Deuterium effect on ionization and fragmentation patterns of monosaccharides ionized by atmospheric pressure chemical ionization

Characterization of the fragmentation behaviors of protonated α-cyclodextrin generated by electrospray ionization

RATIONALE

Electrospray ionization (ESI) of an aqueous solution of α-cyclodextrin (α-CD) gave a protonated molecule, [α-CD + H]⁺ . The fragmentation behavior of the protonated molecule, including direct decomposition and ring cleavage, was investigated by varying the source fragmentor voltage. The possible chemical structures of the product ions were also examined using electronic structure calculations.

METHODS

An aqueous α-CD solution was ionized by ESI and the source fragmentor voltage was varied to examine changes of the product ions depending on collision energy. The structures and energies of the precursor and product ions were obtained by electronic structure calculations using Spartan'10 and Gaussian09.

RESULTS

The major product ions were [M + H - 162m]⁺ (where m = 1-5), with the most abundant being [M + H - 162 × 4]⁺ . The product ions had two chemical structures of the cationic site in the ether linkage ([DPDn - OH]⁺ ) and in the terminal oxonium ion ([DPn - OH]⁺ ) formed by direct decomposition of [α-CD + H]⁺ and fragmentation of the open structure ([DP6 - OH]⁺ ), respectively. The [DP6 - OH]⁺ ion is more stable than the [α-CD + H]⁺ ion.

CONCLUSIONS

The fragmentation behavior of protonated α-CD was characterized by two pathways: direct decomposition of [α-CD + H]⁺ and decomposition of the [DP6 - OH]⁺ open structure. Differences in the relative abundances of product ions were explained by the different fragmentation pathways of [α-CD + H]⁺ and [DP6 - OH]⁺ .

Laser Ablation Electrospray Ionization Hydrogen/Deuterium Exchange Ambient Mass Spectrometry Imaging

Identification and confirmation of known as well as unknown (bio)chemical entities in ambient mass spectrometry (MS) and MS imaging (MSI) mostly involve accurate mass determination, often in combination with MS/MS or MSⁿ work flows. To further improve structural assignment, additional molecular information is required. Here we present an ambient hydrogen/deuterium exchange (HDX) laser ablation electrospray ionization (LAESI) MS method in which, apart from the accurate mass and MS/MS data, the number of exchangeable protons in (un)known molecules is obtained. While eventually presenting ambient HDX-LAESI-MSI, samples were not preincubated with deuterated solvents, but instead HDX occurred following fusion of ablated sample material with microdroplets generated by ESI of deuterated solvents. Therefore, the degree of HDX was first studied following ablation of nondeuterated sample solutions of melamine and monosaccharides. From these experiments, it was concluded that the set-up used could provide meaningful HDX data in support of molecular structure elucidation by significantly reducing the number of structure options from a measured elemental composition. This reduction was demonstrated with an unknown accurate m/z value obtained in the analysis of an orange slice, reducing the possible number of molecular structures having the same elemental composition by 87% due to the number of H/D exchanges observed. Next, deuterated and nondeuterated MS/MS experiments showed the number of exchangeable protons in the substructures from deuterated neutral losses in the product ion spectra, confirming the compound to be arginine. Finally, the potential of ambient HDX-LAESI-MSI was demonstrated by the imaging of (secondary) plant metabolites in a Phalaenopsis petal.

Hydrogen/deuterium exchange in mass spectrometry

The isotopic exchange approach is in use since the first observation of such reactions in 1933 by Lewis. This approach allows the investigation of the pathways of chemical and biochemical reactions, determination of structure, composition, and conformation of molecules. Mass spectrometry has now become one of the most important analytical tools for the monitoring of the isotopic exchange reactions. Investigation of conformational dynamics of proteins, quantitative measurements, obtaining chemical, and structural information about individual compounds of the complex natural mixtures are mainly based on the use of isotope exchange in combination with high resolution mass spectrometry. The most important reaction is the Hydrogen/Deuterium exchange, which is mainly performed in the solution. Recently we have developed the approach allowing performing of the Hydrogen/Deuterium reaction on-line directly in the ionization source under atmospheric pressure. Such approach simplifies the sample preparation and can accelerate the exchange reaction so that certain hydrogens that are considered as non-labile will also participate in the exchange. The use of in-ionization source H/D exchange in modern mass spectrometry for structural elucidation of molecules serves as the basic theme in this review. We will focus on the mechanisms of the isotopic exchange reactions and on the application of in-ESI, in-APCI, and in-APPI source Hydrogen/Deuterium exchange for the investigation of petroleum, natural organic matter, oligosaccharides, and proteins including protein-protein complexes. The simple scenario for adaptation of H/D exchange reactions into mass spectrometric method is also highlighted along with a couple of examples collected from previous studies.

Optimization and Application of APCI Hydrogen-Deuterium Exchange Mass Spectrometry (HDX MS) for the Speciation of Nitrogen Compounds

A systematic study was performed to investigate the utility of atmospheric pressure chemical ionization hydrogen-deuterium exchange mass spectrometry (APCI HDX MS) to identify the structures of nitrogen-containing aromatic compounds. First, experiments were performed to determine the optimized experimental conditions, with dichloromethane and CH(3)OD found to be good cosolvents for APCI HDX. In addition, a positive correlation between the heated capillary temperature and the observed HDX signal was observed, and it was suggested that the HDX reaction occurred when molecules were contained in the solvent cluster. Second, 20 standard nitrogen-containing compounds were analyzed to investigate whether speciation could be determined based on the different types of ions produced from nitrogen-containing compounds with various functional groups. The number of exchanges occurring within the compounds correlated well with the number of active hydrogen atoms attached to nitrogen, and it was confirmed that APCI HDX MS could be used to determine speciation. The results obtained by APCI HDX MS were combined with the subsequent investigation of the double bond equivalence distribution and indicated that resins of shale oil extract contained mostly pyridine type nitrogen compounds. This study confirmed that APCI HDX MS can be added to previously reported chemical ionization, electrospray ionization, and atmospheric pressure photo ionization-based HDX methods, which can be used for structural elucidation by mass spectrometry.

Formation of deaminated dimer species of amino acids by atmospheric pressure chemical ionization

RATIONALE

Interactions of biological molecules to form cluster species play a key role in biological processes and investigation of non-covalent complexes is one of the research fields using mass spectrometry. Atmospheric pressure chemical ionization mass spectrometry (APCI-MS) is a useful method for the investigation of cluster formation of amino acids (AAs) by ion-molecule reactions.

METHODS

A mixture of 20 protein AAs was ionized by APCI and the product ions were analyzed. The ionization was performed in the positive and negative ion modes. Formation of the homo- and heterocluster ions of AAs was investigated. Mechanism for the formation of AA homo- and heterocluster ions was examined using hydrogen/deuterium (H/D)-exchange experiments.

RESULTS

In the positive ion mode, of the dimer species only the [2Pro+H](+) ion was detected. In the negative ion mode, the [2M - H](-) ions of His, Val, Ser, and Gln were observed. The deaminated dimers such as the [2Gln - H - NH3](-) and [His + Gln - H - NH3](-) ions were also observed. In the negative ion mass spectra of the His/Arg, His/Asn, and His/Lys binary mixture solutions, the [His + AA - H - NH3](-) ions of Asn, Arg, and Lys were also detected.

CONCLUSIONS

The number and abundances of the negative product ions were much greater than those of the positive ones. Mechanism for the formation of [2Gln - H - NH3](-) and [His+AA - H - NH3](-) was examined by deuterium replacement of the amine and hydroxide groups to distinguish the deamination and dehydration reactions with a single quadrupole mass spectrometer. The [His + AA - H - NH3](-) ion is formed by ion-molecule reaction between the [His-H](-) ion and a neutral AA.

Differentiation of underivatized monosaccharides by atmospheric pressure chemical ionization quadrupole time-of-flight mass spectrometry

RATIONALE

Differentiation of underivatized monosaccharides is essential in the structural elucidation of oligosaccharides which are closely involved in many life processes. So far, such differentiation has been usually achieved by electrospray ionization mass spectrometry (ESI-MS). As an alternative to ESI-MS, atmospheric pressure chemical ionization mass spectrometry (APCI-MS) should provide complementary results.

METHODS

A quadrupole time-of-flight (QTOF) mass spectrometer with accurate mass measurement ability was used with an APCI heated nebulizer ion source because we believe that a recently published article using a single quadrupole mass spectrometer assigned incorrect identities for APCI ions from hexoses. Using APCI-QTOF, the MS(2) and pseudo-MS(3) mass spectra of 11 underivatized monosaccharides were obtained under various collision voltages. The mass spectra were carefully interpreted after accurate mass measurement.

RESULTS

Differentiation of three hexoses was achieved by different MS(2) spectra of their [M + NH(4)](+) and [M - H](-) ions. The MS(2) spectra of the [M + NH(4)](+) ions were also used to distinguish methyl α-D-glucose and methyl β-D-glucose, while the pseudo-MS(3) spectra of the [M + H](+) ions were utilized to differentiate the three hexosamine and N-acetylhexosamine stereoisomers. Unique [M + O(2)](-) ions were observed and their distinctive fragmentation patterns were utilized to differentiate the three hexosamine stereoisomers.

CONCLUSIONS

Although ESI coupled with single or triple quadrupole and ion trap mass spectrometers has been widely utilized in the differentiation of monosaccharides, this report demonstrated that APCI-QTOF-MS had its own advantages in achieving the same goal.

Optimization of direct analysis in real time (DART) linear ion trap parameters for the detection and quantitation of glucose

Presented here are findings for the development and optimization of a simple, high-throughput, and rapid method for the analysis of glucose. Because the applications of glucose and other six-carbon sugars is a growing field of interest especially in the production of biofuels, an efficient and rapid method for their quantitation from lignocelluloses is necessary. Glucose was analyzed using direct analysis in real time (DART) ionization and formed adducts (along with fragmentation) were observed with a linear ion trap (LIT) mass spectrometer. Since DART can be considered a complex thermal desorption ionization process, an optimization study of the helium gas temperature and introduction into the ionization region was performed. It was observed these parameters have a significant effect on the overall signal intensity as well as the signal-to-noise ratios in DART mass spectra. Using these optimized parameters, a set of different glucose concentrations (ranging from 10 to 3000  μM) were analyzed and used to determine a linear dynamic range (with the use of an internal standard). The analysis of the samples was done with minimal sample preparation and found to be reproducible on different days.