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Jacob Schaefer – One of the best experts on this subject based on the ideXlab platform.

  • Determination of the Extent of Reaction of Amine Cross-Linked Epoxy Resins by Solid-State 13C and 15N NMR
    Macromolecules, 1997
    Co-Authors: Matthew E. Merritt, Laurent Heux, Jean Louis Halary, Jacob Schaefer

    The chemical substitution of amine Nitrogens in cured, 15 N-labeled epoxy resins has been determined by a combination of rotational-echo double-resonance 13 C NMR and dipolar rotational spin-echo 15 N NMR. AmineNitrogen substitution is at least 90% (that is, no more than 10% of all Nitrogens have a directly bonded hydrogen) for resins formed from stoichiometric amounts of epoxide and either hexamethylenediamine or a 1:3 molar mixture of hexamethylenediamine and hexylamine. This direct measure of curing and cross-linking is consistent with indirect Fourier-transform infrared estimates of curing based on the disappearance of the deformation band of the epoxide ring.

T. C. O’connell – One of the best experts on this subject based on the ideXlab platform.

  • ‘Trophic’ and ‘source’ amino acids in trophic estimation: a likely metabolic explanation
    Oecologia, 2017
    Co-Authors: T. C. O’connell

    Amino acid Nitrogen isotopic analysis is a relatively new method for estimating trophic position. It uses the isotopic difference between an individual’s ‘trophic’ and ‘source’ amino acids to determine its trophic position. So far, there is no accepted explanation for the mechanism by which the isotopic signals in ‘trophic’ and ‘source’ amino acids arise. Yet without a metabolic understanding, the utility of Nitrogen isotopic analyses as a method for probing trophic relations, at either bulk tissue or amino acid level, is limited. I draw on isotopic tracer studies of protein metabolism, together with a consideration of amino acid metabolic pathways, to suggest that the ‘trophic’/‘source’ groupings have a fundamental metabolic origin, to do with the cycling of amino-Nitrogen between amino acids. ‘Trophic’ amino acids are those whose amino-Nitrogens are interchangeable, part of a metabolic amino-Nitrogen pool, and ‘source’ amino acids are those whose amino-Nitrogens are not interchangeable with the metabolic pool. Nitrogen isotopic values of ‘trophic’ amino acids will reflect an averaged isotopic signal of all such dietary amino acids, offset by the integrated effect of isotopic fracfractionation from Nitrogen cycling, and modulated by metabolic and physiological effects. Isotopic values of ‘source’ amino acids will be more closely linked to those of equivalent dietary amino acids, but also modulated by metabolism and physiology. The complexity of Nitrogen cycling suggests that a single identifiable value for ‘trophic discrimination factors’ is unlikely to exist. Greater consideration of physiology and metabolism should help in better understanding observed patterns in Nitrogen isotopic values.

John Roboz – One of the best experts on this subject based on the ideXlab platform.

  • Fast atom bombardment and tandem mass spectrometry of quaternary pyridinium salt-type tryptophan derivatives
    Journal of Mass Spectrometry, 1993
    Co-Authors: Laszlo Prokai, Katalin Prokai-tatrai, Nicholas Bodor, Jozsef Lango, John Roboz

    Fast atom bombardment and collision-induced dissdissociation tandem mass spectrometry were used to study the fragmentation of quaternary pyridinium salt-type amides of tryptophan (α-amino-3-indolepropionic acid) esters and their analogs which incorporate the α-Nitrogen into the quaternary pyridinium structure. By cleavage directly at the pyridine Nitrogen, the 1-alkyl-substituted nicotinamides decompose exclusively to a carbocation, which then becomes the intermediate to further fragments. Rearrangement of the 3-indolepropionate-2-yl carbocations may involve a five- to seven-membered ring expansion, which generates alternative fragmentation pathways; the formation of an even-electron and a radical cation, respectively. In trigonellyl amide-type tryptophan derivatives, fragmentation of the pyridinium ion proceeds on multiple pathways induced by the positive charge which may not be localized on the quaternary Nitrogen, and isomerization to a dihydropyridinyl structure is probably involved. Besides the formation of protonated nicotinamide and alkene from tryptophan amides that contain methylene or ethylene units between the amino and the quaternary pyridinium Nitrogens, a fragmentation route leading to the carbocation identical with that of the 1-alkyl-substituted nicotinamides has also been revealed.