Direct mass spectrometry of polymers. VII. Primary thermal fragmentation processes in polycarbonates

The primary fragmentation mechanisms in the thermal decomposition of several polycarbonates were studied by direct pyrolysis into the mass spectrometer. Our results indicate that ester exchange reactions predominate in the primary thermal fragmentation process of polycarbonates, causing the formation of cyclic oligomers.     

Direct mass spectrometry of polymers. XI. Primary thermal fragmentation processes in aromatic–aliphatic polyesters

The thermal decomposition of two series of isomeric aromatic–aliphatic polyesters was studied by direct pyrolysis-mass spectrometry. The results indicate that intramolecular exchange reactions predominate in the primary thermal fragmentation processes to cause the formation of cyclic oligomers. Several secondary thermal processes may occur after the primary step: hydrolytic cleavage of the ester bond, decarboxylation, and β-hydrogen transfer.     

Direct mass spectrometry of polymers. IX. Copoly(carbonate-urethanes) prepared by reorganization of polycarbonates with piperazine

A detailed analysis of the reaction products generated by reorganization of polycarbonates with piperazine, performed by direct mass spectrometry methods, has shown that this reaction actually follows the pathway postulated and that, at 50% piperazine incorporation level, urethanediphenol compounds are almost exclusively produced. This allowed us to obtain, by repolymerization with phosgene, alternating copoly(carbonate-urethanes) with thermal stabilities comparable to those of the parent polyurethanes. The primary fragmentation mechanisms in the thermal decomposition of these copolymers were studied by direct pyrolysis into the mass spectrometer. Ester exchange reactions predominate here, causing the formation of cyclic oligomers, which are subsequently cleaved to open-chain oligomers containing hydroxyl end groups.     

Direct mass spectrometry of polymers. XII. Thermal fragmentation processes in poly-α-aminoacids

The thermal fragmentation processes in poly-α-aminoacids have been investigated by direct pyrolysis–Mass Spectrometry. The mass spectral data show that the pyrolytic breakdown of polyglycine, polysarcosine, and polyproline leads to the formation of cyclic oligomers. Polyalanine, polyphenylalanine, and polytyrosine decompose yielding compounds with olefin and nitrile end-groups. Finally, in the case of poly-α-methylglutamate, the primary thermal process is the loss of methanol with consequent formation, along the polymer chain, of pyroglutamic units, which yield cyclic dimer as main pyrolysis product.     

Direct mass spectrometry of polymers. X. Primary thermal fragmentation processes in totally aromatic polyesters

The thermal decomposition of a series of isomeric poly-(oxphthaloyloxyphenylenes) (I–IV) and poly(m-hydroxybenzoic acid) (V) was studied by Direct Pyrolysis–Mass Spectrometry. The results indicate that intramolecular exchange reactions predominate in the primary thermal fragmentation processes, causing the formation of cyclic oligomers which are subsequently cleaved to open-chain fragments. The size and relative abundance of the cycles produced appear to be strongly influenced by steric factors, i.e., by the structure (para or meta) of the repeating unit in each polymer. Remarkably, in the case of poly(m-hydroxybenzoic acid) the formation of cyclic oligomers containing up to seven repeating units is observed.     

Primary thermal fragmentation processes in poly(lactic acid) investigated by positive and negative chemical ionization mass spectrometry

The mechanism of thermal decomposition of poly(lactic acid) was studied by direct pyrolysis-mass spectrometry using EI, CI and NCI ionization methods. It was found that the thermally formed cyclic oligomers of lactic acid are not stable under EI conditions. The results obtained by CI and NCI indicate that intramolecular exchange reactions predominate in the primary thermal fragmentation processes.     

Direct mass spectrometry of polymers. XIII.. Thermal fragmentation processes in copoly-α-amino acids

The thermal fragmentation processes in L-Proline—Sarcosine and in L-Proline—L-Alanine copolymers have been investigated by direct pyrolysis—mass spectrometry. The mass spectral data show that proline—sarcosine copolymers decompose in three steps, yielding cyclic oligomers. The first step is characterized by the presence of the Sar—Sar, Sar—Pro, and Pro—Pro cyclic dimers. In the second step the formation of several series of cyclic oligomers up to nonamers—decamers can be observed, while the third step of thermal decomposition process leads to the almost exclusive formation of Pro—Pro cyclic dimer. The proline—alanine copolymers, instead, show a single-step thermal degradation process leading to the formation both of cyclic oligomers and compounds with olefin and nitrile end-groups. Our results show that the thermal decompostion of alanine units is affected by the neighboring units along the polymer chain. Sarcosine units act as thermally labile probes, causing the selective thermal cleavage of the Pro—Sar copolymer.     

Understanding Amyloid‐β Oligomerization at the Molecular Level: The Role of the Fibril Surface

The aggregation of the amyloid β‐peptide into fibrils is a complex process that involves mechanisms such as primary and secondary nucleation, fibril elongation and fibril fragmentation. Some of these processes generate neurotoxic Aβ oligomers, which are involved in the development of Alzheimer's disease. Recent experimental studies have emphasized the role of the fibril as a catalytic surface for the production of highly toxic oligomers during secondary nucleation. By using molecular dynamics simulations, we show that it is the hydrophobic fibril region that causes the structural changes required for catalyzing the formation of β‐sheet‐rich Aβ1‐42 oligomers on the fibril surface. These results reveal, for the first time, the molecular basis of the secondary nucleation pathway.     

Oligosaccharide characterizations by fast atom bombardment mass spectrometry. Fragmentation study in relation to the substituent at the reducing end

A systematic investigation of oligosaccharides (cello-oligosaccharides, malto-oligosaccharides, their corresponding alditols and methylmaltosides) was undertaken using fast atom bombardment mass spectrometry. Experimental conditions were defined in order to obtain characteristic fragmentation in relation to the structure of the oligomers. The role of substituents at the reducing end in the fragmentation process was demonstrated.     

Matrix-assisted laser desorption/ionization time-of-flight/time-of-flight tandem mass spectra of poly(butylene adipate)

Matrix-assisted laser desorption/ionization time-of-flight/time-of-flight tandem mass spectrometry (MALDI-TOF/TOF-MS/MS) was employed to analyze four poly(butylene adipate) (PBAd) oligomers and to investigate their fragmentation pathways as a continuation of our work on the MALDI-TOF/TOF-MS/MS study of synthetic polymers. MALDI-TOF/TOF-MS/MS analysis was performed on oligomers terminated by carboxyl and hydroxyl groups, methyl adipate and hydroxyl groups, dihydroxyl groups, and dicarboxyl groups. The sodium adducts of these oligomers were selected as precursor ions. Different end groups do not influence the fragmentation of sodiated polyester oligomers and similar series of product ions were observed in all the MALDI-TOF/TOF-MS/MS spectra. According to the structures of the most abundant product ions identified in the present work, three fragmentation pathways have been proposed to occur most frequently in PBAd: beta-hydrogen-transfer rearrangement, leading to the selective cleavage of the --O--CH(2)-- bonds; --CH(2)--CH(2)-- (beta--beta) bond cleavage in the adipate moiety; and ester bond scission.     

Thermal degradation behavior of polymethacrylates containing amine side groups

The thermal degradation behavior of polymethacrylates containing amine groups such as poly(N,N-diethyl aminoethyl methacrylate), PDEAEM, and poly(N-ethyl-m-tolyl-aminoethyl methacrylate), PMEET, has been studied using thermogravimetry coupled with infrared spectroscopy (TGA/FTIR). PDEAEM showed two degradation stages whereas PMEET displayed only one. The thermal degradation of PDEAEM initially takes place through ester cleavage of the polymethacrylate, generating volatile tertiary amines and alcohols and polymethacrylic anhydride in the remaining solid material. This is followed by further fragmentation of the modified polymeric chain formed. It was also observed that storage of the original polymer affected the thermal decomposition behavior of PDEAEM. The main thermal degradation pathway for PMEET is an immediate backbone chain scission to yield oligomers.     

Direct mass spectrometry of polymers. XIV. Thermal fragmentation processes in poly-schiff bases

The thermal fragmentation processes in poly-Schiff bases have been investigated by direct pyrolysis–mass spectrometry. The mass spectral data show that the thermal fragmentation occurring in the polymers under investigation is characterized by hydrogen transfer reactions. In the case of a totally aromatic poly-Schiff base (polymer I ), the thermal fragmentation process involves hydrogen transfer irom the methyne group with formation of fragments bearing nitrile and/or phenyl end groups. In the case of aromatic-aliphatic poly-Schiff bases (polymers II–IV ), the hydrogen transfer process occurs from the aliphatic methylene groups. The latter process involves a lower energy and therefore occurs at lower temperatures with respect to the totally aromatic polymer I , with formation of thermal fragments bearing olefin and/or imine end groups. Beside these fragments, several thermal fragmentation compounds are also evolved by multiple hydrogen transfer reactions.     

Nucleated polymerization with secondary pathways. III. Equilibrium behavior and oligomer populations

We explore the long-time behavior and equilibrium properties of a system of linear filaments growing through nucleated polymerisation. We show that the length distribution for breakable filaments evolves through two well defined limiting cases: first, a steady state distribution determined by the balance of breakage and elongation is reached; upon monomer depletion at the end of the growth phase, an equilibrium length distribution biased towards smaller filament fragments emerges. We furthermore compute the time evolution of the concentration of small oligomeric filament fragments. For frangible filaments, oligomers are present both at early times and at equilibrium, whereas in the absence of fragmentation, oligomers are only present in significant quantities at the beginning of the polymerisation reaction. Finally, we discuss the significance of these results for the biological consequences of filamentous protein aggregation.     

Mass spectral characterization of polymers. Primary thermal fragmentation processes in polyureas

The primary fragmentation processes in the thermal decomposition of polymers were studied in detail on a series of structurally related polyureas by direct pyrolysis with a mass spectrometer. Our results indicate that polyureas I–III undergo a quantitative depolycondensation process analogous to that observed for N-monosubstituted polyurethanes. The thermal decomposition of polyureas IV–VI proceeds by intramolecular hydrogen transfer processes that occur at higher temperatures with respect to depolycondensation. Polycarboxypiperazine VI is decomposed by a single-stage decomposition mechanism that leads to fragments with amino end groups and carbon oxide.     

Spontaneous crosslinking of poly(1,5‐dioxepan‐2‐one) originating from ether bond fragmentation

The spontaneous reaction of unsaturated double bonds induced by the fragmentation of ether bonds is presented as a method to obtain a crosslinked polymer material. Poly(1,5‐dioxepan‐2‐one) (PDXO) was synthesized using three different polymerization techniques to investigate the influence of the synthesis conditions on the ether bond fragmentation. It was found that thermal fragmentation of the ether bonds in the polymer main chain occurred when the synthesis temperature was 140 °C or higher. The double bonds produced reacted spontaneously to form crosslinks between the polymer chains. The formation of a network structure was confirmed by Fourier transform infrared spectrometry and differential scanning calorimetry. In addition, the low molar mass species released during hydrolysis of the DXO polymers were monitored by ESI‐MS and MALDI‐TOF‐MS. Ether bond fragmentation also occurred during the ionization in the electrospray instrument, but predominantly in the lower mass region. No fragmentation took place during MALDI ionization, but it was possible to detect water‐soluble DXO oligomers with a molar mass up to approximately 5000 g/mol. The results show that ether bond fragmentation can be used to form a network structure of PDXO. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7258–7267, 2008     

Elucidation of the initial step of oligonucleotide fragmentation in matrix-assisted laser desorption/ionization using modified nucleic acids

To probe the mechanism of gas-phase oligonucleotide ion fragmentation, modified oligonucleotides were studied using matrix-assisted laser desorption/ionization. The oligonucleotides were of the form 5'-TTTTXTTTTT, where X was a modified nucleotide. Modifications included substitution of hydroxy, methoxy, amino, and allyl groups at the 2'-position of the deoxyribose. The modified ribose contained adenine, guanine, cytosine, or uracil bases. For comparison, we studied oligomers where X was an unmodified adenosine, guanosine, cytidine, thymidine, or uridine deoxyribonucleotide. We found a very strong dependence of the matrix-to-analyte ratio on fragmentation for these oligomers. Analysis of these modifications suggests that the initial fragmentation step in MALDI-MS involves a two-step (E1) elimination of the base.     

Mass spectrometry for the characterization of unsulfated chondroitin oligosaccharides from 2-mers to 16-mers. Comparison with hyaluronic acid oligomers

This study reports for the first time the complete liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) and tandem mass spectrometry (MS/MS) analyses performed in negative ion mode of saturated unsulfated chondroitin oligosaccharides up to 16-mers and comparison with hyaluronic acid (HA) oligomers differing only in the nature of the hexosamine residue. MS/MS of the chondroitin disaccharide on the singly charged precursor at m/z 396.1 afforded a glycosidic cleavage C1 product ion at m/z 192.9. In the tetrasaccharide, C2 (m/z 396.0) and C3 (m/z 572.0) product anions were generated by glycosidic cleavage. A C5 [M-2H]2- product ion at m/z 475.1 was generated by the glycosidic cleavage of the hexasaccharide, and a C7 ion (m/z 664.6, charge state of -2) was produced from the octasaccharide. The same fragmentation pattern of deprotonated oligomers was observed for the largest oligosaccharides, from 10- to 16-mers. There has been no previous report of MS/MS spectra for unsulfated chondroitin oligomers of these sizes. Unsulfated saturated chondroitin oligosaccharides with x-mer units and larger than a tetrasaccharide dissociate to almost exclusively form CX-1-type ions. Saturated HA oligomers also afforded the same fragmentation pattern as deprotonated oligomers by ESI-MS and MS/MS analyses. Thus, under the experimental conditions used in the current study, we were unable to distinguish between unsulfated chondroitin and HA.     

Funktionell substituierte zinnorganische Verbindungen. III. (2-Aminoethyl)-triorganostannane

Functionally Substituted Organotin Compounds. III. (2-Aminoethyl)-triorganostannanes The preparation of (2-aminoethyl)-triorganostannanes 1 from 2-chloroethylamin and triorganotin halides is reported. Like primary amines the compounds 1 react with aldehydes, ketones, acyl chlorides or acetic anhydride, isocyanates, isothiocyanates, and hydrogen halides yielding the corresponding triorganostannylethyl substituted azomethines 2 , amides 3 , and 5 , ureas and thioureas 6 , and ammonium halides 7 . Compounds 7 undergo thermal induced fragmentation reaction studied by DTA/TG analysis.     

Post-source decay time-of-flight study of fragmentation mechanisms of protonated synthetic polymers under matrix-assisted laser desorption/ionization conditions

Post-source decay (PSD) of three different nylon oligomers desorbed under matrix-assisted laser desorption/ionization (MALDI) conditions was studied and their fragmentation pathways were investigated. The fragmentation of the protonated oligomers is very similar to that of peptides. The b(n)(+), y(n)(+) and z(n)(+) series of ions were observed in abundance in the PSD spectrum. The end groups and the length of the spacer in the repeating unit influence the fragmentation of the different polyamides and the relative abundances of the product ions. Competitive dehydration and deamination reactions were observed, and depend on the nature of the end groups and the repeating units. The PSD spectra are very similar to collision-induced dissociation (CID) spectra obtained under low-energy conditions, implying that the selected precursor ions possess similar average internal energies. All the peaks observed in the PSD spectrum can be rationalized by reasonable fragmentation mechanisms.     

Fundamentals of the application of matrix-assisted laser desorption-ionization mass spectrometry to low mass poly(methylmethacrylate) polymers

Matrix-assisted laser desorption-ionization (MALDI) time of flight is shown to give a molar peak area response for isolated methylmethacrylate oligomers that have 25 and 50 repeat units when run on three different instruments in reflectron or linear mode and using three different matrix materials. In addition, fragmentation was not observed in any of the three different matrices or at higher laser power. No spectral differences were observed for syndiotactic and isotactic methylmethacrylate oligomers. These results suggest that the low most probable peak values observed for narrow distribution poly(methylmethacrylate) standards by MALDI mass spectrometry are not the result of mass discrimination or fragmentation.