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.

First generation and characterization of the enol of glycine, H(2)N--CH=C(OH)(2), in the gas phase.

The enol of glycine, H(2)N-CH&dbond;C(OH)(2), is generated in the gas phase by neutralization of the corresponding radical cation, which is available by dissociative electron ionization of isoleucine. Reionization approximately 0.6 micros later shows that the isolated enol (2) exists and does not isomerize to the significantly more stable glycine molecule, H(2)N--CH(2)--COOH (1); hence the intramolecular tautomerization 2-->1 must be associated with high barriers. The neutralization-reionization reactivity of 1(+*) further confirms that neutral glycine has a canonical structure (1) and is not a zwitterion. The unimolecular chemistry of 1(+*) is dominated by C--C bond cleavage to the immonium ion (+)H(2)NCH(2); in sharp contrast, 2(+*) primarily loses H(2)O. The ylide ion (+)H(3)N--CH(*)--COOH, an intermediate in the water loss from 2(+*), is found to readily equilibrate to 2(+*) prior to dissociation. Tautomers 1(+*) and 2(+*) differ in their charge-stripping behavior, with only 2(+*) forming a stable dication. The radical anions 1(-*) and 2(-*), formed by charge reversal of 1(+*) and 2(+*), respectively, dissociate extensively to (mainly) different closed-shell fragment anions. An important channel is H(*) loss; 1(-*) yields the carboxylate ion H(2)N--CH(2)--COO(-) whereas 2(-*) yields the enolate ion H(2)N--CH=C(OH)O(-).

Dissociation of the peptide bond in protonated peptides.

The dissociation of the amide (peptide) bond in protonated peptides, [M + H](+), is discussed in terms of the structures and energetics of the resulting N-terminal b(n) and C-terminal y(n) sequence ions. The combined data provide strong evidence that dissociation proceeds with no reverse barriers through interconverting proton-bound complexes between the segments emerging upon cleavage of the protonated peptide bond. These complexes contain the C-terminal part as a smaller linear peptide (amino acid if one residue) and the N-terminal part either as an oxazolone or a cyclic peptide (cyclic amide if one residue). Owing to the higher thermodynamic stability but substantially lower gas-phase basicity of cyclic peptides vs isomeric oxazolones, the N-terminus is cleaved as a protonated oxazolone when ionic (b(n) series) but as a cyclic peptide when neutral (accompanying the C-terminal y(n) series). It is demonstrated that free energy correlations can be used to derive thermochemical data about sequence ions. In this context, the dependence of the logarithm of the abundance ratio log[y(1)/b(2)], from protonated GGX (G, glycine; X, varying amino acid) on the gas-phase basicity of X is used to obtain a first experimental estimate of the gas-phase basicity of the simplest b-type oxazolone, viz. 2-aminomethyl-5-oxazolone (b(2) ion with two glycyl residues).

Characterization of neutral fragments in tandem mass spectrometry: a unique route to mechanistic and structural information.

The neutral species eliminated upon fragmentation of fast-moving mass-selected ions can be directly identified by collisional ionization and detection in neutral fragment reionization (Nf R) mass spectra. Establishment of the identity of neutral fragments yields valuable insight into the decomposition mechanism of a precursor ion, as demonstrated for fullerene and alkali metal iodide cluster ions as well as metal ion adducts of amino acids. In addition, neutral fragment reionization also provides structural information that may not be available from the complementary ionic fragments alone; this is illustrated in the differentiation of isomeric mononucleotides. The parameters influencing the appearance of Nf R spectra are discussed and the scope and general applicability of the method are briefly evaluated.

Distonic ion .CH2CH2SCH2+ and the isomeric trimethylene and propylene sulfide radical cations. Assessment of structures and reactivities via decomposition and redox reactions.

The distonic radical ion .CH2CH2SCH2+ (1+.), generated by dissociative electron ionization of 1,4-dithiane or 1,4-thioxane, is identified in the gas phase by its collisionally activated dissociation (CAD), neutralization -reionization (+NR+) and charge-reversal (+CR-) mass spectra. The unimolecular chemistry of 1+. is shown to be substantially different from that of the isomeric, ring-closed, trimethylene sulfide ion (2+.). Hence, a substantial isomerization barrier must separate 1+. from the thermodynamically more stable 2+.. Charge permutation (i.e. charge-stripping, +NR+ and +CR-) are far superior, compared to collision-induced fragmentation, for distinguishing 1+. from 2+., mainly because the oxidized (1++ and 2++) and reduced forms (1 and 2 as well as 1-. and 2-.) of these cations have much lower tendencies for isomerization that 1+. and 2+. themselves. The diradical .CH2CH2SCH2. (1), formed by neutralization of 1+., is found to exist as a bound species, requiring appreciable activation energies for both decomposition to CH2CH2 plus SCH2 and ring-closure to 2. The dissociations and redox reactions of the propylene sulfide ion (3+.) are also assessed in this study and clearly indicate that 3+. is a stable C3H6S+. isomer. Further, the C3H6S+. ions from thiane, 1,3-dithiane and 2-methyl-1,3-dithiane are characterized based on their combined CAD, +NR+ and +CR- spectra. The two 1,3-dithianes produce ionized trimethylene sulfide, 2+.. In contrast, thiane gives rise to a C3H6S+. isomer other than 1+.(-3t); the data strongly suggest that this isomer is the 1-propene-1-thiol radical cation, namely CH3CH = CH-SH+..

The distonic ion (·)CH 2CH 2CH (+)OH, keto ion CH 3CH 2CH=O (+·), enol ion CH 3CH=CHOH (+·), and related C 3H 6O (+·) radical cations. Stabilities and isomerization proclivities studied by dissociation and neutralization-reionization.

Metastable ion decompositions, collision-activated dissociation (CAD), and neutralization-reionization mass spectrometry are utilized to study the unimolecular chemistry of distonic ion (·)CH2CH2CH(-)OH (2(+·)) and its enol-keto tautomers CH3CH=CHOH(-·) (1 (+·)) and CH3CH2CH=O (+·) (3(+·)). The major fragmentation of metastable 1(+·)-3(+·) is H(·) loss to yield the propanoyl cation, CH3CH2C≡O(+). This reaction remains dominant upon collisional activation, although now some isomeric CH2=CH-CH(+) OH is coproduced from all three precursors. The CAD and neutralization-reionization ((+)NR(+)) spectra of keto ion 3 (+·) are substantially different from those of tautomers 2(+·) and 1(+·). Hence, 3(+·) without sufficient energy for decomposition (i. e. , "stable" 3(+·)) does not isomerize to the ther-modynamically more stable ions 2(+·) or 1(+·), and the 1,4-H rearrangement H-CH2CH2CH=O(+·)(3 (+·)) → CH2CH2CH(+) O-H (2 (+·)) must require an appreciable critical energy. Although the fragment ion abundances in the (+) NR (+) (and CAD) spectra of 1 (+·) and 2 (+·) are similar, the relative and absolute intensities of the survivor ions (recovered C3H6O(+·) ions in the (+)NR(+) spectra) are markedly distinct and independent of the internal energy of 1 (+·) and 2 (+·). Furthermore, 1 (+·) and 2 (+·) show different MI spectra. Based on these data, distonic ion 2 (+·) does not spontaneously rearrange to enol ion 1 (+·) (which is the most stable C3H6O(+·) of CCCO connectivity) and, therefore, is separated from it by an appreciable barrier. In contrast, the molecular ions of cyclopropanol (4 (+·)) and allyl alcohol (5 (+·)) isomerize readily to 2 (+·), via ring opening and 1,2-H(-) shift, respectively. The sample found to generate the purest 2 (+·) is α-hydroxy-γ-butyrolactone. Several other precursors that would yield 2 (+·) by a least-motion reaction cogenerate detectable quantities of enol ion 1 (+·), or the enol ion of acetone (CH2=C(CH3)OH(+·), 6 (+·)), or methyl vinyl ether ion (CH3OCH=CH 2 (+·) , 7 (+·)). Ion 6 (+·) is coproduced from samples that contain the -CH2-CH(OH)-CH2- substructure, whereas 7 (+·) is coproduced from compounds with methoxy substituents. Compared to CAD, metastable ion characteristics combined with neutralization-reionization allow for a superior differentiation of the ions studied.

Characterization of the C3H 6O (+·) ion from 2-methoxyethanol. Mixture analysis by dissociation and neutralization-reionization.

The C3H6O(+·) ion formed upon the dissociative ionization of 2-methoxyethanol is identified by a combination of several tandem mass spectrometry methods, including metastable ion (MI) characteristics, collisionally activated dissociation (CAD), and neutralization-reionization mass spectrometry (NRMS). The experimental data conclusively show that 2-methoxyethanol molecular ion, namely, HOCH2CH2OCH 3 (+·) , loses H2O to yield mainly the distonic radical ion ·CH2CH2OCH 2 (+) along with a smaller amount of ionized methyl vinyl ether, namely, CH2=CHOCH 3 (+·) . Ring-closed products, such as the oxetane or the propylene oxide ion are not observed. The proportion of ·CH2CH2OCH 2 (+) increases with decreasing internal energy of the 2-methoxyethanol ion, which indicates a lower critical energy for the pathway leading to this product than for the competitive generation of CH2=CHOCH 3 (+·) . The present study also uses MI, CAD, and NRMS data to assess the structure of the distonic ion(+) (CH3)CHOCH2· (ring-opened ionized propylene oxide) and evaluate its isomerization proclivity toward the methyl vinyl ether ion.