Determining the shape of plant cellulose microfibrils is critical for understanding plant cell wall molecular architecture and conversion of cellulose into biofuels. Only recently has it been determined that these cellulose microfibrils are composed of 18 cellulose chains rather than 36 polymers arranged in a diamond-shaped pattern. This study uses density functional theory calculations to model three possible habits for the 18-chain microfibril and compares the calculated energies, structures, 13C NMR chemical shifts and WAXS diffractograms of each to evaluate which shape is most probable. Each model is capable of reproducing experimentally-observed data to some extent, but based on relative theoretical energies and reasonable reproduction of all variables considered, a microfibril based on 5 layers in a 34443 arrangement is predicted to be the most probable. A habit based on a 234432 arrangement is slightly less favored, and a 6 × 3 arrangement is considered improbable.
We report a macroscopically anisotropic hydrogel developed by the facile dialysis of a synthetic semirigid polyanion in aqueous solution of multivalent cations. By the uniaxial diffusion of Ca2+ into two ends of a thin rectangular reaction cell containing semirigid polyanion poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide) (PBDT) aqueous solution, centimeter-scale anisotropic hydrogels with the PBDT molecules and their self-assembled fibrous bundles align in perpendicular to the Ca2+ diffusion direction are obtained. The anisotropic gel shows a higher elastic modulus and tensile fracture stress/strain in the direction parallel to the PBDT orientation than that of perpendicular direction. By observing in situ the gelation process, an extraordinary molecular reorientation of PBDT at the Ca2+ diffusion flux front is observed for the first time. The mechanism for the molecular reorientation is discussed in terms of complexation and gelation.
BackgroundObtaining a better understanding of the complex mechanisms occurring during lignocellulosic deconstruction is critical to the continued growth of renewable biofuel production. A key step in bioethanol production is thermochemical pretreatment to reduce plant cell wall recalcitrance for downstream processes. Previous studies of dilute acid pretreatment (DAP) have shown significant changes in cellulose ultrastructure that occur during pretreatment, but there is still a substantial knowledge gap with respect to the influence of lignin on these cellulose ultrastructural changes. This study was designed to assess how the presence of lignin influences DAP-induced changes in cellulose ultrastructure, which might ultimately have large implications with respect to enzymatic deconstruction efforts.ResultsNative, untreated hybrid poplar (Populus trichocarpa x Populus deltoids) samples and a partially delignified poplar sample (facilitated by acidic sodium chlorite pulping) were separately pretreated with dilute sulfuric acid (0.10 M) at 160°C for 15 minutes and 35 minutes, respectively . Following extensive characterization, the partially delignified biomass displayed more significant changes in cellulose ultrastructure following DAP than the native untreated biomass. With respect to the native untreated poplar, delignified poplar after DAP (in which approximately 40% lignin removal occurred) experienced: increased cellulose accessibility indicated by increased Simons’ stain (orange dye) adsorption from 21.8 to 72.5 mg/g, decreased cellulose weight-average degree of polymerization (DPw) from 3087 to 294 units, and increased cellulose crystallite size from 2.9 to 4.2 nm. These changes following DAP ultimately increased enzymatic sugar yield from 10 to 80%.ConclusionsOverall, the results indicate a strong influence of lignin content on cellulose ultrastructural changes occurring during DAP. With the reduction of lignin content during DAP, the enlargement of cellulose microfibril dimensions and crystallite size becomes more apparent. Further, this enlargement of cellulose microfibril dimensions is attributed to specific processes, including the co-crystallization of crystalline cellulose driven by irreversible inter-chain hydrogen bonding (similar to hornification) and/or cellulose annealing that converts amorphous cellulose to paracrystalline and crystalline cellulose. Essentially, lignin acts as a barrier to prevent cellulose crystallinity increase and cellulose fibril coalescence during DAP.
Quorum sensing (QS) systems have been proposed in a wide variety of bacteria. The AI-2-based QS system represents the most studied of these proposed interspecies systems, and has been shown to regulate diverse functions such as bioluminescence, expression of virulence factors, and biofilm formation. As such, the development of modulatory compounds, both agonists and antagonists, is of great interest for the study of unknown AI-2 based QS systems and the potential treatment of bacterial infections. The fimbrolide class of natural products has exhibited excellent inhibitory activity against AI-2-based QS, and as such may be considered the “gold-standard” of AI-2 inhibitors. Thus, we sought to include a fimbrolide as a control compound for our recently developed alkyl-DPD panel of AI-2 modulators. Herein, we present a revised synthesis of a commonly studied fimbrolide, as well as a direct comparison between the fimbrolide and alkyl-DPD analogs. We demonstrate that our alkyl-DPD analogs are more potent inhibitors of QS in both Vibrio harveyi and Salmonella typhimurium, the two organisms with defined AI-2 systems, and in doing so, call into question the widely accepted use of fimbrolide-derived compounds as the “gold standard” of AI-2 inhibition.
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