Investigation of porphyrins and metalloporphyrins using two-step laser mass spectrometry

Characterization of Sulfur-Rich Microbial Organic Matter in Jurassic Carbonates Using Laser-Assisted Mass Spectrometry

Laser desorption-ionization mass spectrometry (MS) shows great potential for in situ molecular analysis of planetary surfaces and microanalysis of space-returned samples or (micro)fossils. Coupled with pyrolysis gas chromatography-mass spectrometry (Py-GC-MS) in ESA's ExoMars project, this technique could help assess further the origin of sulfur-bearing organic matter (OM) recently detected on Mars. To unravel this potential, we analyzed sulfurized microbial OM from ca. 150 million year-old carbonates with laser desorption-ionization mass spectrometry (single- and two-step: LDI-MS and L2MS), in comparison with time-of-flight secondary-ion mass spectrometry (ToF-SIMS), gas chromatography-mass spectrometry (GC-MS), and Py-GC-MS. We show that LDI-MS and L2MS readily detect sulfur-bearing moieties such as (alkyl)thiophenes and (alkyl)benzothiophenes. The mineral matrix, however, made the identification of sulfur-bearing molecules challenging in our L2MS experiment. The dominance of small aromatic hydrocarbons (≤14 carbons) in the LDI-MS and L2MS of the extracted soluble and insoluble OM and of the bulk rock is consistent with the low thermal maturity of the sediment and contrasts with the predominance of larger polycyclic aromatic structures commonly observed in meteorites with these techniques. We detected inorganic ions, in particular VO+, in demineralized OM that likely originate from geoporphyrins, which derive from chlorophylls during sediment diagenesis. Finally, insoluble OM yielded distinct compositions compared with extracted soluble OM, with a greater abundance of ions of mass-to-charge ratio (m/z) over 175 and additional N-moieties. This highlights the potential of laser-assisted MS to decipher the composition of macromolecular OM, in particular to investigate the preservation of biomacromolecules in microfossils. Studies comparing diverse biogenic and abiogenic OM are needed to further assess the use of this technique to search for biosignatures.

A new update of MALDI-TOF mass spectrometry in lipid research

Matrix-assisted laser desorption and ionization (MALDI) mass spectrometry (MS) is an indispensable tool in modern lipid research since it is fast, sensitive, tolerates sample impurities and provides spectra without major analyte fragmentation. We will discuss some methodological aspects, the related ion-forming processes and the MALDI MS characteristics of the different lipid classes (with the focus on glycerophospholipids) and the progress, which was achieved during the last ten years. Particular attention will be given to quantitative aspects of MALDI MS since this is widely considered as the most serious drawback of the method. Although the detailed role of the matrix is not yet completely understood, it will be explicitly shown that the careful choice of the matrix is crucial (besides the careful evaluation of the positive and negative ion mass spectra) in order to be able to detect all lipid classes of interest. Two developments will be highlighted: spatially resolved Imaging MS is nowadays well established and the distribution of lipids in tissues merits increasing interest because lipids are readily detectable and represent ubiquitous compounds. It will also be shown that a combination of MALDI MS with thin-layer chromatography (TLC) enables a fast spatially resolved screening of an entire TLC plate which makes the method competitive with LC/MS.

Direct and comprehensive analysis of dyes based on integrated molecular and structural information via laser desorption laser postionization mass spectrometry

Laser desorption laser postionization time-of-flight mass spectrometry (LDPI-TOFMS) was employed for direct analysis and determination of typical basic dyes. It was also used for the analysis and comprehensive understanding of complex materials such as blue ballpoint pen inks. Simultaneous emergences of fragmental and molecular information largely simplify and facilitate unambiguous identification of dyes via variable energy of 266nm postionization laser. More specifically, by optimizing postionization laser energy with the same energy of desorption laser, the structurally significant results show definite differences in the fragmentation patterns, which offer opportunities for discrimination of isomeric species with identical molecular weight. Moreover, relatively high spectra resolution can be acquired without the expense of sensitivity. In contrast to laser desorption/ionization mass spectrometry (LDI-MS), LDPI-MS simultaneously offers valuable molecular information about dyes in traces, solvents and additives about inks, thereby offering direct determination and comprehensive understanding of blue ballpoint inks and giving a high level of confidence to discriminate the complicated evidentiary samples. In addition, direct analysis of the inks not only allows the avoidance of the tedious sample preparation processes, significantly shortening the overall analysis time and improving throughput, but allows minimized sample consumption which is important for rare and precious samples.

Extensive fragmentation of pheophytin-a by infrared multiphoton dissociation tandem mass spectrometry

RATIONALE

In a previous tandem mass spectrometry (MS/MS) study on chlorophyll-a, infrared multiphoton dissociation (IRMPD) was demonstrated as a more effective fragmentation method than collision-induced dissociation (CID) and electron-induced dissociation (EID), where odd-electron product ions were observed ubiquitously in CID and IRMPD. To further understand the role of the macrocycle and the central Mg atom in the MS/MS process, the fragmentation behaviour of pheophytin-a, the Mg-free chlorophyll-a, was investigated.

METHODS

CID, IRMPD, and EID were applied to the singly protonated pheophytin-a using an ultra-high-resolution Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. The result is compared with the MS/MS study of chlorophyll-a.

RESULTS

For pheophytin-a, some different fragmentation patterns from chlorophyll-a were obtained by all three MS/MS methods, but IRMPD still appears the most efficient method of generating product ions. The detection of odd-electron fragments in the CID and IRMPD spectra of protonated pheophytin-a suggests that the macrocyclic structure effectively stabilizes radicals, and these radical ions seem to have a relatively higher abundance in the presence of the central Mg atom.

CONCLUSIONS

The strong absorption in the infrared region of pheophytin-a and secondary free radical rearrangement are proposed to explain the extensive frgmentation in IRMPD spectra. In addition, a comparison of the IRMPD spectra of chlorophyll-a and pheophytin-a shows that the macrocycle in the absence of the Mg atom is much more fragile.

Mass spectrometry for malaria diagnosis

A physical method currently being developed for malaria parasite detection and diagnosis in blood is reviewed in this article. The method - direct laser desorption mass spectrometry - is based on the detection of heme (iron protoporphyrin) as a unique qualitative and quantitative molecular biomarker for malaria. In infected erythrocytes, the parasite sequesters heme in a molecular crystal (hemozoin) - a volume of highly concentrated and purified biomarker molecules. Laser desorption mass spectrometry detects only heme from hemozoin in parasite-infected blood, and not heme that is bound to hemoglobin or other proteins in uninfected blood samples. The method requires only a drop of blood with minimal sample preparation. Laser desorption mass spectrometry may become a rapid and high-throughput tool for specific and sensitive pan-malaria detection at levels below 10 parasites/mul of blood.

Structural characterization of chlorophyll-a by high resolution tandem mass spectrometry

A high resolution Fourier transform ion cyclotron resonance (FTICR) mass spectrometer is used for characterizing the fragmentation of chlorophyll-a. Three tandem mass spectrometry (MS/MS) techniques, including electron-induced dissociation (EID), collisionally activated dissociation (CAD), and infrared mutiphoton dissociation (IRMPD) are applied to the singly protonated chlorophyll-a. Some previously unpublished fragments are identified unambiguously by utilizing high resolution and accurate mass value provided by the FTICR mass spectrometer. According to this research, the two long aliphatic side chains are shown to be the most labile parts, and favorable cleavage sites are proposed. Even though similar fragmentation patterns are generated by all three methods, there are much more abundant peaks in EID and IRMPD spectra. The similarities and differences are discussed in detail. Comparatively, cleavage leading to odd electron species and H(•) loss both seem more common in EID experiments. Extensive loss of small side groups (e.g., methyl and ethyl) next to the macrocyclic ring was observed. Coupling the high performance FTICR mass spectrometer with contemporary MS/MS techniques, especially IRMPD and EID, proved to be very promising for the structural characterization of chlorophyll, which is also suitable for the rapid and accurate structural investigation of other singly charged porphyrinic compounds.

9,10-Diphenylanthracene as a matrix for MALDI-MS electron transfer secondary reactions

The most common secondary-ionization mechanism in positive ion matrix-assisted laser desorption/ionization (MALDI) involves a proton transfer reaction to ionize the analyte. Peptides and proteins are molecules that have basic (and acidic) sites that make them susceptible to proton transfer. However, non-polar, aprotic compounds that lack basic sites are more difficult to protonate, and creating charged forms of this type of analyte can pose a problem when conventional MALDI matrices are employed. In this case, forming a radical molecular ion through electron transfer is a viable alternative, and certain matrices may facilitate the process. In this work, we investigate the performance of a newly developed electron-transfer secondary reaction matrix: 9,10-diphenylanthracene (9,10-DPA). The use of 9,10-DPA as matrix for MALDI analysis has been tested using several model compounds. It appears to promote ionization through electron transfer in a highly efficient manner as compared to other potential matrices. Thermodynamic aspects of the observed electron transfers in secondary-ionization reactions were also considered, as was the possibility for kinetically controlled/endothermic, electron-transfer reactions in the MALDI plume.

Liquid Secondary Ion Mass Spectrometry and Collision-induced Dissociation Mass Spectrometry of Sulfonamide Derivatives of meso-Tetraphenylporphyrin

Liquid secondary ion mass spectrometry (LSIMS) and collision-induced dissociation (CID) were used for the characterization of sulfonamide derivatives of meso-tetraphenylporphyrin (TPP). The spectra obtained using LSIMS show abundant molecular ions and fragment ions from losses of the sulfonamide moieties. The main fragmentation observed in the LSI mass spectra and in the CID spectra of the protonated or cationized molecules involves the loss of one sulfonamide group. In addition, in the CID spectra of these compounds the fragment ions formed by the elimination of two, three and/or four sulfonamide groups are also observed. The CID spectra of the protonated or cationized molecules of these derivatives do not display the ions formed by the cleavage of the S - N bond which have been reported for other sulfonamide compounds. The LSI mass spectra and CID spectra of sulfonamide derivatives of meso-tetraphenylporphyrin provide an easy and reliable means of identification of the number and nature of sulfonamide groups in the porphyrinic ring.

Investigation of MALDI-TOF Mass Spectrometry of Diverse Synthetic Metalloporphyrins, Phthalocyanines and Multiporphyrin Arrays

We investigated the utility of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) for analyzing porphyrinic compounds using a variety of different synthetic porphyrins, azaporphyrins, phthalocyanines and multiporphyrin arrays. Comparisons of spectra obtained from these analytes deposited either as neat samples or codeposited with neutral or acidic matrices have been made with the goal of identifying conditions that yield minimal demetalation, transmetalation, adduct formation and fragmentation. It was found that the molecular masses of many porphyrins can be successfully measured from neat sample preparations and do not require a matrix to facilitate desorption and ionization, although the measurement of large multiporphyrin arrays was facilitated by the use of matrices. Demetalation of magnesium porphyrins occurred in the presence of acidic matrices, but not with neutral matrices such as 1,4-benzoquinone. Positive ion spectra were obtained for each compound and negative ion spectra were also collected for the azaporphyrins and phthalocyanines. Examination of selected samples (prepared neat, with 1,4-benzoquinone, 2,3,5,6-tetrachloro-1,4-benzoquinone or α-cyano-4-hydroxycinnamic acid) showed that the dominant process of ionization involved oxidation yielding the radical cation M+· rather than the protonated molecule [M+H]+. MALDI-TOF-MS is shown to be a powerful analytical tool for the characterization of diverse synthetic porphyrinic compounds.

The spectroscopy of jet-cooled porphyrins: an insight into the vibronic structure of the Q band

We will present resonant two-photon ionization spectra for meso-tetraphenylporphyrin, H 2 TPP , measured under isolated conditions. The polycrystalline compound was vaporized, in vacuo, using both thermal and laser desorption, and seeded into a supersonic expansion of an inert-carrier gas. The molecules remain largely intact in the gaseous phase. However, the two techniques for vaporizing H 2 TPP give different internal temperatures for the isolated substrate, with greater vibrational cooling achieved using laser desorption. A comparison of the peak positions and intensities in the resonant two-photon ionization spectra of thermal- and laser-desorbed molecules provides an insight into the vibrational structure of the Q band. In particular, the greater contribution made by electronic transitions originating from higher vibrational levels in the ground state of H 2 TPP is emphasized. We conclude that vibronic coupling in the ground electronic state plays an important role in a quantum-mechanical interpretation of the Q band.

An update of MALDI-TOF mass spectrometry in lipid research

Although matrix-assisted laser desorption and ionization (MALDI) mass spectrometry (MS)--often but not exclusively coupled with a time-of-flight (TOF) mass analyzer--is primarily established in the protein field, there is increasing evidence that MALDI MS is also very useful in lipid research: MALDI MS is fast, sensitive, tolerates sample impurities to a relatively high extent and provides very simple mass spectra without major fragmentation of the analyte. Additionally, MALDI MS devices originally purchased for "proteomics" can be used also for lipids without the need of major system alterations. After a short introduction into the method and the related ion-forming process, the MALDI mass spectrometric characteristics of the individual lipid (ranging from completely apolar hydrocarbons to complex glycolipids with the focus on glycerophospholipids) classes will be discussed and the progress achieved in the last years emphasized. Special attention will be paid to quantitative aspects of MALDI MS because this is normally considered to be the "weak" point of the method, particularly if complex lipid mixtures are to be analyzed. Although the detailed role of the matrix is not yet completely clear, it will be also explicitly shown that the careful choice of the matrix is crucial in order to be able to detect all compounds of interest. Two rather recent developments will be highlighted: "Imaging" MS is nowadays widely established and significant interest is paid in this context to the analysis of lipids because lipids ionize particularly well and are, thus, more sensitively detectable in tissue slices than other biomolecules such as proteins. It will also be shown that MALDI MS can be very easily combined with thin-layer chromatography (TLC) allowing the spatially-resolved screening of the entire TLC plate and the detection of lipids with a higher sensitivity than common staining protocols.

Generation and dissociation of oxygen- and chloride-bridged iron(III) and manganese(III) tetraphenylporphyrin dimer ions in the gas phase

Electrospray ionization mass spectrometry (ESI/MS) has allowed the discovery of novel dimer ions emerging from solutions of metalloporphyrin salts and their investigation by collision-induced dissociation (CID) with N(2) molecules. ESI mass spectra have been recorded for the formation of the oxygen or chloride-bridged dimer ions [(FeTPP)(2)OH](+), [(MnTPP)(2)OH](+), [(FeTPP)(2)Cl](+) and [(MnTPP)(2)Cl](+) derived from various solutions of FeTPPCl and MnTPPCl salts. The CID of [(FeTPP)(2)OH](+) proceeds mainly by neutral loss of (FeTPP)OH to form [FeTPP](+) and, to a minor extent, to form the charge-reversed products. The CID of [(MnTPP)(2)OH](+) exhibits exclusively the product ion [MnTPP](+) by loss of neutral (MnTPP)OH. [(FeTPP)(2)Cl](+) and [(MnTPP)(2)Cl](+) dissociate by loss of (Fe/MnTPP)Cl to give rise to [Fe/MnTPP](+). [(FeTPP)(2)O](+) and [(FeTPP)(2)OH](+) were generated from a solution of the dimer, (FeTPP)(2)O. Dissociation of [(FeTPP)(2)O](+) yields two product ions, [FeTPP](+) and [(FeTPP)O](+), with higher onsets compared to the equivalent fragments formed from [(FeTPP)(2)OH](+).

Unexpected fragmentation of beta-substituted meso-tetraphenylporphyrins induced by high-energy collisional activation

The protonated molecules and radical cations of meso-tetraphenylporphyrins with beta-pyrrolic substituents, when formed by fast atom bombardment (FAB) and subjected to high-energy collisions, give rise to unexpected fragment ions. The reaction involves hydrogen migration from the ortho position of the phenyl ring to the a atom of the substituent, with formation of an intramolecular, six-membered ring. The process is analogous to condensed-phase cyclizations described for the same type of compounds. The fragmentation requires the presence of a double bond in the substituent group attached to the pyrrolic ring. A rearrangement process involving anchimeric assistance by the phenyl group (analogous to an ortho effect) is proposed for the formation of these ions.

Laser desorption electron attachment time-of-flight mass spectrometry: a new approach to detection of involatile compounds

We report initial results of a new method for obtaining mass spectra of involatile compounds: laser desorption electron attachment time-of-flight mass spectrometry. With this approach, laser desorbed neutral molecules are entrained in a molecular beam and subsequently ionized by low energy electron attachment. Mass analysis is carried out by a linear time-of-flight. We present a description of the apparatus and a number of examples of our early results on: van der Waals condensates of SF6, Fullerenes, derivatized Fullerenes, perfluorinated polyethers, polyphenylethers, and 5-bromouracil.

A fast atom bombardment and matrix-assisted laser desorption/ionization mass spectrometry study of doubly charged porphyrins

In this mass spectrometry (MS) study of doubly charged porphyrin salts, fast atom bombardment (FAB) and matrix-assisted laser desorption/ionization (MALDI) MS techniques are utilized to examine several unique ionic species. The predominant transformation of preformed doubly charged ions in the desorption/ionization mechanism of FAB and MALDI is the result of deprotonation reactions to form singly charged ions of the type [M(2+) - H(+)](+) and of one-electron reductions to form radical cations [M(2+) + e(-)](+.). The dependence of this phenomenon and the formation of a number of additional ionic species on the different matrices and the FAB-matrix additive benzoquinone is examined. The significant analogous behavior of doubly charged porphyrins in FAB- and MALDI-MS leads to the conclusion that one-electron reductions are of distinct relevance in the desorption/ionization mechanism of MALDI.

Determination of porphyrins and metalloporphyrins using liquid chromatography-diode array detection and mass spectrometry

A HPLC procedure has been optimized and successfully applied to porphyrins of environmental interest, such as etio and octaethylporphyrins and their VO and Ni compounds. The use of UV-Vis diode array allowed the detection of the analytes within the 5-15 ng/ml range. In order to achieve greater specificity and some structural information, the coupling of liquid chromatography with mass spectrometry was investigated, and the particle beam interface conditions were optimized. Electron impact (EI) spectra, comparable to those reported in the literature were obtained. The entire procedure has been applied to a real marine sediment, previously spiked with porphyrins to resemble oil-contaminated samples. The results pointed out that the method is suitable for such levels of analytes (5-10 microg/ml), allowing their identification and quantification with no need for purification steps.

Do charge-remote fragmentations occur under matrix-assisted laser desorption ionization post-source decompositions and matrix-assisted laser desorption ionization collisionally activated decompositions?

The precursor ions of tetraphenylporphyrins that are substituted with fatty acids can be introduced into the gas phase by matrix-assisted laser desorption ionization (MALDI) and undergo post-source and collisionally activated decompositions (CAD) in a time-of-flight mass spectrometer. The goal of the research is to obtain a better understanding of post-source decompositions (PSD); specifically, we asked the question of whether ions undergoing PSD have sufficient energy to give charge-remote fragmentations along an alkyl chain. We chose the porphyrin macrocycle because we expected it to act as an inert "support," allowing the molecule to be desorbed by MALDI and to be amenable to charge-remote fragmentation. MALDI-PSD and MALDI-CAD spectra are similar to high-energy CAD spectra and considerably more informative than low-energy CAD spectra, showing that charge-remote fragmentations of the fatty acid moieties do occur upon MALDI-PSD and MALDI-CAD.

Investigation of a series of synthetic cationic porphyrins using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) was used to study a series of synthetic cationic porphyrins as the perchlorate and bromide salts. This work presents the analytical results for the porphyrins obtained using 2, 5-dihydroxybenzoic acid (DHB) and 1,8,9-anthratriol as matrices. The selective use of matrix affects ion formation from these porphyrins. By using DHB as the matrix, we not only observed [M - nClO(4)](+) (n = 1-4) ions, but also obtained [2M - nClO(4)](+) (n = 2-7) ions from the synthetic cationic porphyrins. The space volume of the side chains (R groups) and the nature of the anions (Br(-) or ClO(4)(-)) affected the relative importance of monomeric and dimeric ions of the porphyrin. The possible mechanisms of desorption and ionization of these cationic porphyrins were also considered in this study. MALDI-TOFMS proved to be a very useful method for obtaining structural information on these synthetic cationic porphyrins. Copyright 1999 John Wiley & Sons, Ltd.

High- and low-energy collisionally activated decompositions of octaethylporphyrin and its metal complexes

High-energy (HE) and low-energy (LE) collisionally activated decompositions of octaethylporphyrin (OEP) and its metal complexes (ZnOEP and CuOEP) depend on whether the precursor is produced by electrospray ionization as protonated molecules or by fast atom bombardment as radical cations or protonated molecules. LE activation leads to such simple product-ion spectra that a complete picture of fragmentation emerges only after nine stages of tandem mass spectrometry (MS). HE activation, on the other hand, gives product-ion spectra that afford an integrated view of all the decomposition channels in a single MS/MS experiment. These results are the basis of a recommendation that OEP is an appropriate model compound for investigating energy effects in the collisional activation of organic and bioorganic molecule ions.