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Dietrich A Volmer

Publications and source records attributed to Dietrich A Volmer.

At least 19 recordsLinked to original sources
PubMed2007

Infrared multiphoton dissociation of the siderophore enterobactin and its Fe(III) complex. Influence of Fe(III) binding on dissociation kinetics and relative energetics.

The dissociation pathways of the siderophore enterobactin and its complex with Fe(III) were examined using infrared multiphoton dissociation (IRMPD). Under experimental conditions (pH = 3.5), both compounds' electrospray spectra exhibited exclusively singly-charged anions. The compositions of the dissociation products were characterized by accurate mass measurements using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The primary dissociation channel for both species was determined to be the loss of one serine group from the precursor molecules. To further investigate the influence of Fe(III) binding on the intramolecular interactions, dissociation kinetics and relative energetics for the loss of this serine group were determined using the focused radiation for gaseous multiphoton energy-transfer (FRAGMENT) method. From the kinetic data, it was found that enterobactin was approximately seven times more reactive than its Fe(III) complex over the range of laser intensities investigated. The relative activation energies, however, exhibited similar values, approximately 7 kcal.mol(-1). These results suggest that at pH = 3.5, Fe(III) interacts with only two of the three serine groups. The results from the present work are believed to be valuable for the characterization of novel siderophores as well as their associated metabolites and synthetic analogues.

Enterobactin↗
PubMed2006

Different iron-chelating properties of pyochelin diastereoisomers revealed by LC/MS.

Pyochelin is a siderophore and virulence factor common to Burkholderia cepacia and several Pseudomonas strains. It is isolated from bacterial media as a mixture of two epimers, which readily equilibrate in most solvents. Experiments based on high-performance liquid chromatography/electrospray ionization mass spectrometry are reported here, allowing the investigation of the different Fe(III)-chelating properties of pyochelin diastereomers in solution without the need for labourious isolation. It is demonstrated in this study that only one of the two pyochelin diastereomers is able to chelate Fe(III); no Fe(III) complexes of the other diastereomer could be detected. The Fe(III)-pyochelin complex exhibited a 1:1 metal-to-siderophore ratio and no evidence for other stoichiometries was found.

Chromatography, Liquid↗
PubMed2006

Assessing the properties of internal standards for quantitative matrix-assisted laser desorption/ionization mass spectrometry of small molecules.

Growing interest in the ability to conduct quantitative assays for small molecules by matrix-assisted laser desorption/ionization (MALDI) has been the driving force for several recent studies. This present work includes the investigation of internal standards for these analyses using a high-repetition rate MALDI triple quadrupole instrument. Certain physicochemical properties are assessed for predicting possible matches for internal standards for different small molecules. The importance of similar molecular weight of an internal standard to its analyte is seen through experiments with a series of acylcarnitines, having a fixed charge site and growing alkyl chain length. Both acetyl- and hexanoyl-carnitine were systematically assessed with several other acylcarnitine compounds as internal standards. The results clearly demonstrate that closely matched molecular weights between analyte and internal standard are essential for acceptable quantitation results. Using alpha-cyano-4-hydroxycinnamic acid as the organic matrix, the similarities between analyte and internal standard remain the most important parameter and not necessarily their even distribution within the solid sample spot. Several 4-quinolone antibiotics as well as a diverse group of pharmaceutical drugs were tested as internal standards for the 4-quinolone, ciprofloxacin. Quantitative results were shown using the solution-phase properties, log D and pKa, of these molecules. Their distribution coefficients, log D, are demonstrated as a fundamental parameter for similar crystallization patterns of analyte and internal standard. In the end, it was also possible to quantify ciprofloxacin using a drug from a different compound class, namely quinidine, having a similar log D value as the analyte.

Carnitine↗
PubMed2006

Mass spectral characterization of phloroglucinol derivatives hyperforin and adhyperforin.

Active phloroglucinol constituents of Hypericum perforatum (St. John's wort) extracts, hyperforin and adhyperforin, have been studied following ion activation using tandem mass spectrometry (MS/MS) and complemented by accurate mass measurements. These two compounds were readily analyzed as protonated and deprotonated molecules with electrospray ionization. MS/MS and MS3 data from a quadrupole-linear ion trap tandem mass spectrometer were employed to elucidate fragmentation pathways. Fourier transform ion cyclotron resonance measurements afforded excellent mass accuracies for the confirmation of elemental formulae of product ions formed via infrared multiphoton dissociation and sustained off-resonance irradiation collision-induced dissociation. Fragmentation schemes have been devised for the dissociation of hyperforin and adhyperforin in negative and positive ion modes. This information is expected to be especially valuable for the characterization of related compounds, such as degradation products, metabolites and novel synthetic analogs of hyperforin.

Bridged Bicyclo Compounds↗
PubMed2005

Toxin screening in phytoplankton: detection and quantitation using MALDI triple quadrupole mass spectrometry.

The investigation of a MALDI triple quadrupole instrument for the analysis of spirolide toxins in phytoplankton samples is described in this study. A high-frequency (kHz) laser was employed for MALDI, generating a semicontinuous ion beam, thus taking advantage of the high duty cycle obtained in sensitive triple quadrupole MRM experiments. Initially, several experimental parameters such as type of organic matrix and concentration, solvent composition, and matrix-to-analyte ratio were optimized, and their impact on sensitivity and precision of the obtained ion currents for a reference spirolide, 13-desmethyl-C, was studied. In all quantitative experiments, excellent linearities in the concentration range between 0.01 and 1.75 microg/mL were obtained, with R2 values of 0.99 or higher. The average precision of the quantitative MALDI measurements was 7.4+/-2.4% RSD. No systematic errors were apparent with this method as shown by a direct comparison to an electrospray LC/MS/MS method. Most importantly, the MALDI technique was very fast; each sample spot was analyzed in less than 5 s as compared to several minutes with the electrospray assay. To demonstrate the potential of the MALDI triple quadrupole method, its application to quantitative analysis in several different phytoplankton samples was investigated, including crude extracts and samples from mass-triggered fractionation experiments. 13-Desmethyl spirolide C was successfully quantified in these complex samples at concentration levels from 0.05 to 90.4 microg/mL (prior to dilution to have samples fall within the dynamic range of the method) without extensive sample preparation steps. The versatility of the MALDI triple quadrupole method was also exhibited for the identification of unknown spirolide analogues. Through the use of dedicated linked scan functions such as precursor ion and neutral loss scans, several spirolide compounds were tentatively identified directly from the crude extract, without the usual time-consuming chromatographic preseparation steps. Moreover, high-quality CID spectra were obtained for low-abundant spirolides present in the phytoplankton samples.

Animals↗
PubMed2005

Periodicity in collision-induced and remote-bond activation of alkali metal ions attached to polyether ionophores.

The collision-induced dissociation of lasalocid and monensin A bound to alkali metal cations has been investigated using electrospray MS/MS. The binding affinity for the metal cations, as measured with collision-induced dissociation, was found to depend on their ionic radius, decreasing with increasing radius. Monensin A was observed to have a greater binding affinity for alkali metal cations than lasalocid acid.

Animals↗
PubMed2005

Rapid identification of siderophores by combined thin-layer chromatography/matrix-assisted laser desorption/ionization mass spectrometry.

The investigation of a combined thin-layer chromatography/matrix-assisted laser desorption/ionization mass spectrometry (TLC/MALDI-MS) method for the analysis of siderophores from microbial samples is described. The investigated siderophores were enterobactin, ferrioxamine B, ferrichrome, ferrirhodin, rhodotorulic acid and coprogen. Solid-phase extraction was employed to recover the siderophores from the microbial samples. After visualization of the spots via spraying with ferric chloride or chrome azurol sulfonate assay solution, the MALDI matrix was applied to the gel surface. Several TLC/MALDI experimental parameters were optimized, such as type and concentration of MALDI matrix, as well as the type and composition of solvent to facilitate analyte transport from the inside of the TLC gel to the surface. The impact of these parameters on sensitivity, precision and ion formation of the various siderophores was studied. The detection limits for the investigated siderophores were in the range 1-4 pmol. These values were about 4-24 times higher than the detection limits obtained directly from stainless steel MALDI targets. The differences were most likely due to incomplete transport of the 'trapped' analyte molecules from the deeper layers of the TLC gel to the surface and into the matrix layer. In addition, chromatographic band broadening spread the analyte further in TLC as compared with the steel plates, resulting in less analyte per surface area. The identification of the siderophores was aided by concurrently applying a Ga(III) nitrate solution to the TLC plate during the visualization step. The resulting formation of Ga(III) complexes lead to distinctive (69)Ga/(71)Ga isotope patterns in the mass spectra. The versatility of the TLC/MALDI-MS assay was demonstrated by using it to analyze siderophores in a Pseudomonas aeruginosa sample. An iron-binding compound was identified in the sample, namely pyochelin (2-(2-o-hydroxyphenyl-2-thiazolin-4-yl)-3-methylthiazolidine-4-carboxylic acid).

Chromatography, High Pressure Liquid↗
PubMed2005

Some fundamental and technical aspects of the quantitative analysis of pharmaceutical drugs by matrix-assisted laser desorption/ionization mass spectrometry.

The purpose of the present paper was to study some of the underlying physical and technical aspects of high-throughput quantitative matrix-assisted laser desorption/ionization (MALDI) of small drug molecules. A prototype MALDI-triple quadrupole instrument equipped with a high repetition rate laser was employed. Initially, the detection limits and dynamic ranges for the quantitation of four drugs (quinidine, danofloxacin, ramipril and nadolol) were determined. Internal standards were carefully chosen for each of these analytes in terms of structure similarity and fragmentation pathways. Three organic matrices were tested for these assays, resulting in different crystallization behaviors and measurement reproducibilities. alpha-Cyano-4-hydroxycinnamic acid yielded the best results and was subsequently employed for the quantitative determination of all four analytes. Further experiments considered the role of laser energy and pulse rate on the ablated areas as well as ion signals. Light microscope and scanning electron microscope images allowed the examination of the ablated area of the MALDI spots. The images showed convincing evidence that the ablated area was virtually void of crystals after analysis, with no preferential removal of material in the center of the laser's path. Average values for the amount of material ablated were determined to be 3.9+/-0.5% of the total spot size, and as low as 19.5 attomoles of analyte were detectable for our most sensitive analyte, ramipril. It was calculated that, under these assay conditions, it was possible to accurately quantify less than 1 femtomole of all analytes with the use of appropriately pure internal standards. These studies showed very promising results for the quantitative nature of MALDI for small molecules with molecular weights less than 500 Da.

Adrenergic alpha-Antagonists↗
PubMed2005

Assigning product ions from complex MS/MS spectra: the importance of mass uncertainty and resolving power.

This study offers a unique insight into the mass accuracy and resolving power requirements in MS/MS analyses of complex product ion spectra. In the examples presented here, accurate mass assignments were often difficult because of multiple isobaric interferences and centroid mass shifts. The question then arose whether the resolving power of a medium-resolution quadrupole time-of flight (QqTOF) is sufficient or high-resolution Fourier-transform ion cyclotron resonance (FT-ICR) is required for unambiguous assignments of elemental compositions. For the comparison, two paralytic shellfish poisons (PSP), saxitoxin (STX) and neosaxitoxin (NEO), with molecular weights of 299 and 315 g x mol(-1), respectively, were chosen because of the high peak density in their MS/MS spectra. The assessment of QqTOF collision-induced dissociation spectra and FT-ICR infrared multiphoton dissociation spectra revealed that several intrinsic dissociation pathways leading to isobaric fragment ions could not be resolved with the QqTOF instrument and required FT-ICR to distinguish very close mass differences. The second major source of interferences was M + 1 species originating from coactivated 13C12Cc-1 ion contributions of the protonated molecules of the PSPs. The problem in QqTOF MS results from internal mass calibration when the MH+ ions of analyte and mass calibrant are activated at the same time in the collision or trapping cell. Although FT-ICR MS readily resolved these interfering species, the QqTOF did not provide resolving power >20,000 (full width at half maximum) required to separate most isobaric species. We were able to develop a semi-internal QqTOF calibration technique that activated only the isolated 12C isotope species of the protonated molecules, thus reducing the M + 1 interferences significantly. In terms of overall automated elemental formulas assignment, FT-ICR MS achieved the first formula hit for 100% of the product ions, whereas the QqTOF MS hit rate was only 56 and 65% for STX and NEO product ions, respectively. External mass calibration from commercial FT-ICR and QqTOF instruments gave similar results.

Calibration↗
PubMed2005

A comparison of data analysis methods for determining gas phase stabilities by CID: alkali metal complexes of polyether ionophore antibiotics.

The gas phase stabilities of Group I metal complexes of the polyether ionophore antibiotics lasalocid and monensin were investigated by collision induced dissociation mass spectrometry. Electrospray ionization was used with a triple quadrupole mass spectrometer for the determination of threshold dissociation energies upon application of increasing collision energies. Various data analysis techniques for the determination of dissociation energies are discussed to assess the most suitable method for determining the stabilities of the ionophore-metal complexes studied here. In all cases only the relative stabilities of different complexes may be obtained by the method presented in this study, which does not assess absolute gas phase dissociation energies. Correction factors have been applied, however, to account for the energy conversion during collisions of different metal complexes and the varying degrees of freedom of different sized ligands, allowing for the comparison of the stabilities of different ionophores with like-metals. The measured threshold dissociation energies were compared with respect to the ionic radius of the metal cation, revealing a maximum stability for the K+ complexes of both lasalocid and monensin. A striking decrease in the stabilities of the Rb+ and Cs+ complexes was observed and is believed to be related to a decreasing degree of coordination that the ionophores can accomplish with the larger metals.

Algorithms↗
PubMed2004

Evaluation of a cyano stationary phase for the determination of tacrolimus, sirolimus and cyclosporin A in whole blood by high-performance liquid chromatography-tandem mass spectrometry.

The potential of a cyano HPLC column for the analysis of three immunosuppressants is investigated. Tacrolimus, sirolimus and cyclosporin A, were used to probe differences in the retention and efficiency of a cyano column compared to the more widely used C18 column. The cyano column showed comparable retention for all three compounds, whereas the C18 column showed stronger retention, especially for cyclosporin A. Furthermore, the efficiencies at 50 degrees C were up to 12 times higher on the cyano column. As a result, a baseline separation was achieved in less than three minutes with the cyano column, using an isocratic mobile phase of 52/48 (v/v) acetonitrile/water at 0.45 mL/min. The analysis of immunosuppressant drugs in human whole blood was performed with the cyano column using a selected reaction monitoring (SRM) method for each analyte with negative ion mode electrospray ionization on a triple quadrupole mass spectrometer. Detection limits were 0.05 ng/mL for sirolimus, 0.1 ng/mL for cyclosporin A and 0.2 ng/mL for tacrolimus. Calibration curves were linear over three orders of magnitude. Good agreement was obtained with the actual levels of immunosuppressant drugs in patient samples with an average error of less than 10%.

Chromatography, High Pressure Liquid↗
PubMed2004

Hyperforin modifies neuronal membrane properties in vivo.

Hyperforin, the major active constituent of St. John Wort (SJW) extract, affects several neurotransmitter systems in the brain putatively by modulation of the physical state of neuronal membranes. Accordingly, we tested the effects of SJW extract and of hyperforin on the properties of murine brain membrane fluidity. Oral administration of SJW extract and of hyperforin sodium salt results in significant hyperforin brain levels. Treatment of mice with hyperforin leads to decreased annular- and bulk fluidity and increased acyl-chain flexibility of brain membranes. All hyperforin related changes of membrane properties were significantly correlated with the corresponding hyperforin brain levels. Our data emphasises a membrane interaction of hyperforin that possibly contributes to its pharmacological effects.

Administration, Oral↗
PubMed2004

Ion activation methods for tandem mass spectrometry.

This tutorial presents the most common ion activation techniques employed in tandem mass spectrometry. In-source fragmentation and metastable ion decompositions, as well as the general theory of unimolecular dissociations of ions, are initially discussed. This is followed by tandem mass spectrometry, which implies that the activation of ions is distinct from the ionization step, and that the precursor and product ions are both characterized independently by their mass/charge ratios. In collision-induced dissociation (CID), activation of the selected ions occurs by collision(s) with neutral gas molecules in a collision cell. This experiment can be done at high (keV) collision energies, using tandem sector and time-of-flight instruments, or at low (eV range) energies, in tandem quadrupole and ion trapping instruments. It can be performed using either single or multiple collisions with a selected gas and each of these factors influences the distribution of internal energy that the activated ion will possess. While CID remains the most common ion activation technique employed in analytical laboratories today, several new methods have become increasingly useful for specific applications. More recent techniques are examined and their differences, advantages and disadvantages are described in comparison with CID. Collisional activation upon impact of precursor ions on solid surfaces, surface-induced dissociation (SID), is gaining importance as an alternative to gas targets and has been implemented in several different types of mass spectrometers. Furthermore, unique fragmentation mechanisms of multiply-charged species can be studied by electron-capture dissociation (ECD). The ECD technique has been recognized as an efficient means to study non-covalent interactions and to gain sequence information in proteomics applications. Trapping instruments, such as quadrupole ion traps and Fourier transform ion cyclotron resonance instruments, are particularly useful for the photoactivation of ions, specifically for fragmentation of precursor ions by infrared multiphoton dissociation (IRMPD). IRMPD is a non-selective activation method and usually yields rich fragmentation spectra. Lastly, blackbody infrared radiative dissociation is presented with a focus on determining activation energies and other important parameters for the characterization of fragmentation pathways. The individual methods are presented so as to facilitate the understanding of each mechanism of activation and their particular advantages and representative applications.

Chemistry, Clinical↗
PubMed2004

Gas-phase dissociation reactions of protonated saxitoxin and neosaxitoxin.

The aim of this study was to investigate the behavior of the protonated paralytic shellfish poisons saxitoxin (STX) and neosaxitoxin (NEO) in the gas-phase after ion activation using different tandem mass spectrometry techniques. STX and NEO belong to a group of neurotoxins produced by several strains of marine dinoflagellates. Their chemical structures are based on a tetrahydropurine skeleton to which a 5-membered ring is fused. STX and NEO only vary in their substituent at N-1, with STX carrying hydrogen and NEO having a hydroxyl group at this position. The collision-induced dissociation (CID) spectra exhibited an unusually rich variety and abundance of species due to the large number of functional groups within the small skeletal structures. Starting with triple-quadrupole CID spectra as templates, linked ion-trap MSn data were added to provide tentative dissociation schemes. Subsequent high-resolution FTICR experiments gave exact mass data for product ions formed via infrared multiphoton dissociation (IRMPD) from which elemental formulas were derived. Calculations of proton affinities of STX and NEO suggested that protonation took place at the guanidinium group in the pyrimidine ring for both molecules. Most of the observed parallel and consecutive fragmentations could be rationalized through neutral losses of H2O, NH3, CO, CO2, CH2O and different isocyanate, ketenimine and diimine species, many of which were similar for STX and NEO. Several exceptions, however, were noted and differences could be readily correlated with reactions involving NEO's additional hydroxyl group. A few interesting variations between CID and IRMPD spectra are also highlighted in this paper.

Animals↗
PubMed2003

Structural study of spirolide marine toxins by mass spectrometry. Part I. Fragmentation pathways of 13-desmethyl spirolide C by collision-induced dissociation and infrared multiphoton dissociation mass spectrometry.

A novel group of toxins, the spirolides, has been investigated by several mass spectrometric (MS) methods to enable structure elucidation and metabolite identification. These macrocyclic compounds, produced by the dinoflagellate Alexandrium ostenfeldii, are a new class of marine phycotoxin with characteristic spiro-linked tricyclic ether and imine moieties. A crude phytoplankton extract has been shown to contain known spirolides and several unknown compounds, present at low yet significant levels. This study has focused on mass spectrometric characterization of the main component of this extract, 13-desmethyl spirolide C. Collision-induced dissociation (CID) spectra were collected on triple-quadrupole and quadrupole linear ion-trap instruments. High-resolution Fourier-transform ion cyclotron resonance MS data revealed the accurate masses of the protonated molecule and the product ions formed by infrared multiphoton dissociation. A fragmentation scheme for this toxin has been proposed to explain the formation of the collision-induced fragments. Charge-remote fragmentations dominate the CID spectra, because there is only one predominantly basic site in this molecule, and prove to be structurally informative. Extensive MS characterization of 13-desmethyl spirolide C will undoubtedly be useful in the characterization of known and unknown spirolides and other related compounds.

Animals↗
PubMed2003

Structural study of spirolide marine toxins by mass spectrometry. Part II. Mass spectrometric characterization of unknown spirolides and related compounds in a cultured phytoplankton extract.

The spirolides are a family of marine biotoxins derived from the dinoflagellate Alexandrium ostenfeldii, recently isolated from contaminated shellfish and characterized. A crude phytoplankton extract has been extensively studied for mass spectrometric determination and characterization of several known spirolides and previously unreported compounds. The complex sample was initially analyzed by full-scan mass spectrometry in an ion-trap instrument, enabling identification of several components. Subsequent analysis by selected-ion monitoring in a triple-quadrupole instrument resulted in the confirmation of the identities of the compounds detected in the ion trap. Purification of the crude extract was performed using an automated mass-based fractionation system, yielding several fractions with different relative contributions of the spirolide components. Collision-induced dissociation (CID) in the triple-quadrupole instrument produced significant fragment ions for all identified species. Selective enrichment of some minor compounds in certain fractions enabled excellent CID spectra to be generated; this had previously been impossible, because of interferences from the major toxins present. Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry was then performed for accurate determination of the masses of MH+ ions of all the species present in the sample. Additionally, infrared multiphoton dissociation in the FTICR instrument generated elemental formulae for product ions, including those formed in the previous collisional activation experiments. Collection of these results and the fragmentation scheme proposed for the main component of the extract, 13-desmethyl spirolide C, from part I of this study, enabled elucidation of the structures of some uncharacterized spirolides and some biogenetically related compounds present at previously unreported masses.

Animals↗