Protonating polymer oligomers in the gas phase to change fragmentation pathways

Ionization of polymers in mass spectrometry is usually achieved by forming metal ion adducts. The metal ion has been shown by Wesdemiotis to often play a spectator role in the collision-induced dissociation (CID) chemistry of these species, wherein they fragment according to a free-radical mechanism similar to that found in their pyrolysis. The result is a predominance of low-mass ions in the CID mass spectrum. We have changed this behavior by generating protonated oligomers in the gas phase by first forming proton-bound complexes of the oligomers with amino acids or peptides by electrospray ionization. These complexes dissociate first by loss of the amino acid/peptide to form protonated oligomers, which then undergo a unique fragmentation chemistry. In this article we discuss the results for poly(methyl methacrylate) (PMMA) and poly(butyl acrylate) (PBA). Initially, protonated PMMA and PBA lose methanol and butanol, respectively, from the side chains of the respective monomers. The resulting PMMA-derived ion then undergoes a series of neutral losses corresponding to 32 and 28 Da, methanol and carbon monoxide. This continues as collision energy increases until a final, carbon-rich backbone ion is formed, which then undergoes a classic hydrocarbon fragmentation pattern. The PBA-derived ions are proposed to fragment by the loss of butylether molecules to form anhydride rings along the oligomer chain. The number of ether molecules lost corresponded to half the number of available side chains in the oligomer. The resulting poly-anhydride ion dissociates by small molecule loss. Mechanisms have been suggested for the fragmentation chemistry of these two classes of oligomers.     

Ionized o-, m-, and p-difluorobenzene dissociate through ring-opened intermediates: A TPEPICO investigation

Threshold photoelectron photoion coincidence (TPEPICO) experiments have shown that o-, m-, and p-difluorobenzene ions dissociate via a common, ring-opened intermediate and not via ionized p-difluorobenzene. Rice-Ramsperger-Kassel-Marcus (RRKM) modeling of the experimental breakdown curves yields activation energies for the initial isomerization of 4.48 +/- 0.05, 4.55 +/- 0.05, and 4.68 +/- 0.05 eV for o-, m-, and p-difluorobenzene, respectively. These values place each ion at a similar absolute energy and thus similar transition states. A large positive DeltaS(double dagger) for each ion (ca 100 J K(-1) mol(-1)) suggests a ring-opened structure for these transition states.     

Photophysical processes in dimers of thiacarbocyanine cations

The results of quantum-chemical calculations of absorption, fluorescence, and induced T-T absorption spectra and the rate constants of photophysical processes for the monomer and dimers of the 3,3′-diethylthiacarbocyanine cation are presented. It has been shown that the formation of the dimers leads to a substantial reduction in the efficiency of the radiative pathways and enhancement of the efficiency of the non-radiative pathways of deactivation of the fluorescent state, thereby resulting in a decrease in the quantum yield of fluorescence. The fluorescence quantum yield depends in this case on the relative orientation of the molecules in the dimer with respect to each other. The calculation results satisfactorily describe experimental data on the spectral and luminescent properties of the dimers of the 3,3′-diethylthiacarbocyanine cation and their photophysical processes.     

A new ultra-pressure liquid chromatography method for the determination of biogenic amines in cheese

A fast and reliable ultra-performance liquid chromatography (UPLC™) method for the determination of biogenic amines (ethanolamine, methylamine, agmatine, histamine, dimethylamine, ethylamine, octopamine, pyrrolidine, dopamine, isopropylamine, propylamine, tyramine, putrescine, butylamine, cadaverine, tryptamine, 2-phenylethylamine, 3-methylbutylamine, spermidine, spermine) in cheese was established. After pre-column derivatization with 6-aminoquinolyl-N-hydroxy-succinimidyl carbamate (AQC), 20 primary and secondary biogenic amines were separated on an Acquity™ UPLC™ column (BEH C₁₈, 1.7 μm; 2.1 mm × 50 mm) within 9 min. Limits of detection (mg/100 g cheese) ranged from 0.04 (ethanolamine) to 1.62 (spermine), and limits of quantification were between 0.16 (ethanolamine) and 6.09 (spermine). The UPLC™ method was applied to the analysis of 58 cheese samples as retailed in Austria. About 13.8% of samples had a histamine content above 10 mg/100 g, and 22.4% had a tyramine content above 10 mg/100 g. Moreover, 8.6% of samples had a putrescine or cadaverine content higher than 10 mg/100 g. The total concentration of biogenic amines in two cheese samples was about 194 mg/100 g. Thus, obligatory monitoring of biogenic amines should be considered to ensure quality of cheese in future.     

Electric field dependence of ion mobilities of aromatic compounds with different ionic mass and different functional groups

Ion mobility spectrometry with high electric fields complements the information from the spectra obtained using conventional time-of-flight ion mobility spectrometry by providing the field dependence of ion mobility. The formation and dissociation of ion clusters depending on the differences between high and low field strength is one process which has a considerable influence on the peak position in the resulting spectra. This process can be affected by the properties of substances. Therefore, one objective of our study was to determine possible influences of the nature and position of functional groups on the field dependence of ion mobilities. For this purpose, we investigated sets of compounds with different functional groups including chlorobenzenes, toluenes, anilines and phenols. Furthermore, it is known that ionic mass is an important parameter influencing the field dependence of ion mobility. Therefore, we also investigated unsubstituted aromatic hydrocarbons with different ionic masses. Although an influence of functional groups on the field dependence of ion mobilities was found, the ionic mass has a more significant influence. However, some isomers can be separated by applying high field amplitudes. Furthermore, the influence of ionic mass on the peak position is also more significant using high field amplitudes.     

Toward the total synthesis of maoecrystal V: establishment of contiguous quaternary stereocenters

A synthetic strategy toward maoecrystal V has been identified. It has been shaped by the necessity to maneuver in sterically hindered molecular environments.     

Coordination of 4-Aminoantipyrine to Rhenium(V)

Abstract  

The reactions of trans-[ReOX₃(PPh₃)₂] (X = Cl, Br) with 4-aminoantipyrine (H₂pap) were studied, and the complexes cis-[ReX₂(pap)(H₂pap)(PPh₃)](ReO₄) (X = Cl (1), Br (2)) were isolated. The crystal structure of 2 was determined by single-crystal X-ray diffraction. It crystallizes in the monoclinic space group P2₁ with a = 11.5145(6) ?, b = 11.5330(5) ?, c = 17.5492(9) ?, β = 105.322(2)°, V = 2247.6(2) ?³, Z = 2, C₄₀H₃₉N₆O₆PBr₂Re₂, M _r = 1262.97, D _c = 1.866 g cm⁻³, F(000) = 1208 and μ(Mo K _α) = 7.244 mm⁻¹, the final R = 0.0545 and wR = 0.1314. The ligand pap is coordinated through the doubly deprotonated amino nitrogen as an imide, and H₂pap acts as a neutral bidentate chelate, with coordination through the neutral amino nitrogen and the ketonic oxygen.     

Product line pricing for services with capacity constraints and dynamic substitution

In this paper, we address a service provider’s product line pricing problem for substitutable products in services, such as concerts, sporting events, or online advertisements. For each product, a static price is selected from a pre-defined set such that the total revenue is maximised. The products are differentiated by some of their attributes, and their availability is restricted due to individual capacity constraints. Furthermore, they are simultaneously sold during a common selling period at the end of which the service is delivered. Consumers differ from one another with respect to their willingness to pay, and, hence, their reservation prices vary depending on the product. In the event of a purchase, they choose the product that maximises their consumer surplus.     

Transfers of metabelian p-groups

Explicit expressions for the transfers V _i from a metabelian p-group G of coclass cc(G)?=?1 to its maximal normal subgroups M ₁, . . . , M _p+1 are derived by means of relations for generators. The expressions for the exceptional case p?=?2 differ significantly from the standard case of odd primes p?≥ 3. In both cases the transfer kernels Ker(V _i ) are calculated and the principalisation type of the metabelian p-group is determined, if G is realised as the Galois group ${{\rm{Gal}}({F}_p^2(K)\vert K)}$ of the second Hilbert p-class field ${{F}_p^2(K)}$ of an algebraic number field K. For certain metabelian 3-groups G with abelianisation G/G′ of type (3, 3) and of coclass cc(G)?=?r?≥ 3, it is shown that the principalisation type determines the position of G on the coclass graph ${\mathcal{G}(3,r)}$ in the sense of Eick and Leedham-Green.     

An approach to aminonaphthoquinone ansamycins using a modified Danishefsky diene

A robust and scalable synthesis of a novel, cyano-substituted Danishefsky-type diene and its use in the Diels-Alder reaction with various dienophiles is reported. The diene allows for the rapid construction of highly substituted aminonaphthoquinones that occur in numerous ansamycin antibiotics.