Ionization Of Metastable Atoms Research Articles

A unified statistical RRKM approach to the fragmentation and autoneutralization of metastable molecular negative ions of hexaazatrinaphthylenes.

For the compounds promising for use as n-type semiconductors in organic electronics and energy storage devices, hexaazatrinaphthylene (HATNA) and its derivative hexamethoxy-hexaazatrinaphthylene (HMHATA), the monomolecular processes occurring under the exposure of molecules to low-energy (0-15 eV) free electrons were studied by means of resonant electron capture negative ion mass spectrometry. Resonant electron attachment results in the formation of eminently long-lived molecular negative ions (MNIs) in an abnormally wide range of incident electron energy (Ee) from 0 to 5-7 eV. For both compounds, this observation serves as an indication of the strong electron-accepting properties and high stability of MNIs against electron autodetachment. A weak yield of the only fragment NIs, dehydrogenated anions, was detected for HATNA at Ee > 6 eV. MNIs of HMHATA are less stable to dissociative decay because of the presence of weakly bound terminal substituents. This is evidenced by the mass spectral observation of intense fragmentation occurring above Ee≈ 1 eV and leading to a loss of up to 3 methyl groups as the Ee increases. A series of metastable NI peaks observed in the mass spectra testify to the delayed and sequential nature of fragmentation. Based on the principles of statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory, the theoretical model of dissociative decay of NIs was developed and then adopted to quantify the rates of ground-state anion decay via electron autodetachment. The experimentally measured electron autodetachment lifetimes and fragmentation rates were best reproduced by the model at molecular adiabatic electron affinities preset to 2.15 eV for HATNA and 1.88 eV for HMHATA, in reasonable agreement with the quantum chemical DFT PBE/3ζ predictions.

Coulomb explosion dynamics of triatomic molecules in laser pulses ranging from 7 to 200fs

Femtosecond laser pulses from 7fs to 200fs have been used to explore the ionization process and dissociation dynamics of triatomic molecules. Time and position sensitive detection allows us to detect each fragment ion in coincidence. We observe and characterize which ionization channels are Coulombic and which are non-Coulombic, and we observe concerted and stepwise processes which involve metastable fragment ions.

The unimolecular dissociation of protonated glyoxylic acid: Structure and dynamics of a step-by-step process

The unimolecular chemistry of protonated glyoxylic acid [HCOCOOH]H +, has been investigated by analyzing the fragmentation of metastable ions (MI) during their flight through a sector-field mass spectrometer. The only significant ionic product in the MI experiments is H 3O +, indicating the loss of two carbon monoxide molecules. High-level ab initio calculations have been used to model the relevant parts of the potential energy surface (PES). Starting from the most stable isomer, H(CO)C(OH) 2 + ( 1), fragmentation takes place as the result of three successive intramolecular proton transfer reactions. The first CO molecule is liberated in-between the second and the third proton transfer and moves slowly away. This mechanism is fully confirmed by ab initio direct dynamics calculations. In addition, the dynamics calculations reveal that the majority of the potential energy, which is liberated ends up as OH vibrations of the H 3O + moiety. After passage through the critical transition state, dissociation of the H 2OH +⋯CO ( 6) complex appears to be a limiting step. In order to acquire sufficient relative translational energy to depart, the hydroxonium ion and carbon monoxide have to reside together for 1–2 ps.

Photoexcitation of tetrathiafulvalene radical cations in a storage ring: Kinetics and energetics of the dissociation process

In this work, we report on the dissociation kinetics of tetrathiafulvalene radical cations (TTF +). In collisions with helium, the ions were first thermalised at room temperature or cooled to −65 °C in an ion trap before injection into an ion storage ring. Delayed dissociation of photoexcited ions was measured, and the kinetics were found to be non-trivial. The time spectrum obtained after 390 nm photoexcitation was not a single exponential decay normally observed for the dissociation of ions with a narrow energy distribution. We consider two interpretations of the time spectrum, either a multiple-exponential decay or a power-law decay. The physical implication of a multiple-exponential decay requires the presence of at least three isomers that interconvert slowly on the timescale for dissociation. In contrast, a power-law model implies a broad internal energy distribution of the excited ions. The origin of a broad distribution is, however, not obvious since we believe the width of the distribution to be narrow before excitation, about 0.1 eV, and TTF + is non-fluorescent. We, therefore, discard the power-law model in favour of the multiple-isomer model. The dominant dissociation reaction at low internal energies is loss of SCH as measured in collision-induced dissociation experiments and fragmentation of metastable ions. We suggest that excited TTF + rearranges via local minima on the potential energy surface to higher-energy isomers either before or after the photoexcitation. Isomerisation may be entropy-driven, which reduces the rate for reverse reactions to prevent equilibrium before the dissociation step. DFT calculations were applied in the search for possible structures of isomers.

Decomposition of protonated formic acid: one transition state—two product channels

The unimolecular chemistry of protonated formic acid, [HCOOH]H +, has been investigated by analyzing the fragmentation of metastable ions (MI) during flight in a sector mass spectrometer, and by proton transfer to formic acid in a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. High level ab initio calculations have been used to model the relevant parts of the potential energy surface (PES). In addition, ab initio direct dynamics calculations have been conducted, tracing out 60 different reaction trajectories. The only stable isomer in the mass spectrometric experiments is HC(OH) 2 + , which is the precursor to both observed ionic products, HCO + and H 3O +, via the same saddle point of the potential energy surface. The detailed motion of the dissociating molecule during passage of the post-transition state region of the PES therefore determines which product ion is formed. After passing the TS a transient HC(O)OH 2 + molecule is first formed. High total energy increases the probability that the nascent water molecule will have sufficient speed to escape the HCO + moiety. Otherwise, typically at low energies, the two units recombine, upon which intra-complex proton transfer is very likely. Eventually, this will give the more stable H 3O +.

Fragmentation pathways of N G-methylated and unmodified arginine residues in peptides studied by ESI-MS/MS and MALDI-MS

Protein methylation at arginine residues is a prevalent posttranslational modification in eukaryotic cells that has been implicated in processes from RNA-binding and transporting to protein sorting and transcription activation. Three main forms of methylarginine have been identified: N G-monomethylarginine (MMA), asymmetric N G,N G-dimethylarginine (aDMA), and symmetric N G,N′ G-dimethylarginine (sDMA). To investigate gas-phase fragmentations and characteristic ions arising from methylated and unmodified arginine residues in detail, we subjected peptides containing these residues to electrospray triple-quadrupole tandem mass spectrometry. A variety of low mass ions including (methylated) ammonium, carbodiimidium, and guanidinium ions were observed. Fragment ions resulting from the loss of the corresponding neutral fragments (amines, carbodiimide, and guanidine) from intact molecular ions as well as from N- and C-terminal fragment ions were also identified. Furthermore, the peptides containing either methylated or unmodified arginines gave rise to abundant fragment ions at m/ z 70, 112, and 115, for which cyclic ion structures are proposed. Electrospray ionization tandem mass spectra revealed that dimethylammonium ( m/ z 46) is a specific marker ion for aDMA. A precursor ion scanning method utilizing this fragment ion was developed, which allowed sensitive and specific detection of aDMA-containing peptides even in the presence of a five-fold excess of phosphorylase B digest. Interestingly, regular matrix-assisted laser desorption/ionization mass spectra recorded from aDMA- or sDMA-containing peptides showed metastable fragment ions resulting from cleavages of the arginine side chains. The neutral losses of mono- and dimethylamines permit the differentiation between aDMA and sDMA.

Theoretical calculation of isotope effects, kinetic energy release and effective temperatures for alkylamines

The kinetic isotope effect (KIE) and kinetic energy release (KER) of protonated alkylamine dimers were studied by theoretical modeling. In the calculations on reaction kinetics one empirical parameter was used to describe the looseness of the transition state. Calculations are compared to experiments described by Norrman and McMahon [Int. J. Mass. Spetrom. 182/183 (1999) 381]. In the case of experiments using a high-pressure ion source the Rice–Ramsperger–Kassel–Marcus (RRKM) model, taking into account energy distributions and the time scale of metastable ion fragmentation, accurately describes the experimentally observed KIE’s of α-deuterated amines. The KER (available experimentally in one case only) is also correctly calculated, using no further parameters. In the case of low-pressure ion source, the internal energy distribution (IED) is not thermal, so it was empirically estimated based on the experimentally observed KIE. Using this estimate, it was possible to calculate the KER of α-deuterated amines accurately. Based on theoretical expectations it was found that the mean KER value is equal to 3/2 kT eff. This allows estimation of the KER in cases where it is determined by statistical factors. Energy distributions of various fragmenting ion populations are discussed in some detail. These may be helpful for a qualitative understanding of mass spectrometric processes and the theoretical basis of the kinetic method.

Fragmentation reactions of protonated peptides containing glutamine or glutamic acid.

A variety of protonated dipeptides and tripeptides containing glutamic acid or glutamine were prepared by electrospray ionization or by fast atom bombardment ionization and their fragmentation pathways elucidated using metastable ion studies, energy-resolved mass spectrometry and triple-stage mass spectrometry (MS(3)) experiments. Additional mechanistic information was obtained by exchanging the labile hydrogens for deuterium. Protonated H-Gln-Gly-OH fragments by loss of NH(3) and loss of H(2)O in metastable ion fragmentation; under collision-induced dissociation (CID) conditions loss of H-Gly-OH + CO from the [MH - NH(3)](+) ion forms the base peak C(4)H(6)NO(+) (m/z 84). Protonated dipeptides with an alpha-linkage, H-Glu-Xxx-OH, are characterized by elimination of H(2)O and by elimination of H-Xxx-OH plus CO to form the glutamic acid immonium ion of m/z 102. By contrast, protonated dipeptides with a gamma-linkage, H-Glu(Xxx-OH)-OH, do not show elimination of H(2)O or formation of m/z 102 but rather show elimination of NH(3), particularly in metastable ion fragmentation, and elimination of H-Xxx-OH to form m/z 130. Both the alpha- and gamma-dipeptides show formation of [H-Xxx-OH]H(+), with this reaction channel increasing in importance as the proton affinity (PA) of H-Xxx-OH increases. The characteristic loss of H(2)O and formation of m/z 102 are observed for the protonated alpha-tripeptide H-Glu-Gly-Phe-OH whereas the protonated gamma-tripeptide H-Glu(Gly-Gly-OH)-OH shows loss of NH(3) and formation of m/z 130 as observed for dipeptides with the gamma-linkage. Both tripeptides show abundant formation of the y(2)'' ion under CID conditions, presumably because a stable anhydride neutral structure can be formed. Under metastable ion conditions protonated dipeptides of structure H-Xxx-Glu-OH show abundant elimination of H(2)O whereas those of structure H-Xxx-Gln-OH show abundant elimination of NH(3). The importance of these reaction channels is much reduced under CID conditions, the major fragmentation mode being cleavage of the amide bond to form either the a(1) ion or the y(1)'' ion. Particularly when Xxx = Gly, under CID conditions the initial loss of NH(3) from the glutamine containing dipeptide is followed by elimination of a second NH(3) while the initial loss of H(2)O from the glutamic acid dipeptide is followed by elimination of NH(3). Isotopic labelling shows that predominantly labile hydrogens are lost in both steps. Although both [H-Gly-Glu-Gly-OH]H(+) and [H-Gly-Gln-Gly-OH]H(+) fragment mainly to form b(2) and a(2) ions, the latter also shows elimination of NH(3) plus a glycine residue and formation of protonated glycinamide. Isotopic labelling shows extensive mixing of labile and carbon-bonded hydrogens in the formation of protonated glycinamide.

Modeling Metastable Ion Time-of-Flight Peaks

An analytical model is presented to describe time-of-flight (TOF) peak shapes of ionized metastable species emitted from solid surfaces under particle or photon bombardment. The model complements and consolidates the formalism for determination of metastable ion mean lives, release energies (Q-values), and fragmentation patterns by using linear-type TOF spectrometer data. Effects of fragmentation in the field free region of the TOF spectrometer are discussed. The analysis of phenylalanine TOF spectrum is performed to illustrate the determination of Q and mean life values.

Fragmentation reactions of protonated peptides containing phenylalanine: a linear free energy correlation in the fragmentation of H–Gly–Xxx–Phe–OH

The fragmentation reactions of a variety of protonated tri- and tetra-peptides containing phenylalanine have been examined using metastable ion studies and energy-resolved collision-induced dissociation studies. For peptides with the sequence H–Gly–Xxx–Phe–OH (Xxx=Gly,Ala,Val,Leu,Phe) the major primary fragmentation of MH + involves cleavage of the C-terminus amide bond to form either the b 2 ion or the y 1″ ion. For metastable ion fragmentation it is found that log([b 2]/[ y 1″]) increases linearly with the increase in the gas phase basicity of H–Xxx–OH. This linear free energy correlation is in contrast to the lack of such a correlation in the fragmentation of protonated H–Gly–Xxx–Gly–OH [J. Mass Spectrom. 30 (1995) 290]. When Phe is the central residue in tripeptides, the major primary fragmentation reaction involves formation of the b 2 ion which fragments further to the a 2 ion; at higher internal energies the a 2 ion fragments to give the phenylalanine immonium ion which becomes the dominant fragment. When Phe is in the N-terminus position, as in Phe–Gly–Gly–OH, the phenylalanine immonium ion is the dominant fragment and is formed, in part, directly by fragmentation of MH +. The fragmentation of the tetrapeptides H–Gly–Gly–Phe–Leu–OH, H–Phe–Gly–Gly–Phe–OH and H–Val–Ala–Ala–Phe–OH are more complex but show a substantial directional effect of the phenylalanine residue(s).

Dissociation of fast ions analyzed by time-of-flight

The fragmentation of metastable ions, having keV of kinetic energy, is analyzed by time-of-flight technique. Assuming isotropic distribution of fragments in a free field region, it is deduced an analytical expression to describe the corresponding peak shapes in linear TOF spectrometers. Metastable ion mean-life and the kinetic energy release (Q-value) are the quantities extracted from data fitting. As an illustration, the dissociation of C8H10N+ metastable ions, desorbed by 252Cf ssion fragment impact on organic target, is studied.

Distinguishing of linkage isomers of lactotetra oligosaccharides by using the relative ion intensity analysis of post-source decay fragment ions in curved-field reflectron matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

The native oligosaccharides of lacto-N-neotetraose (Gal beta1-4GlcNAc beta1-3Gal beta1-4Glc; LNnT) and lacto-N-tetraose (Gal beta1-3GlcNAc beta1-3Gal beta1-4Glc; LNT) were analyzed by using curved-field reflectron matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). Since a curved-field reflectron TOFMS enables a simultaneous focusing of a wide mass range of metastable fragment ions, the relative ion intensities in the post-source decay (PSD) mass spectra can be discussed. The PSD mass spectra of LNnT and LNT were distinguishable in their relative ion intensities. In the case of LNT, beta-elimination could occur in the N-acetyl glucosamine (GlcNAc) at the C-3 position, which was bonded by galactose (Gal); however, it did not occur in LNnT. The 3-O elimination caused a difference in the relative ion intensities in the PSD mass spectra of LNnT and LNT. The beta1-3 glycosyl linkage cleaved more easily than the beta1-4 glycosyl linkage in the MALDI-PSD fragmentation. An analysis of the relative ion intensities in the MALDI-PSD mass spectra of oligosaccharides was very useful for distinguishing the linkage isomers and for characterizing the types of glycosyl linkages.

Site of protonation of N-alkylanilines

The MH + ions of N-methylaniline, N,N-dimethylaniline, N-ethylaniline, and N-N-diethylaniline, when prepared by CH 4 chemical ionization, show significant elimination of H 2 in both metastable ion and low energy collision-induced fragmentation. In contrast, the MH + ions of the same species produced by fast atom bombardment (FAB) ionization show no elimination of H 2 in either metastable ion or collision-induced fragmentation. Other metastable ion fragmentation reactions involve loss of CH 3 for the methylanilines and loss of C 2H 4 and C 2H 5 for the ethylanilines. The MH + ions prepared using CH 3OH or (CH 3) 2CO as Brønsted acid reagents also show H 2 elimination whereas protonation using i-C 4H 9NH 2 as reagent produces MH + ions that show essentially no H 2 elimination. It is concluded that FAB ionization leads to the N-protonated species whereas protonation in the gas phase using CH 4, CH 3OH, or (CH 3) 2CO as reagents leads to substantial formation of the ring-protonated tautomer. Protonation of the ring is endothermic for i-C 4H 9NH 3 + as reagent ion. Because N-protonation is thermodynamically favoured, the results provide further evidence for kinetic control in gas-phase Brønsted acid chemical ionization.

Unimolecular decomposition of ethyl lactate, CH 3CH(OH)COOC 2H 5, upon electron impact

The spontaneous unimolecular fragmentation of metastable ions of ethyl lactate, CH 3CH(OH)COOC 2H 5, 1, (MW:118) by electron ionization has been investigated by using mass-analyzed ion kinetic energy (MIKE) spectrometry and D labeling. The fragmentation of 1 is compared with those of methyl lactate, CH 3CH(OH)COOCH 3, 2, (MW:104), and ethyl glycolate, CH 2(OH)COOC 2H 5, 3, (MW:104), which are lower homologues of 1. The metastable ions of 1 +· decompose to give the three fragment ions m/ z 89 (Ċ 2H 5 loss), 75 (CH 3ĊO loss) and 45 (ĊOOC 2H 5 loss). The first fragmentation is accompanied with a large kinetic energy release (KER) in analogy with that for the case of ĊH 3 loss from 2 +·. Double hydrogen atom migration occurs in the second reaction in a similar manner as those in 2 and 3. The last fragment ion is CH 3CH +OH and this is also generated by the other two processes. These fragmentations of 1 +· are similar to those of 2 +·, except for CO loss observed in 2 +·, however, are different from those of 3 +·.

Functional group interactions in the fragmentation of protonated and methylated succinic acid derivatives

The methane chemical ionization (CI) mass spectra of succinic acid monoamide and monomethyl succinate have been determined. In addition, the unimolecular metastable ion fragmentation reactions and low-energy collision-induced dissociation reactions of the MH + and M.CH 3 + ions of these compounds have been determined. In the CH 4 CI mass spectra extensive fragmentation of the MH + ions is observed, indicative of functional group interactions. An important fragmentation reaction for the protonated monoamide involves formation of NH 4 + and it is suggested that this occurs through the intermediacy of a protonated succinic anhydride/ammonia ion-neutral complex. The [MH–H 2O] + and [MH–CH 3OH] + fragment ions formed in the CH 4 CI of monomethyl succinate give collision-induced dissociation (CID) spectra which are in agreement with the spectra obtained for methylated and protonated succinic anhydride, respectively; these results strongly support the suggestions that the formation of stable cationated cyclic anhydride structures are one of the factors which result in extensive fragmentation of protonated and cationated dicarboxylic acid derivatives. The M.CH 3 + adduct of succinic acid monoamide gives a CID spectrum which is distinctly different from that obtained for the protonated monomethyl ester of succinic acid monoamide. This result indicates that little addition of CH 3 + to the free COOH group of the monoamide occurs. The CID spectrum of the M.CH 3 + adduct of monomethyl succinate also shows substantial differences from the CID mass spectrum of protonated dimethyl succinate.

Stereochemical applications of Mass Spectrometry. Part 5—fragmentation of protonated and methylated maleic and fumaric acid and derivatives

The unimolecular metastable ion and collision-induced dissociation (CID) fragmentation reactions of protonated and methylated monoamides, monomethyl esters and methyl esters of the monoamides of maleic and fumaric acids were studied. In addition, some studies of the fragmentation of protonated and methylated maleic and fumaric acids were carried out. The [MH - H 2 O] + ions derived from protonated maleic and fumaric acids show distinctly different CID mass spectra, that for the [MH - H 2 O] + ion from the maleic acid being the same as that of protonated maleic anhydride; the results show that the stereochemistry about the double bond is retained in the [MH - H 2 O] + ions. Fragmentation of specifically deuterium-labelled and protonated or deuterated maleic acids show that the added proton becomes scrambled with the carboxylic hydrogens prior to loss of H 2 O. The fragmentation of similarly labelled fumaric acids show that a 1,3-H + migration followed by elimination of H 2 O is not the only pathway to water elimination; the results implicate proton migration from one carboxyl group to the other as well as some involvement of the C-bonded hydrogens in the water-loss reaction. A major fragmentation reaction of protonated maleamic acid forms NH 4 + ; this reaction is of only minor importance for protonated fumamic acid. Other primary fragmentation reactions involve elimination of NH 3 and H 2 O from the protonated species, The protonated monomethyl esters fragment initially by loss of H 2 O or loss of CH 3 OH; the former is more prominent for the maleate whereas the latter dominates for the fumarate. Protonation of methyl maleamate and methyl fumamate results in loss of NH 3 or CH 3 OH as primary fragmentation reactions; these primary fragment ions undergo less facile further fragmentation for the maleamate than for the fumamate. The CH 3 + adducts of the monoamides fragment by loss of NH 3 , H 2 O and CH 3 OH; the CID spectra of the adducts are distinctly different from those of the protonated methyl esters of the monoamides, indicating predominant addition of the methyl to the amide oxygen. The CH 3 + adduct of monomethyl maleate fragments primarily by loss of methanol, the two methyl groups having become equivalent prior to fragmentation. A minor fragmentation route involves loss of dimethyl ether, a reaction not observed for protonated dimethyl maleate. Elimination of dimethyl ether is a major fragmentation channel for the CH 3 + adduct of monomethyl fumarate. Since this reaction channel is not observed for protonated dimethyl fumarate, the results indicate predominant CH 3 + addition to the carbomethoxy group of the monoester.

Loss of H 2 from CH 2NH 2+ and NHNH 2+. Reaction mechanisms and dynamics from observation of metastable ion fragmentations and ab initio calculations

On the basis of the measurement of the translational energy release distribution, previous isotope labelling experiments, extensive ab initio calculations of relevant parts of the potential energy surface and an ab initio direct dynamics calculation, it is concluded that the loss of H 2 from CH 2NH 2 + ( 1) occurs in two steps. The first step, which is irreversible, is a hydrogen rearrangement giving the high energy isomer CH 3N + ( 2). The existence of this isomer is transient and it decomposes “on the fly” to give the products CHNH + ( 3) and H 2 ( 4) via the transition structure TS2 3 + 4 . Passage of this transition structure brings the reacting system to a part of the potential energy surface which provides a strong repulsive force between the two fragments. This gives rise to the observed large translational energy release. The mechanism also explains why the reaction is a specific 1,2-H 2 elimination. NHNH 2 + ( 5) decomposes in one step via TS5 6 + 4 to give NNH + ( 6) and H 2 ( 4), also with a large translational energy release. There is an alternative higher energy route which occurs in two steps via the isomer NNH 3 + ( 7).

Unimolecular reactions of protonated hydrazine. Reaction mechanisms and dynamics from observation of metastable ion fragmentations and ab initio calculations

The unimolecular chemistry of the system NH 2NH 3 + has been investigated experimentally and theoretically. In accordance with experiment losses of H 2 and H • are calculated to be the dominating low energy processes on the singlet potential energy surface. A weak signal due to loss of NH with formation of NH 4 + is also observed. This reaction probably takes place by cross-over to the triplet surface. The reaction dynamics of the H 2 loss is investigated in great detail. This process has a substantial reverse barrier, and a large non-statistical translational energy release. An ab initio direct dynamics calculation provides good insight into the process, and is in agreement with the experimentally determined translational energy release. The proton affinity of NH 2NH 2 was calculated, and found to reproduce the experimental figure very well.

C-C and C-H bond activation in the fragmentation of the [M + Ni] + adducts of aliphatic amino acids

The major metal-containing species formed upon fast atom bombardment of amino acid/Ni+2 mixtures is the [M + Ni]+ adduct, involving reduction of the Ni+2 to the +1 oxidation state. By contrast, electrospray ionization of amino acid/Ni+2 mixtures produces predominantly [Ni(M − H)M]+; this species, on collisional activation, produces predominantly [M + Ni]+ by elimination of [M - H], presumably a carboxylate radical. The unimolecular fragmentation reactions occurring on the metastable ion time scale for the [M + Ni]+ adducts of a variety of α-amino acids have been recorded. The adducts with phenylalanine, α-aminoisobutyric acid and α-aminobutyric acid fragment by elimination of H2O, H2O + CO and, to a minor extent, by elimination of CO2. These reactions are similar to those observed for the [M + Cu]+ adducts of α-amino acids. A reaction distinctive for the [M + Ni]+ adducts involves formation of the immonium ion RCH=NH 2 + . By contrast, the [M + Ni]+ adducts with leucine, isoleucine, and norleucine show extensive metastable ion fragmentation by elimination of H2, CH4, C2H4, C3H6, and C4H8, with the relative importance of the different fragmentation channels depending on the configuration of the C4H9 side chain. These results are interpreted in terms of C-C and C-H bond activation of the C4H9 side chain by the Ni+. The adducts with valine and norvaline fragment in a fashion similar to the adduct with phenylalanine, except that minor elimination of C3H6 is observed.

Synthesis of Monodisperse Macromolecular Bicyclic and dendritic compounds from (R)‐3‐hydroxybutanoic acid and benzene‐1,3,5‐tricarboxylic acid and analysis by fragmenting MALDI‐TOF mass spectroscopy

AbstractAs previously shown, oligo‐ and poly(β‐hydroxyalkanoates) have a high tendency to form lamellar crystallites with ca. 50‐Å thickness which corresponds to chain lengths of 16 units (Fig. 1). To have monodisperse model compounds, we have now prepared bicyclic derivatives with three parallel (27–29) or two parallel and an antiparallel chain (68–70) consisting of up to 16 3‐hydroxybutanoate (3‐HB) units. We also prepared dendritic compounds (71–75, 82–85) containing oligo(3‐HB) chains which cannot possibly be arranged as in the lamellae; the branching units were prepared from trimesic acid (= benzene‐1,3,5‐tricarboxylic acid). So far, none of the prepared samples formed crystals or contained crystalline domains which would have been suitable for single‐crystal or powder‐diffraction X‐ray analysis. The terminally deprotected dendrimers (74, 75, and 85) are multi‐anionic (up to 12 peripheral CO2H groups) and biodegradable. The macromolecular HB derivatives (molecular weight up to 10150 Da) have been fully characterized by IR, 1H‐ and 13C‐NMR, [α]D, and elemental analysis. Especially important is the analysis by mass spectrometry with the MALDI‐TOF technique (Fig. 2), proving that the products are monodisperse; application of a new variation of this MS method (post source decay = PSD or fragment analysis by structural time of flight = FASTTM) allows for the observation of metastable fragment ions and, thus, is a tool for structural oligomer analysis (Fig. 3).