Recent experiments claiming that Naf-BBL protein follows a global downhill folding raised an important controversy as to the folding mechanism of fast-folding proteins. Under the global downhill folding scenario, not only do proteins undergo a gradual folding, but folding events along the continuous folding pathway also could be mapped out from the equilibrium denaturation experiment. Based on the exact calculation using a free energy landscape, relaxation eigenmodes from a master equation, and Monte Carlo simulation of an extended Muñoz–Eaton model that incorporates multiscale-heterogeneous pairwise interactions between amino acids, here we show that the very nature of a two-state cooperative transition such as a bimodal distribution from an exact free energy landscape and biphasic relaxation kinetics manifest in the thermodynamics and folding–unfolding kinetics of BBL and peripheral subunit-binding domain homologues. Our results provide an unequivocal resolution to the fundamental controversy related to the global downhill folding scheme, whose applicability to other proteins should be critically reexamined. 相似文献
A scaling relation for the disjoining pressure of strongly confined polymer fluids is proposed for the first time, which yields directly the scaling exponent ν of the radius of gyration for polymers. To test the proposed scaling relation we performed extensive particle‐based, coarse‐grained computer simulations of polymers confined under θ‐solvent conditions and found that the value of ν agrees with the expected value for strictly two dimensional chains, ν = 4/7, which points towards the essential correctness of our scaling relation. New approaches are suggested for experimental tests of this scaling law. This work opens up the way to look for other scaling exponents that may reveal new physical regimes and it constitutes an efficient route to determine ν because the scaling exponent can be obtained with chains of a single polymerization degree by simply reducing the distance between the confining plates.
Experimental mixed‐gas sorption/dilation data and mixture densities estimated by the fitted Sanchez–Lacombe equation of state have been used to estimate the partial molar volumes (PMV) of gases and polymers in multicomponent mixtures (i.e., ternary) at conditions of industrial relevance. The method developed estimates accurately the PMV and volumetric thermal expansion coefficients of various highly soluble gases and polymers in multicomponent mixtures over a wide range of temperatures, pressures, and gas phase compositions. A comparison of solubility, volumetric thermal expansion coefficients, and PMVs of the gases involved in the studied ternary mixtures reveal that, irrespective of the polymer nature, co‐solvent effect is caused by the gas with higher solubility in the polymer phase and higher thermal expansion coefficient, which provides an explanation to the occurrence of co‐solubility effects in multicomponent gases/polymer mixtures. It has also been shown that the PMV behavior of gases in the ternary mixtures with polymers is different from their PMV behavior in the corresponding binary gas/polymer mixtures, and that the PMV of a gaseous penetrant in a multicomponent system depends on its gas phase concentration.
The homopolymerizations of D ,L ‐lactide and glycolide catalyzed by ZnOct2 were studied by DSC. The kinetic parameters obtained are compared with those obtained from conversion data of bulk polymerizations. For the glycolide, the respective values of the calculated rate constants were quite similar. For the lactide polymerization, however, the agreement between the values obtained was not so good because the kinetics is affected by thermodynamics in the DSC experiments. As a result, the kinetics can not be studied by DSC. The degree of aggregation of the catalyst, the reaction order with respect to it, and the initiator influence were determined for glycolide homopolymerization.
Ultrathin films of end-tethered polymers, generally indicated as polymer brushes, have gained great interest for their ability to modify the surface interaction with the surrounding environment. One of the most common strategies to obtain polymer brushes is based on the reaction of end-functional polymers with the substrate surface, in a process defined as grafting to. The two advantages of grafting to reactions are i) the functional polymer is prepared before the grafting reaction and therefore can be fully characterized and ii) the reaction has a self-limiting nature, meaning that the number of grafted chains converges to a plateau value after a sufficiently long reaction time. The limit value of grafted chains is governed by the molecular weight of the employed polymer, thus allowing a fine control of the brush thickness. In this work, the origin of the self-limiting nature of the grafting to reaction is discussed comparing theoretical models with experimental data. Furthermore, the kinetics of the process is discussed, considering both polymer diffusion and end-group reactivity as the main drivers of the grafting to process. Finally, the effect of molecular weight dispersity on the brush characteristics is highlighted. 相似文献
Rare-earth elements (REEs) are critical components of many modern technologies, but their efficient separation and purification continue to pose significant challenges. Metal-binding polymers may enable the development of promising alternative approaches to conventional separation techniques (e.g., solvent extraction), and insights into the interactions between the metal ions and polymers in solution are crucial to such development. In this work, the effect of ionic strength on the chelation of REEs by poly(acrylic acid) is investigated using isothermal titration calorimetry (ITC) to measure the metal-binding thermodynamics. ITC experiments revealed that when ionic strength increases from increasing the sodium acetate buffer strength, polymer-REE binding becomes increasingly disfavored. However, when ionic strength is increased by the addition of sodium chloride, binding again becomes increasingly disfavored, but to a lesser extent than when ionic strength increases from increasing the buffer strength. Additionally, the reduction in binding affinity is enthalpically driven when ionic strength is raised with sodium chloride, while it is entropically driven when raised with sodium acetate buffer, suggesting competive binding by the acetate ions. These results highlight the importance of ionic strength in polymer-metal binding thermodynamics and will guide the development of new processes for the purification of critical metals. 相似文献