Robert Erb

Mass spectrometric methods for monitoring redox processes in electrochemical cells

Electrochemistry (EC) is a mature scientific discipline aimed to study the movement of electrons in an oxidation–reduction reaction. EC covers techniques that use a measurement of potential, charge, or current to determine the concentration or the chemical reactivity of analytes. The electrical signal is directly converted into chemical information. For in-depth characterization of complex electrochemical reactions involving the formation of diverse intermediates, products and byproducts, EC is usually combined with other analytical techniques, and particularly the hyphenation of EC with mass spectrometry (MS) has found broad applicability. The analysis of gases and volatile intermediates and products formed at electrode surfaces is enabled by differential electrochemical mass spectrometry (DEMS). In DEMS an electrochemical cell is sampled with a membrane interface for electron ionization (EI)-MS. The chemical space amenable to EC/MS (i.e., bioorganic molecules including proteins, peptides, nucleic acids, and drugs) was significantly increased by employing electrospray ionization (ESI)-MS. In the simplest setup, the EC of the ESI process is used to analytical advantage. A limitation of this approach is, however, its inability to precisely control the electrochemical potential at the emitter electrode. Thus, particularly for studying mechanistic aspects of electrochemical processes, the hyphenation of discrete electrochemical cells with ESI-MS was found to be more appropriate. The analytical power of EC/ESI-MS can further be increased by integrating liquid chromatography (LC) as an additional dimension of separation. Chromatographic separation was found to be particularly useful to reduce the complexity of the sample submitted either to the EC cell or to ESI-MS. Thus, both EC/LC/ESI-MS and LC/EC/ESI-MS are common.

Association of polymorphisms in pharmacogenetic candidate genes (OPRD1, GAL, ABCB1, OPRM1) with opioid dependence in European population: a case-control study.

It is becoming increasingly evident that genetic variants contribute to the development of opioid addiction. An elucidation of these genetic factors is crucial for a better understanding of this chronic disease and may help to develop novel therapeutic strategies. In recent years, several candidate genes were implicated in opioid dependence. However, most study findings have not been replicated and additional studies are required before reported associations can be considered robust. Thus, the major objective of this study was to replicate earlier findings and to identify new genetic polymorphisms contributing to the individual susceptibility to opioid addiction, respectively. Therefore, a candidate gene association study was conducted including 142 well-phenotyped long-term opioid addicts undergoing opioid maintenance therapy and 142 well-matched healthy controls. In both study groups, 24 single nucleotide polymorphisms predominantly located in pharmacogenetic candidate genes have been genotyped using an accurate mass spectrometry based method. The most significant associations with opioid addiction (remaining significant after adjustment for multiple testing) were observed for the rs948854 SNP in the galanin gene (GAL, p = 0.001) and the rs2236861 SNP in the delta opioid receptor gene (OPRD1, p = 0.001). Moreover, an association of the ATP binding cassette transporter 1 (ABCB1) variant rs1045642 and the Mu Opioid receptor (OPRM1) variant rs9479757 with opioid addiction was observed. The present study provides further support for a contribution of GAL and OPRD1 variants to the development of opioid addiction. Furthermore, our results indicate a potential contribution of OPRM1 and ABCB1 SNPs to the development of this chronic relapsing disease. Therefore it seems important that these genes are addressed in further addiction related studies.

Ascorbic acid for homogenous redox buffering in electrospray ionization-mass spectrometry

AbstractElectrospray ionization (ESI) involves the dispersion of a liquid containing analytes of interest into a fine aerosol by applying a high potential difference to the sample solution with respect to a counter electrode. Thus, from the electrochemical point of view, the ESI source represents a two-electrode controlled-current electrochemical flow cell. The electroactive compounds part of the solvent sprayed may be altered by occurring electrolysis (oxidation in positive ion mode and reduction in negative ion mode). These reactions can be troublesome in the context of unknown identification and quantification. In the search for a simple, inexpensive, and efficient way to suppress electrochemical oxidation in positive ESI, the usability of ascorbic acid, hydroquinone, and glutathione for homogenous redox buffering was tested. Performance of the antioxidants was assessed by analyzing pharmaceutical compounds covering a broad range of functional groups prone to oxidation. Different emitter setups were applied for continuous infusion, flow injection, and liquid chromatography/mass spectrometry experiments. Best performance was obtained with ascorbic acid. In comparison to hydroquinone and glutathione, ascorbic acid offered superior antioxidant activity, a relatively inert oxidation product, and hardly any negative effect on the ionization efficiency of analytes. Furthermore, ascorbic acid suppressed the formation of sodiated forms and was able to induce charge state reduction. Only in the very special case of analyzing a compound isobaric to ascorbic acid, interference with the low-abundant [ascorbic acid+H]+ signal may become a point of attention. FigureAscorbic acid efficiently suppresses analyte oxidation and formation of sodiated forms in positive electrospray ionization

An optimized electrochemistry-liquid chromatography-mass spectrometry method for studying guanosine oxidation

Oxidative stress can disrupt the integrity of genetic material. Due to its importance in the pathogenesis of different kinds of disease, including neurodegenerative disease, cardiovascular disease and cancer, major efforts are put into the elucidation of mechanisms involved. Herein, the combination of electrochemistry/liquid chromatography/mass spectrometry (EC/LC/MS) is presented as convenient, fast and simple method to study nucleic acids oxidation. Guanosine was selected as test compound. 8‐Hydroxyguanosine and (guanosine‐H)2 were identified as primary oxidation products. Oxidation was accomplished in an electrochemical thin‐layer cell integrated in the flow path of the autosampler of the chromatographic system. The reaction mixture was separated and mass analyzed by LC/MS. The use of LC was found to be particularly beneficial to resolve isobaric oxidation products. Another advantage of the setup used was the ability to decouple the electrochemical cell and the electrospray ionization source from each other eliminating any kind of cell potential interaction. Separation of EC from LC/MS, furthermore, facilitates method optimization. Experimental parameters were optimized for both techniques independently. Highest yields and best detectability of oxidation products were obtained with 10 mM ammonium formate at physiological pH delivered at a flow rate of 2.5‐5 μL/min through the electrochemical cell.

CYP2D6 genotyping by liquid chromatography-electrospray ionization mass spectrometry

AbstractGenetic polymorphisms can significantly affect the enzyme activity of the drug metabolizing enzyme Cytochrome P450 2D6 (CYP2D6; OMIM 124030). Accordingly, CYP2D6 genotyping is considered as a valid approach to predict the individual CYP2D6 metabolizing status. We introduce ion-pair reversed-phase high-performance liquid chromatography-electrospray ionization mass spectrometry (ICEMS) as method for the characterization of single base variants, small deletions, and insertions in the CYP2D6 gene. A two-step polymerase chain reaction (PCR) was developed for the simultaneous amplification of nine polymorphic regions within the CYP2D6 gene. Cleanup, separation, and denaturation of PCR amplicons were achieved by high-performance liquid chromatography. High-performance molecular mass measurements provided nucleotide composition profiles that principally enable the resolution of 37 reported CYP2D6 alleles. The developed assay was applied to the genotyping of 93 unrelated Austrian individuals. For validation, a selected number of samples and polymorphic sites were retyped by alternative genotyping technologies. The PCR-ICEMS assay turned out to be an accurate, robust, and cost-effective CYP2D6 genotyping strategy. FigureOutline of the principal steps of the PCR-ICEMS assay developed for CYP2D6 genotyping. A two-step PCR is used for the simultaneous amplification of nine polymorphic regions within the CYP2D6 gene. PCR amplicons are analyzed by ion-pair reversed-phase high-performance liquid chromatography-electrospray ionization mass spectrometry (ICEMS). High-performance molecular mass measurements provide nucleotide composition profiles that enable the identification of CYP2D6 alleles

Formation and characterization of covalent guanosine adducts with electrochemistry—liquid chromatography–mass spectrometry ☆

Chemicals can interact with the genetic material giving rise to the formation of covalent adducts. These alterations can lead to adverse consequences, including cancer, reproductive impairment, development anomalies, or genetic diseases. In search for an assay allowing identification of hazardous compounds that might form covalent adducts with nucleic acids, electrochemistry (EC)/liquid chromatography (LC)/mass spectrometry (MS) is presented. EC/LC/MS is a purely instrumental approach. EC is used for oxidative activation, LC for the fractionation of the reaction mixture, and MS for the detection and characterization of the reaction products. To test the system capabilities, we investigated the formation of covalent adducts produced by guanosine and acetaminophen (APAP). Electrochemical activation of mixtures of guanosine and APAP gave rise to the formation of four isomers of (guanosine + APAP-2H). Mass voltammograms as well as dose–response-curves were used to obtain insights in the mechanism of adduct formation. These experiments revealed that a mechanism involving radical intermediates is favored. The initial step of adduct formation is the conversion of both APAP and guanosine into radicals via one-electron–one-proton reactions. Among different competing reaction pathways, the generated radical intermediates undergo intermolecular reactions to form covalent adducts between guanosine and APAP.

Comparison of mobile-phase systems commonly applied in liquid chromatography-mass spectrometry of nucleic acids.

LC‐MS represents an important technology for the qualitative and quantitative analysis of nucleic acids. For MS, ESI in negative ion mode is used. The chromatographic method of choice is ion‐pair (IP) RP chromatography. Chromatographic separations are usually accomplished by gradients of an organic modifier in aqueous solutions of IP reagents. Commonly applied IP reagents are 2.3 mM triethylamine/400 mM 1,1,1,3,3,3‐hexafluoro‐2‐propanol (TEA/HFIP, pH 7.0) and 10–25 mM cyclohexyldimethylammonium acetate (CycHDMAA, pH 8.4). Direct comparison of mass spectrometric performance of the two solvent systems revealed that the TEA/HFIP system offers better detection sensitivity than the CycHDMAA system. This is mainly attributable to the depletion of HFIP during droplet formation and solvent evaporation. Removal of the anionic counterion facilitates oligonucleotide ionization, and the oligonucleotides are desorbed as highly charged ions into the gas phase. TEA/HFIP‐based mobile phases are recommended for developing quantitative assays targeting defined oligonucleotides. The CycHDMAA system allows the formation of cyclohexyldimethylammonium adducts. These adducts are cleaved in the gas phase, and this decomposition gives rise to charge state reduction. Ammonium adduct formation is of particular importance in preventing adducting with metal ions. Thus, adducts with metal ions are efficiently suppressed with CycHDMAA. For the TEA/HFIP system, however, such adducting represents a severe problem particularly if large oligonucleotides are analyzed. Thus, CycHDMAA‐based mobile phases are recommended for qualitative assays such as LC‐MS‐based genotyping.

Studying the reducing potencies of antioxidants with the electrochemistry inherently present in electrospray ionization-mass spectrometry

AbstractIn this proof-of-principle study, the applicability of electrospray ionization-mass spectrometry (ESI-MS) to characterize the reducing potencies of natural antioxidants is demonstrated. The ESI source represents a controlled-current electrochemical cell. The interfacial potential at the emitter electrode will be at or near the electrochemical potential of those reactions that sufficiently supply all the required current for the ESI circuit. Indicator molecules prone to oxidation in ESI such as amodiaquine were used to visualize the impact of reducing compounds on the interfacial potential. The extent of inhibition of the oxidation of the indicator molecule was found to be dependent on the kind and amount of antioxidant added. Concentration–inhibition curves were constructed and used to compare reducing potencies and to rank antioxidants. This ranking was found to be dependent on the electrode material–indicator molecule combination applied. For fast and automated characterization of the reducing potencies of electrochemically active molecules, a flow-injection system was combined with ESI-MS. Liquid chromatography was used to process complex biological samples, such as red and white wine. Due to their high content of different polyphenols, red wine fractions were found to exhibit higher reducing potencies than the corresponding white wine fractions. Furthermore, for 14 important natural antioxidants, the results obtained with the controlled-current EC–ESI-MS assay were compared to those obtained with chemical antioxidant assays. Irrespectively of the kind of assay used to test the reducing potency, gallic acid, quercetin, and epicatechin were found to be potent reductants. Other antioxidants performed well in one particular assay only. This observation suggests that different kinds of redox and antioxidant chemistry were assessed with each of the assays applied. Therefore, several assays should be used to comprehensively study antioxidants and their reducing potencies. FigureFractions of a red wine sample were screened by ESI-MS for compounds showing reducing potency.

A novel amplification strategy for genotyping with liquid chromatography-electrospray ionization mass spectrometry

Among numerous available genotyping techniques, mass spectrometry (MS) based methods play a major role in providing high quality genotype data at reasonable costs for research and diagnostics, e.g. for pharmacogenetic applications. Ion-pair reversed-phase liquid chromatography hyphenated to electrospray ionization time-of-flight MS (ICEMS) is, for example, a powerful instrument that allows a direct characterization of complex mixtures of polymerase chain reaction (PCR) amplified DNA fragments. Current limitations of PCR-ICEMS genotyping are mainly concerned with the multiplex PCR set-up. Assay development often requires time-consuming primer design and intensive optimization of PCR conditions. To overcome this restraint, a robust amplification strategy originally combined with arrayed primer extension genotyping was transferred and adapted to ICEMS genotyping. The modifications involved limitation of the primer length, application of two universal sequences and amplification with an appropriate DNA polymerase. To demonstrate the applicability of the novel amplification strategy for ICEMS, a 23-plex pharmacogenetic genotyping assay was developed. After slight optimization steps, an efficient and quantitatively balanced amplification of all targeted markers was achieved, resulting in a convenient characterization of the multiplexed PCR fragments with ICEMS. Expenditure of time, costs and hands-on work associated with assay design and optimization was dramatically lowered compared to previous multiplex PCR-ICEMS assays. The developed 23-plex assay was applied in a pharmacogenetic study including 284 individuals (genotype call rate 99.0%). A total of 399 SNPs were retyped by Sanger sequencing (concordance rate 99.8%). The PCR-ICEMS assay turned out to be an accurate, reliable, cost-effective and a ready-to-use tool for pharmacogenetic genotyping.