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Entangled polymers are an important class of materials for their toughness, processability, and functionalizability. During processing, deformation introduces elastic stresses due to a combination of polymer orientation and polymer stretching. For many flows, however, the elastic contribution from chain stretching is not significant, and so-called “nonstretching” approximations have been developed to help explain and interpret experimental observations. Unfortunately, these nonstretching models tend to be limited to simple polymer formulations (linear and monodisperse) and are not useful for understanding any effects from marginal chain stretching that may be present. In this paper, we show that nonstretching approximations can be formally constructed as a perturbation expansion starting from a fully stretching constitutive equation. We apply this framework to the Rolie-Poly model, deriving the existing nonstretching variation and expanding to the second order. The second-order continuation provides quantitatively improved accuracy for both steady and unsteady flows and prevents a pathological/nonphysical blowup that can occur when effects from marginal chain stretching are ignored. We also derive and discuss leading order nonstretching approximations for more complex models of entangled polymers, accounting for disentanglement dynamics, polydispersity, and reversible scission reactions. Alternatives to the formal perturbation framework are also discussed, with potential trade-offs between accuracy, versatility, and computational cost. 

Chlorogenic acid esterases (ChlEs) are a useful class of enzymes that hydrolyze chlorogenic acid (CGA) into caffeic and quinic acids. ChlEs can break down CGA in foods to improve their sensory properties and release caffeic acid in the digestive system to improve the absorption of bioactive compounds. This work presents the structure, molecular dynamics, and biochemical characterization of a ChlE fromLactobacillus helveticus(Lh). Molecular dynamics simulations suggest that substrate access to the active site ofLhChlE is modulated by two hairpin loops above the active site. Docking simulations and mutational analysis suggest that two residues within the loops, Gln₁₄₅and Lys₁₆₄, are important for CGA binding. Lys₁₆₄provides a slight substrate preference for CGA, whereas Gln₁₄₅is required for efficient turnover. This work is the first to examine the dynamics of a bacterial ChlE and provides insights on substrate binding preference and turnover in this type of enzyme.