Unfolding of Lignin Structure Using Size-Exclusion Fractionation.

The heterogeneous and recalcitrant structure of lignin hinders its practical application. Here, we describe how new approaches to lignin characterization can reveal structural details that could ultimately lead to its more efficient utilization. A suite of methods, which enabled mass balance closure, the evaluation of structural features, and an accurate molecular weight (MW) determination, were employed and revealed unexpected structural features of the five alkali lignin fractions obtained with preparative size-exclusion chromatography (SEC). A thermal carbon analysis (TCA) provided quantitative temperature profiles based on sequential carbon evolution, including the final oxidation of char. The TCA results, supported with thermal desorption/pyrolysis gas chromatography-mass spectrometry (TD-Py-GC-MS) and 31P NMR spectroscopy, revealed the unfolding of the lignin structure as a result of the SEC fractionation, due to the disruption of the interactions between the high- and low-MW components. The "unraveled" lignin revealed poorly accessible hydroxyl groups and showed an altered thermal behavior. The fractionated lignin produced significantly less char upon pyrolysis, 2 vs. 47%. It also featured a higher occurrence of low-MW thermal evolution products, particularly guaiacol carbonyls, and more than double the number of OH groups accessible for phosphitylation. These observations indicate pronounced alterations in the lignin intermolecular association following size-exclusion fractionation, which may be used for more efficient lignin processing in biorefineries.

Pathways toward PAH Formation during Fatty Acid and Triglyceride Pyrolysis.

Molecular beam mass spectrometry was used to follow model triglyceride pyrolysis with temperature. A selectively formed set of PAHs (276, 352, 444 amu) arose with increasing temperature. They were attributed to association of up to five C7-C8 sized fragments (observed in abundance by pyrolysis with gas chromatography), presumably due to their propensity to form stable benzyl radicals. Results were surprisingly similar regardless of triglyceride fatty acids (FAs), containing 0-2 C=C double bonds (14 to 18 carbon atoms). However, the absence of C=C double bonds shifted the process to higher temperatures. Shorter FA chains, particularly 14:0, enhanced generation of shorter size fragments, facilitating the alternate formation of nonselective PAH homology series. An increase in the length of the glass wool filled sample stage enhanced the formation of two more PAHs, 316 and 388 amu. They appear to involve the formation of indenyl in addition to benzyl radicals as key intermediates.

Electrospray Ionization with High-Resolution Mass Spectrometry as a Tool for Lignomics: Lignin Mass Spectrum Deconvolution.

The capability to characterize lignin, lignocellulose, and their degradation products is essential for the development of new renewable feedstocks. Electrospray ionization high-resolution time-of-flight mass spectrometry (ESI-HR TOF-MS) method was developed expanding the lignomics toolkit while targeting the simultaneous detection of low and high molecular weight (MW) lignin species. The effect of a broad range of electrolytes and various ionization conditions on ion formation and ionization effectiveness was studied using a suite of mono-, di-, and triarene lignin model compounds as well as kraft alkali lignin. Contrary to the previous studies, the positive ionization mode was found to be more effective for methoxy-substituted arenes and polyphenols, i.e., species of a broadly varied MW structurally similar to the native lignin. For the first time, we report an effective formation of multiply charged species of lignin with the subsequent mass spectrum deconvolution in the presence of 100 mmol L-1 formic acid in the positive ESI mode. The developed method enabled the detection of lignin species with an MW between 150 and 9000 Da or higher, depending on the mass analyzer. The obtained M n and Mw values of 1500 and 2500 Da, respectively, were in good agreement with those determined by gel permeation chromatography. Furthermore, the deconvoluted ESI mass spectrum was similar to that obtained with matrix-assisted laser desorption/ionization (MALDI)-HR TOF-MS, yet featuring a higher signal-to-noise ratio. The formation of multiply charged species was confirmed with ion mobility ESI-HR Q-TOF-MS. Graphical Abstract ᅟ.

Enantioselective metabolism of trans-4-hydroxy-2-nonenal by brain mitochondria.

Trans-4-hydroxy-2-nonenal (HNE) is a product of lipid peroxidation with many cellular effects. HNE possesses a stereogenic center at the C4 carbon that influences the metabolism and alkylation targets of HNE. We tested the hypothesis that rat brain mitochondria metabolize HNE in an enantioselective manner after exposure to racemic HNE. The study of HNE chirality, however, is hindered by the lack of facile methods to chromatographically resolve (R)-HNE and (S)-HNE. We used a chiral hydrazine, (S)-carbidopa, as a derivatization reagent to form diastereomers with (R)-HNE and (S)-HNE that were separated by reverse-phase HPLC. After exposure to racemic HNE, rat brain mitochondria metabolized HNE enantioselectively with a higher rate of (R)-HNE metabolism. By using the purified enantiomers of HNE, we found that this enantioselective metabolism of HNE was the result of higher rates of enzymatic oxidation of (R)-HNE by aldehyde dehydrogenases compared to (S)-HNE. Conjugation of HNE to glutathione was a minor metabolic pathway and was not enantioselective. These studies demonstrate that the chirality of HNE affects its mitochondrial metabolism and potentially other processes in the central nervous system.

One-pot synthesis of C-glycosylic compounds (C-glycosides) from D-glucal, p-tolylsulfenyl chloride and aromatic/heteroaromatic compounds in the presence of Lewis acids.

In the presence of Zn(CN)(2), benzylated 2-thio-2-S-(p-tolyl)pyranosyl chlorides (2) generated in situ from tri-O-benzyl-D-glucal and p-TolSCl, smoothly react with thiophene, 2-methylthiophene, furan, 2-methylfuran, and N-methylpyrrole to give heteroaryl 2-thio-2-S-(p-tolyl)-C-beta-D-glucopyranosylic compounds (C-glycosides) in good yields. Upon treatment with SnCl(4), the reaction of chlorides 2 with thiophene or 1,4-dimethoxybenzene provides the corresponding benzylated C-beta-D-glucofuranosylic derivatives. Under the same conditions, the use of 2-methylthiophene, furan, 2-methylfuran, or N-methylpyrrole yields (2S,3R,4R,5S)-1,3,4-tribenzyloxy-6,6-diheteroaryl-5-(p-tolylthio)-2-hexanoles. Treatment of 2 and mesitylene with AgBF(4) yielded 1,6-anhydro-3,4-di-O-benzyl-2-thio-2-S-(p-tolyl)-beta-D-glucose.

A one-pot synthesis of beta-C-glucopyranosides from exo-glucal, p-tolylsulfenyl chloride, an alpha-methoxyalkene, and an external nucleophile.

[formula: see text] beta-C-Glycosides were synthesized in one-pot experiments using the following sequence of four reactions: (i) addition of p-TolSCl to an alpha-methoxyalkene, (ii) generation of the episulfonium ion from a beta-(arylsulfanyl)alkyl chloride, (iii) reaction of the episulfonium intermediate with benzylated exo-D-glucal to form a cyclic five-membered sulfonium salt, and (iv) quenching of the sulfonium salt with the external nucleophile: H2O, CH3OH, or NaCNBH3.