Engineered Photoactivatable Genetic Switches Based on the Bacterium Phage T7 RNA Polymerase

Genetic switches in which the activity of T7 RNA polymerase (RNAP) is directly regulated by external signals are obtained with an engineering strategy of splitting the protein into fragments and using regulatory domains to modulate their reconstitutions. Robust switchable systems with excellent dark-off/light-on properties are obtained with the light-activatable VVD domain and its variants as regulatory domains. For the best split position found, working switches exploit either the light-induced interactions between the VVD domains or allosteric effects. The split fragments show high modularity when they are combined with different regulatory domains such as those with chemically inducible interaction, enabling chemically controlled switches. To summarize, the T7 RNA polymerase-based switches are powerful tools to implement light-activated gene expression in different contexts. Moreover, results about the studied split positions and domain organizations may facilitate future engineering studies on this and on related proteins

Nonlinear Rheology of Random Sulfonated Polystyrene Ionomers: The Role of the Sol–Gel Transition

The linear and nonlinear rheological behaviors of nonentangled sulfonated polystyrene (SPS) ionomers near the sol–gel transition were studied. When the degree of sulfonation, <i>p</i>, was below the gel point, the ionomer exhibited sol-like linear viscoelastic (LVE) behavior, and shear thinning was observed for steady shear flow. For <i>p</i> close to the gel point, the ionomer showed power-law-like LVE behavior over a wide frequency range. Strain hardening and shear thickening behavior were observed, and their magnitudes depended on the temperature, molecular weight of the PS precursor, and the Coulomb energy of the ion pair. Above the gel point, a distinct rubbery plateau was observed in the dynamic modulus. Melt fracture occurred upon start-up shear, which prevented quantitative examination of the nonlinear rheology. The possible mechanisms for strain hardening and shear thickening near the gel point are discussed with respect to formation of large clusters that nearly percolate in space

Nanostructures, Linear Rheological Responses, and Tunable Mechanical Properties of Microphase-Separated Cellulose-<i>graft</i>-Diblock Bottlebrush Copolymer Elastomers

A series of cellulose-graft-diblock bottlebrush copolymer elastomers (cellulose-graft-poly(n-butyl acrylate)-block-poly(methyl methacrylate) (Cell-g-PBA-b-PMMA)) with short side chains were synthesized via successive atom transfer radical polymerization (ATRP) to study the influence of varying compositions and lengths of the graft diblock side chains on microphase morphologies and properties. The microphase-separated morphologies from misaligned spheres to cylinders were observed by atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS) measurements. These bottlebrush copolymer elastomers possessed thermal stability and enhanced mechanical properties because the PMMA outer block could self-assemble into hard microdomains, which served as physical cross-links. The viscoelastic responses of these bottlebrush copolymers within the linear viscoelastic (LVE) regime were carried out by the oscillatory shear rheology. The time–temperature superposition (tTs) principle was applied to construct the master curves of the dynamic moduli, and the sequential relaxation of dense bottlebrush copolymers with different PMMA hard outer block lengths was analyzed. The rheological behaviors in this work could be utilized to build up the connection of microstructures and properties for the application of these bottlebrush copolymers as high-performance thermoplastic elastomers

Rheological Behavior of Partially Neutralized Oligomeric Sulfonated Polystyrene Ionomers

The linear viscoelastic (LVE) behavior of partially neutralized oligomeric sulfonated polystyrene (SPS) ionomers with different degrees of sulfonation (<i>p</i>) and degrees of neutralization (<i>x</i>) was investigated. The ionic dissociation time, τ_s, obtained from the reversible gelation model [Chen Macromolecules 2015, 48, 1221−1230] is mainly controlled by the neutralization degree, <i>x</i>, rather than the functional group (i.e., sulfonic acid and metal sulfonate) concentration, <i>p</i>. For a fixed <i>p</i>, increasing <i>x</i> significantly increases τ_s and the zero shear viscosity, η₀, especially near complete neutralization. These results explain the observations reported by Lundberg et al. [Ions in Polymers; American Chemical Society: 1980; Vol. 187, pp 67−76] that the increase of the viscosity of SPS ionomers with neutralization undergoes a substantial increase between 90% and 100% neutralization of the sulfonic acid groups to metal salts. This rapid increase of τ_s and η₀ is probably related to the decrease of sulfonic acid groups in the ionic aggregates with increasing <i>x</i>

Linear Viscoelasticity and Swelling of Polyelectrolyte Complex Coacervates

Mixing oppositely charged hydrophilic polyelectrolytes is the simplest path to constructing a polyampholyte gel that is useful as a soft tissue scaffold for binding enzymes in their native state. The swelling and viscoelastic properties of such a synthetic polyampholyte gel coacervate, constructed from polyions of different charge density, are reported in water with various amounts of NaCl salt. When constructed, this coacervate is roughly 70% water and 15% of each polyion, nearly charge balanced. If salt is removed from the surrounding supernatant, the gel swells owing to the weak charge imbalance because small amounts of salt screen electrostatic repulsions. If instead more salt is added to this coacervate, the gel behaves as any polyampholyte gel, swelling as salt is added because the excess salt screens the electrostatic attractions and eventually this leads to redissolving the coacervate. The amount of salt needed to redissolve the coacervate increases with polyion molecular weight. To our surprise, we discovered that the small charge imbalance within the coacervate grows with the molecular weight of the more strongly charged polyion

Copper Powder and Pd(II) Salts Triggered One-Pot Aromatic Halide Homocoupling via a Radical Pathway

(sp2)­C–(sp2)C bond formation is one of the most utilitarian techniques in target synthesis and material and pharmaceutical production. Biaryls usually emerge with the coupling of aryl halides or pseudohalides and require the prepreparation of an organometallic reagent, which results in low efficiency and atomic economy. The classic Ullmann reactions could be adopted to directly synthesize biaryls from aromatic halides. However, the requirement of extremely high temperatures limits the usage of the methodology in manufacturing. At the same time, the mechanism triggers a wide debate between classic redox and redox reactions involving radicals. In this work, a bimetallic system was demonstrated, referring to stoichiometric copper powder in the presence of a catalytic amount of Pd­(OAc)2, which contributed to delivering various symmetric/asymmetric (sp2)­C–(sp2)C species. It has been proposed that the coupling process might be promoted via radicals produced by redox between Cu(0) and Pd­(IV) species in the heating system. Increasing examples demonstrated the good tolerance of this method for aryl bromide among functional groups

Effect of Various Dissolution Systems on the Molecular Weight of Regenerated Silk Fibroin

Effect of Various Dissolution Systems on the Molecular Weight of Regenerated Silk Fibroi

Nucleophilicity Parameters of Arylsulfonyl-Substituted Halomethyl Anions

The rates of the reactions of the arylsulfonyl-substituted carbanions carrying α-chloro and α-bromo substituents (<b>1a–e</b>) with quinone methides <b>2a–g</b> and benzylidenemalonates <b>2h</b> and <b>2i</b> in DMSO were determined photometrically at 20 °C. The reactions were performed under pseudo-first-order conditions, and the second-order rate constants were obtained as the slopes of the plots of the pseudo-first-order rate constants versus the concentrations of the reactants used in excess. The second-order rate constants correlate linearly with the parameters <i>E</i> of the reference electrophiles according to the linear free energy relationship log <i>k</i>₂(20 °C) = <i>s</i>_N(<i>N</i> + <i>E</i>), which allowed us to derive the nucleophile-specific parameters <i>N</i> and <i>s</i>_N of carbanions <b>1a–e</b>. The resulting nucleophilicity parameters <i>N</i> (23 < <i>N</i> < 29) reveal the title compounds to be among the most reactive nucleophiles so far integrated on our comprehensive nucleophilicity scale

DataSheet1_Morphology and rheology of composites of poly(styrene-co-2-vinyl pyridine) copolymers and phosphotungstic acid.pdf

Morphological and rheological properties are examined for poly(styrene-co-2-vinyl pyridine) (P(S-co-2VP)) copolymers upon introducing phosphotungstic acid, one kind of polyoxometalates (POMs). The phosphotungstic acid protonates the 2VP monomers, and the deprotonated phosphotungstic acid effectively crosslinks the protonated 2VP monomers, inducing phase segregation into the S-rich and 2VP-rich domains. Linear viscoelasticity (LVE) of the composite samples strongly relies on the continuity of the 2VP-rich domains and can be classified into the following three types. (1) For 2VP-rich sphere domains in the S-rich matrix, LVE is akin to the conventional elastomer characterized by a wide rubbery regime before the terminal relaxation. (2) For bicontinuous morphology, where both the 2VP-rich and S-rich domains are continuous, two glassy processes manifest in LVE, and the chain relaxation is controlled by the continuous ion dissociation in the less mobilized 2VP-rich domain. (3) When the 2VP-rich domain is the only continuous phase, only the glassy modulus of the 2VP-rich domain manifests in LVE, and the chain relaxation is activated by the continuous ionic dissociations in the matrix. Surprisingly, the relaxation time obtained for all three abovementioned morphologies can be reduced to a universal behavior once the average glass transition temperature of the 2VP-rich region and the number of effective stickers per chain have been properly normalized, indicating that these two parameters control the chain-dimensional dynamics.</p

Viscoelasticity of Reversible Gelation for Ionomers

Linear viscoelasticity (LVE) of low-ion-content and low-molecular-weight (nonentangled) randomly sulfonated polystyrene shows a sol–gel transition when the average number of ionic groups per chain approaches unity. This transition can be well understood by regarding the number of ionizable sites over a chain as the relevant functionality for cross-linking. For ionomers below but very close to the gel point, the LVE shows power law relaxation similar to gelation of chemical cross-linking. Nevertheless, ionomers near and beyond the gel point also show terminal relaxation not seen in chemically cross-linking systems, which is controlled by ionic dissociation. Careful analysis of the power law region of the frequency dependence of complex modulus close to the gel point shows a change in exponent from ∼1 at high frequency to ∼0.67 at low frequency, which strongly suggests a transition from mean-field to critical percolation known as the Ginzburg point. A mean-field percolation theory by Rubinstein and Semenov for gelation with effective breakup has been modified to include critical percolation close to the gel point and predicts well the observed LVE of lightly sulfonated polystyrene oligomers