is no need to have a makeup solvent. Using this system, we have analyzed a group of amines and peptides. The concentration detection limit of serotonin is in the 10(-)(7) M level. The separation and identification of an amine mixture by CE/MS demonstrates the complementary nature of the information.

Narrow-bore liquid chromatography-tandem mass spectrometry with simultaneous radioactivity monitoring for partially characterizing the biliary metabolites of an arginine fluoroalkyl ketone analog of D-MePhe-Pro-Arg, a potent thrombin inhibitor.

Characterizing components eluting from a HPLC column is enhanced when multiple detectors are incorporated in-line. The performance of a system consisting of a combination of two detectors-electrospray ionization mass spectrometry (and tandem mass spectrometry) and radioactivity monitoring, following gradient separation with a 250 x 2.1 mm I.D. (Vydac Protein and Peptide C18, 5 microns, 300 A) column-is evaluated with respect to chromatographic integrity and detection. The HPLC effluent was split (8:1) and a post-column make-up solvent was added to flow directed towards the radioactivity detector containing a solid glass cell. Trifluoracetic acid (0.1%) was added to the make-up flow solvent to prevent silanol interactions from degrading the profile displayed in the 14C trace. A 14C chromatographic peak representing 550 dpm was detected with signal-to-noise ratio of 3. This system was used for rapidly characterizing the biliary metabolites of an arginine fluoroalkyl ketone analog of D-MePhe-Pro-Arg, a potent thrombin inhibitor currently being evaluated as a drug candidate. These metabolites are shown to comprise of mono- and dihydroxylated drug as well as a reduced ketone form of the drug. Combining the radioactivity monitor in-line with the mass spectrometer ensured that all of the major metabolites (as evident from the 14C profile) were characterized by mass spectrometry.

The effect of valley and ridge on composition profile in an azeotropic continuous distillation column.

Azeotropic mixtures have a strong non-linearity of vapor-liquid equilibria. The non-linearity called the valley or ridge imposes restrictions on the composition profiles of distillation columns. The valley and ridge make it difficult to determine the system structure, the quantity and the location of make-up solvent. In other words, when designing an azeotropic distillation system, engineers should know the relationships between a valley and ridge and composition profiles. But, since the definition of valley and ridge is vague, these relationships have not been known but are merely interpreted empirically. If these relationships are made clear, it will become easy to select a solvent and to design azeotropic distillation systems. In this paper the above-mentioned relationships for homogeneous and heterogeneous azeotropic distillation systems are presented by experiment, and the effective use of a decanter is discussed on the basis of the relationship between a valley and liquid-liquid tie lines.