MALDI Mass Spectra Research Articles

Characterization of DNA adducts and tetraols derived from anti-benzo[ghi]fluoranthane-3,4-dihydrodiol-5,5a-epoxide.

A total of seven DNA adducts and two racemic tetraols derived from anti-benzo[ghi]fluoranthene-3,4-dihydrodiol-5,5a-epoxide (anti-B[ghi]FDE, 2) were characterized by analyses of UV, (1)H NMR, CD, and MALDI mass spectra. The structure of 2 is the first example of a diolepoxide in which a fully fused cyclopentane ring is covalently linked to the saturated ring bearing the epoxide function. Compound 2 is also a conformationally rigid structure analogue of the extensively studied anti-benzo[c]phenanthrene-3,4-dihydrodiol-1,2-epoxide (anti-BcPDE), thus serving as a model for probing the diolepoxide-DNA interaction [Chang et al. (2002) Chem. Res. Toxicol. 15, 198-208 (following paper in this issue)]. The most abundant adducts are formed from trans- or cis-openings of the epoxide by the amino groups of either deoxyguanosine or deoxyadenosine. Adducts of minor abundance formed by the attachment of the diolepoxide to the amino group of deoxycytidine N(4) and guanine N(7) were also isolated. Post-source decay MALDI spectra of the (M + H)(+) molecule ions are consistent with the assigned adduct structures. The lack of a typical benzylic proton at the site of deoxynucleoside attachment necessitated a new NMR assignment strategy. Despite the steric constraint, the epoxide ring opening of 2 occurred exclusively at the dibenzylic C5a, not at C5. The assignments on the trans- and cis-epoxide opening were made based on the molecular modeling structures, i.e., the pseudoaxial H5 in cis-adducts is placed directly under the strong influence of a shielding cone of the aromatic ring system, while the same proton in trans-adducts adopts a pseudoequatorial conformation, thereby protruding away from the aromatic ring system. The absolute configuration at the site of deoxynucleoside attachment (C5a) was tentatively assigned on the basis of the empirical rules that have been established for deoxynucleoside-adducts derived from traditional alternant PAH diolepoxides.

On the nature of the chemical noise in MALDI mass spectra

The so-called “chemical noise background” imposes a major limit on the practical sensitivity of MALDI mass spectrometry. Typically, as the amount of material of interest subjected to MALDI analysis is reduced, the signal decreases to the point where it can no longer be differentiated from the chemical noise. Using a newly designed MALDI-ion trap mass spectrometer, we describe experiments intended to throw light on the nature of the chemical noise background and to reduce its effects. Single-stage mass spectrometric signals from peptides were observed to disappear into the noise when the amount of sample applied to the MALDI sample stage was decreased to less than a femtomole. At these low levels, analysis of the collision-induced fragmentation spectra revealed the presence of ions originating from the peptide as well as cluster ions that originate from the chemical noise. The fragmentation pattern arising from dissociation of the cluster species suggests that they are composed largely of matrix molecules. A significant fraction of these cluster ions can be dissociated at activation energies lower than the threshold for peptide fragmentation. We used this finding to collisionally pre-activate MALDI ions to remove a significant portion of the chemical noise from the spectrum, allowing us to obtain readily discernible single stage MS signals from 100 attomols of peptide. The strategy also yielded high quality MS/MS spectra from 100 attomols of peptide. Different possibilities of collisional pre-activation for improving sensitivity are considered.

Synthesis and biological evaluation of thioglycosylated porphyrins for an application in photodynamic therapy

The aim of this work is the synthesis of a new family of glycosylated porphyrins in which the sugar moieties are linked to the tetrapyrrole ring by a thioglycosidic bond. Two series have been designed. The first one corresponds to meso-aryl porphyrin derivatives. The second one has been obtained from protoporphyrin IX derivatization. Aryl-porphyrins were prepared from tristolyl o- and p-hydroxyporphyrins followed by bromoallylation and thioglycosylation with peracetylated S-glucose, mannose and galactose and deprotection. The other series has been synthesized from protoporphyrin IX dimethylester with a regioselective glycosylation of terminal alkenyl carbon. The UV-visible, NMR and MALDI mass spectra are presented. Photocytotoxicities of the synthesized compounds against K562 chronic leukaemia cell line has been evaluated.

Negative-Mode Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry of Maltoheptaose and Cyclomaltooligosaccharides.

We used norharmane (9H-pyrido[3,4-b]indole) as a matrix for negative-mode ionization of neutral oligosaccharides in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). In-source decay (ISD) fragmentation occurred, producing fragment ions [(M-C4H8O4)-H]- in negative-mode linear MALDI-MS. The ISD product ion [(M-C4H8O4)-H]- of maltoheptaose was predominantly observed but deprotonated molecule was not. In the MALDI mass spectrum of cellohexaose, the ISD product ion [(M-C4H8O4)-H]- was observed, as was deprotonated molecule [M-H]-. In positive-mode MALDI-MS spectra of oligosaccharides, these ISD product ions were not observed. In cyclomaltooligosaccharides of cyclodextrins (CD), only deprotonated molecules [M-H]- were observed clearly and ISD product ions were not seen in negative-mode MALDI-MS. The result of sugar branched cyclodextrin of glucosyl-βCD was the same. The ISD product ion [(M-C4H8O4)-H]- of maltoheptaose would thus occur at the reducing end and corresponded to 2, 4A fragmentation. In the post-source decay (PSD) fragment spectra of cyclodextrins, PSD product ions [(M-C4H8O4)-H]- produced by 2, 4A and Z cleavage were observed. The formation of PSD product ions [(M-C4H8O4)-H]- of cyclodextrins would thus relate to that of ISD product ions of linear oligosaccharides in negative-mode MALDI-MS.

Application of sulfur as a matrix for laser desorption/ionization in the characterization of halogenated fullerenes

The application of sulfur as a matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) analysis of highly chlorinated and fluorinated fullerenes is reported. Control over fluorofullerene fragmentation which resulted in the domination of the molecular peak C(60)F(36)(-) was achieved, with the optimal matrix-to-analyte ratio found to be 1000:1. We suggest the possible mechanism of the molecular ion formation according to the charge transfer between the sulfur anions and C(60)F(36). For the first time the LDI and MALDI mass spectra of the highly chlorinated fullerene C(60)Cl(x)(x(max) approximately 32) are presented. The formation of odd chlorine ions (positive and negative) is observed. We conclude that use of sulfur as a matrix leads to a significant decrease in fragmentation of the halogenated fullerenes.

Analysis of Polymer Additives by Matrix‐Assisted Laser Desorption Ionization/Time of Flight Mass Spectrometer Using Delayed Extractionand Collision Induced Dissociation

AbstractMatrix‐assisted laser desorption/ionization (MALDI) in combination with mass spectrometry proved to be a viable method for identifying additives in polyethylene ex tracts. The MALDI mass spectra of additives standards were found to be very simple consisting of [M+H]+ and/or [M+Na]+ pseudo molecular ions with little fragment ions. For real samples, which of ten contain more than one additive, the production of only one or two ions for each additive makes the tentative assignment much easier. Collision induced dissociation (CID) of the pseudomolecular ion is used to confirm the tentative assignment. The analysis of highmolecular weight additives, such as Chimassorb 944 and Tinuvin 622 indicated that MALDI was superior to other ionization techniques such as electrospray ionization (ESI), desorption chemical ionization (DCI) and fast atom bombardment (FAB) in the analysis of high molecular weight polymer additives. Sample preparation was found to be more critical than DCI in the analysis of real sample. The signals were interfered by the low molecule weight polyethylenemolecules, which were co‐extracted with the additives. This problem was partially overcome by using acetone in stead of methanol to precipitate the low molecular weight polyethylenemolecules.

Size exclusion chromatography/mass spectrometry applied to the analysis of polysaccharides.

A novel method for analysing polysaccharide materials is described which employs size-exclusion chromatography (SEC) followed by detection by on-line electrospray ionisation mass spectrometry (ESI-MS) and off-line matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOFMS). It is demonstrated through SEC/ESI ion trap mass spectrometry that the formation of multiply charged oligomer ions, which bind up to five sodium cations, allows the rapid analysis of polysaccharide ions with molecular weights in excess of 9 kDa. MALDI spectra generated from fractionation of the effluent collected from the same SEC separation are shown to be in good agreement with the ESI spectra with respect to molecular weight distributions and types of ions generated. ESI and MALDI mass spectra of samples obtained from sequential graded ethanol precipitation and SEC fractionation of acid and enzymatically digested arabinoxylan polysaccharides show important structural differences between polysaccharide fragments. In addition, a comparison is made between the mass spectra of native and permethylated SEC-separated fragments of acid and enzymatically treated arabinogalactan. Linkage information of the permethylated arabinogalactan oligomers can be rapidly established through the use of on-line SEC/ESI-MS( n) experiments.

MALDI-MS Evaluation of Poly(3-hexylthiophene) Synthesized by Chemical Oxidation with FeCl3

Poly(3-hexylthiophene) synthesized by chemical oxidation with FeCl3 was investigated with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) using either dithranol or the electron-transfer matrices terthiophene and anthracene. Two to three series of ions were observed in the MALDI mass spectra. These series all have hexylthiophene repeat groups, and the end groups/modifications were identified as two or more chlorine atoms through the analysis of isotopic distributions and exact mass measurements. The chlorine atoms may be end groups (on α- or β-carbons of terminal thiophenes) or substituents on the β-carbons of the thiophene groups along the polymer backbone. Although poly(3-alkylthiophenes) synthesized by this method have been reported to contain iron or chlorine impurities, this is the first demonstration that the polymeric product actually contains chlorine atoms that are covalently bound to the polymer chain (vs residual FeCl3). The FeCl3 oxidizing agentnot the chlorinated solve...

Accurate Molecular Mass Determination of Mycolic Acids by MALDI-TOF Mass Spectrometry

Mycolic acids, major and specific long-chain fatty (C70-C90) acid components of the mycobacterial cell envelope, were analyzed for the first time using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry operating in a reflectron mode. The various types of purified mycolates from representative mycobacterial species were analyzed using 2,5-DHB as matrix, because less than 10 pmol of mycolates was sufficient to obtain well-resolved mass spectra composed exclusively of pseudomolecular [M + Na]+ ions consistent with the structures deduced from the chemical analytical techniques applied to these molecules. Examination of the MALDI mass spectra demonstrated that the chain lengths of the various mycolates correlated with the growth rate of mycobacterial strains. Although slow growers, such as Mycobacterium tuberculosis and Mycobacterium ulcerans, produced a series of odd carbon numbers (C74-C82) of alpha-mycolic acids, rapid growers synthesized both odd and even carbon numbers. In addition, the main chain of oxygenated mycolic acids from slow growers were four to six carbon atoms longer than the corresponding alpha-mycolic acids, whereas rapid growers elaborated oxygenated homologues possessing the same chain lengths as their alpha-mycolic acids. Furthermore, a comparative analysis of the crude fatty acid mixtures from a wild-type strain of M. tuberculosis and its isogenic mutant effected in the synthesis of oxygenated mycolates by MALDI mass spectrometry revealed structural differences between the alpha-mycolates from the two strains. Thus, this technique appeared to be a rapid and highly sensitive technique for the analysis of mycolic acids, not only by providing accurate molecular masses and new structural information, but also by both reducing sample consumption and saving time.

A General Strategy for Epitope Mapping by Direct MALDI-TOF Mass Spectrometry Using Secondary Antibodies and Cross-Linking

The combination of limited proteolysis and MALDI-TOF mass spectrometry has become an important tool for the determination of epitopes but works best with highly purified antibodies. Here we report the use of capture antibodies to reduce the need for purification of the antibody in the mass spectrometric determination of the epitope. In this new method, a secondary Fc-specific antibody, covalently bound to Sepharose beads, is used to capture the primary antibody (the antibody of interest). After capture, the two antibodies are cross-linked. The antigen is then bound to the immobilized antibodies and subjected to proteolysis using several successive proteinases. In this study, this strategy is demonstrated with a crude mouse anti-ACTH IgG solution and adrenocorticotropin (ACTH). Comparing this strategy with previous methods where the antibody is bound directly to activated beads, the new method (1) results in a higher binding capacity of the bound antibody to ACTH, (2) does not require purification of the antibody of interest, and (3) dramatically reduces the chemical background in the MALDI mass spectra.

Characterization of inorganic coordination complexes by matrix-assisted laser desorption/ionization mass spectrometry.

We report the direct laser desorption/ionization (LDI) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometric (MALDI-TOFMS) analysis of four inorganic coordination complexes: monometallic [Ir(dpp)(2)Cl(2)](PF(6)), homonuclear trimetallic ([(bpy)(2)Ru(dpp)](2)RuCl(2))- (PF(6))(4), and heteronuclear [(tpy)Ru(tpp)Ru(tpp)RhCl(3)](PF(6))(4) and ([(bpy)(2)Ru(dpp)](2)IrCl(2))(PF(6))(5) (dpp = 2,3-bis-(2'-pyridyl)pyrazine, bpy = 2,2'-bipyridine, tpy = 2,2',6',2"-terpyradine, tpp = 2,3,5,6,-tetrakis-(2'-pyridyl)pyrazine). Spectral intensities and fragmentation patterns are compared and evaluated for instrument parameters, matrix selection, and matrix-to-analyte ratio. Direct LDI and MALDI mass spectra of the monometallic complex showed the same ion peaks and differed only in the relative peak intensities. Direct LDI of the trimetallic complexes produced only low-mass fragments containing one metal at most. MALDI spectra of the trimetallic complexes exhibited little fragmentation in the high-mass region (>1500 Da) and less fragmentation in the low-mass region compared to direct LDI. Significant fragments of the molecules were detected and identified, including ligand fragments, intermediate-mass fragments such as [Ru(tpy)](+), and molecular ions with varying degrees of PF(6)(-) loss ([M - n(PF(6))](+), where n = 1-3). A correlation exists between the solution-phase electrochemistry and the observed [M - n(PF(6))](+) series of peaks for the trimetallic complexes. Proper matrix selection for MALDI analysis was vital, as was an appropriate matrix-to-analyte ratio. The results demonstrate the applicability of MALDI-TOFMS for the structural characterization of labile inorganic coordination complexes.

UV-MALDI-TOF and ESI-TOF Mass Spectrometry Characterization of Silsesquioxanes Obtained by the Hydrolytic Condensation of (3-Glycidoxypropyl)- trimethoxysilane in an Epoxidized Solvent

Silsesquioxanes obtained by the hydrolytic condensation of (3-glycidoxypropyl)trimethoxysilane (GPMS) in diglycidyl ether of bisphenol A (DGEBA) were characterized by electrospray ionization time-of-flight mass spectrometry (ESI-TOF MS) and matrix-assisted ultraviolet laser desorption/ionization time-of-flight mass spectrometry (UV-MALDI-TOF MS), employing two different matrices and both positive and negative ion modes. A bimodal distribution of molar masses, in the 1300-6400 m/z range, was observed in MALDI mass spectra. This distribution accounted for oligomers formed in two successive generations but did not include a cluster of higher molar-mass species present in SEC chromatograms. Most of the peaks present in ESI and MALDI mass spectra could be described by the generic formula T n -(OCH 3 ) m , with m = 0, 2, and 4 for n = even, m = 1, 3, 5 for n = odd, and T = RSiO (3n-m)/2n . This corresponds to completely condensed polydedra (m = 0), incompletely hydrolyzed polyhedra (m = 1 to 3), and their precursors (m = 4). Predominant species in the first cluster contained 10 to 14 Si atoms whereas those in the second cluster had 18 to 23 Si atoms. Small amounts of the following species: T 8 (OH)(OCH 3 ), T 9 (OH), T 10 (OH)(OCH 3 ), and T 11 (OH) could be identified in MALDI MS, using 9H-pyrido[3,4-b]indole (nor-harmane) as matrix in the negative ion mode. It was inferred that some of these species had a relevant participation in the generation of the second cluster of higher molar masses. The stability of the silsesquioxane solution in DGEBA was the result of the very small concentration of free SiOH groups available for further condensation.

Phosphorylation and oligomerization states of native pig brain HSP90 studied by mass spectrometry

HSP90 is one of the most abundant proteins in the cytosol of eukaryotic cells. HSP90 forms transient or stable complexes with several key proteins involved in signal transduction including protooncogenic protein kinases and nuclear receptors, it interacts with cellular structural elements such as actin-microfilament, tubulin-microtubule and intermediate filaments, and also exhibits conventional chaperone functions. This protein exists in two isoforms alpha-HSP90 and beta-HSP90, and it forms dimers which are crucial species for its biological activity. PAGE, ESI-MS and MALDI-MS were used to study HSP90 purified from pig brain. The two protein isoforms were clearly distinguished by ESI-MS, the alpha isoform being approximately six times more abundant than the beta isoform. ESI-MS in combination with lambda phosphatase treatment provided direct evidence of the existence of four phosphorylated forms of native pig brain alpha-HSP90, with the diphosphorylated form being the most abundant. For the beta isoform, the di-phosphorylated was also the most abundant. MALDI mass spectra of HSP90 samples after chemical cross-linking showed a high percentage of alpha-alpha homodimers. In addition, evidence for the existence of higher HSP90 oligomers was obtained.

Copper(I) chloride: a simple salt for enhancement of polystyrene cationization in matrix-assisted laser desorption/ionization mass spectrometry.

The possibility of using copper(I) chloride as a doping salt to enhance the cationization of polystyrene in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) was investigated. It was shown that copper(I) chloride possesses sufficient solubility in tetrahydrofuran. The parameters of the MALDI mass spectra of different polystyrene samples, such as the number-average (M(n)) and mass-average (M(w)) molecular mass values, obtained by copper(I) cationization were compared with those obtained by means of silver(I) cationization, and good agreement was found. It was also shown that application of copper(I) chloride as a doping salt, and dithranol as a matrix, ensured good MALDI mass spectra of the sample spots even after storage for 1 month.

Structural characterization of quinoxaline homopolymers and quinoxaline/ether sulfone copolymers by matrix-assisted laser desorption ionization mass spectrometry.

Polyphenylquinoxalines (PPQs) are prepared from self-polymerizable quinoxaline monomers that carry fluorine, hydroxyaryl (ArOH), and phenyl substituents. In basic media, these monomers self-polymerize via a series of nucleophilic aromatic substitution reactions (SNAr), in which aromatic enolates (ArO- nucleophiles) attack the electrophilic carbons bearing F leaving groups to effect fluoride displacement. Polyphenylquinoxaline/polyether-sulfone (PPQ/PES) copolymers are synthesized similarly by combining self-polymerizable quinoxaline monomers with a 1:1 molar mixture of 4,4'-dichlorodiphenyl sulfone and bisphenol A. The MALDI mass spectra of the polymers reveal that the major products up to approximately 15,000 Da molecular mass are homo- or copolymeric macrocycles. Linear byproducts are also observed, arising from nucleophilic ring opening of already formed macrocycles. Oligomers containing at least one PPQ unit readily protonate upon MALDI, whereas PES homopolymers require alkali metal ion addition to become detectable. Molecular orbital calculations point out that the nucleophilic and electrophilic reactivities of the PPQ monomer and the PPQ growing chains generated during propagation are comparable, allowing for continued condensations via SN-Ar, until cyclization terminates this process. The calculations also predict a significantly lower electrophilic reactivity for carbons substituted by chlorine instead of fluorine, justifying the discrimination against incorporation of PES units observed for the copolymers. The computationally optimized structures of PPQ and PPQ/PES macrocycles show a diverse array of cavity sizes and geometries which depend on the size of the macrocycle, the sequence of the repeat units, and the position of the substituents in the quinoxaline ring; quinoxaline pendants (phenyl groups) are found to favor helical arrangements in the prepared macrocycles.

Conformation Changes, Complexation, and Phase Transition in Matrix-Assisted Laser Desorption

Using molecular dynamics simulations we have demonstrated that the guest species in matrix-assisted laserdesorption exhibits significantly different conformations in the gas phase, on the crystal surface, and embeddedinto the matrix. Our model fully accounted for the internal structure of matrix and guest molecules; thus, wewere able to follow the details of conformational changes in the guest molecule within the crystal and throughoutthe desorption. During the embedding process, simulated by sinapinic acid crystal growth around the substanceP guest species, changes in the guest species conformation were investigated. While the guest species placedon the surface of the host crystal exhibited two hydrogen bonds on the backbone and one turn, the conformationof the embedded species contained three hydrogen bonds and two turns. Independent of the burial depth, theconformation of the guest species within the crystal remained the same. After laser heating and during thedesorption process, no preferred gas-phase conformation was observed. Investigation of the radius-of-gyrationof the guest species originating from different layers of the crystal revealed that, upon desorption, the guestspecies did not sustain a stable conformation in the gas phase. Liftoff velocities of the guest species embeddedin various layers of the host crystal and on the surface were similar to the liftoff velocities of the matrixmolecules around them. This is an indication of the entrainment of guest species by the matrix plume.Investigation of the energy histories of the matrix and guest species showed lower internal and kinetictemperatures for the latter, which could explain the absence of fragmentation in MALDI. Results also showedthat maximum energy transfer occurred during the phase transition of the host crystal at which time the guestspecies were released from the crystal. Examining the noncovalent complex of the guest species and sinapinicacid anions showed that the complex remained intact throughout the desorption process. A similar phenomenonis often reflected in MALDI mass spectra, in the form of guest-matrix adduct peaks.

First Isolation of Human UDP-d-Xylose: Proteoglycan Core Protein β-d-Xylosyltransferase Secreted from Cultured JAR Choriocarcinoma Cells

Human UDP-d-xylose:proteoglycan core protein beta-d-xylosyltransferase (EC, XT) initiates the biosynthesis of glycosaminoglycan lateral chains in proteoglycans by transfer of xylose from UDP-xylose to specific serine residues of the core protein. In this study, we report the first isolation of the XT and present the first partial amino acid sequence of this enzyme. We purified XT 4,700-fold with 1% yield from serum-free JAR choriocarcinoma cell culture supernatant. The isolation procedure included a combination of ammonium sulfate precipitation, heparin affinity chromatography, ion exchange chromatography, and protamine affinity chromatography. Among other proteins an unknown protein was detected by matrix-assisted laser desorption ionization mass spectrometry-time of flight analysis in the purified sample. The molecular mass of this protein was determined as 120 kDa by SDS-polyacrylamide gel electrophoresis. The isolated protein was enzymatically cleaved by trypsin and endoproteinase Lys-C. Eleven peptide fragments were sequenced by Edman degradation. Searches with the amino acid sequences in protein and EST data bases showed no homology to known sequences. XT was enriched by immunoaffinity chromatography with an immobilized antibody against a synthetic peptide deduced from the sequenced peptide fragments and was specifically eluted with the antigen. In addition, XT was purified alternatively with an aprotinin affinity chromatography and was detected by Western blot analysis in the enzyme-containing fraction.

Structural characterization of polyoxymethylenes by matrix‐assisted laser desorption/ionization mass spectrometry

Several polyoxymethylene (POM) model compounds with relatively low molecular weights were characterized by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) using dithranol matrix, 1,1,1,3,3,3-hexafluoro-2-propanol solvent for matrix/analyte solution, and sodium iodide as cationization reagent. For POM oligomers with thermally unstable hydroxyl end-groups (paraformaldehyde), a series of sodium-cationized molecular ions up to MW ca. 2000 was observed at good sensitivity with little or no fragmentation. The MALDI mass spectra of the trimethylsilylated and acetylated POM samples were interpreted in terms of their end-group structures and molecular weight distributions. Furthermore, the formation mechanisms of the macrocyclic POM oligomers (Mn = 1500–2000), isolated by alkaline degradation of raw bulk POM obtained by cationic polymerization, are discussed by relating them to their lamellar structures on the basis of their MALDI mass spectra. Copyright © 2001 John Wiley & Sons, Ltd.

Characterization of the protein subset desorbed by MALDI from whole bacterial cells.

This study characterizes various features of the proteins that are detected in MALDI mass spectra when whole bacteria cells are analyzed, in an effort to understand why some proteins are successfully detected and many others are not. Forty peaks observed in the mass range 4,000-20,000 Da in the spectra of Escherichia coli K-12 and 11775 are tentatively assigned to proteins in a protein database, and these proteins are characterized by cell location, copy number, pI, and hydropathicity. Those detected originate in the cytosol and generally share the traits of high abundance within the cell, strong bacisity, and medium hydrophilicity.

Structural characterization of polyoxymethylenes by matrix-assisted laser desorption/ionization mass spectrometry

Several polyoxymethylene (POM) model compounds with relatively low molecular weights were characterized by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) using dithranol matrix, 1,1,1,3,3,3-hexafluoro-2-propanol solvent for matrix/analyte solution, and sodium iodide as cationization reagent. For POM oligomers with thermally unstable hydroxyl end-groups (paraformaldehyde), a series of sodium-cationized molecular ions up to MW ca. 2000 was observed at good sensitivity with little or no fragmentation. The MALDI mass spectra of the trimethylsilylated and acetylated POM samples were interpreted in terms of their end-group structures and molecular weight distributions. Furthermore, the formation mechanisms of the macrocyclic POM oligomers (Mn = 1500–2000), isolated by alkaline degradation of raw bulk POM obtained by cationic polymerization, are discussed by relating them to their lamellar structures on the basis of their MALDI mass spectra. Copyright © 2001 John Wiley & Sons, Ltd.