Assessing internal structure of polymer assemblies from 2D to 3D CryoTEM: Bicontinuous micelles

The self-assembly behaviour of block copolymers in solution has been of significant interest over the past two decades for a number of applications — for example, as delivery vectors and micro-reactors. More recently, attention has turned to the formation of aggregates with complex internal structure, such as multi-compartment micelles and the so-called “Janus” particles (biphasic aggregates) for their promising application as vectors for the simultaneous inclusion of chemically-different encapsulates and their possible catalytic and templating properties. The challenge has been to observe these complex aggregates in such a way as to be able to characterise their internal morphology whilst preserving their intricate structure. To this end, cryogenic transmission electron microscopy (cryoTEM) has become a powerful and indeed a necessary tool for the elucidation and observation of self-assembled polymer systems. Through its use, a new class of complex micelles has been discovered: amphiphilic block copolymer nanospheres with internal bicontinuous structure. These structures have been observed from a variety of block copolymer amphiphiles, although rarely. Intriguingly, there is no seemingly obvious unifying blueprint for their formation. This review will present the importance of cryoTEM in the elucidation and characterisation of internally-structured polymeric aggregates in recent years and highlight its significance in the definition of bicontinuous dispersions.     

Birthrates and delay times of Type Ia supernovae

Type Ia supernovae (SNe Ia) play an important role in diverse areas of astrophysics, from the chemical evolution of galaxies to observational cosmology. However, the nature of the progenitors of SNe Ia is still unclear. In this paper, according to a detailed binary population synthesis study, we obtained SN Ia birthrates and delay times from different progenitor models, and compared them with observations. We find that the Galactic SN Ia birthrate from the double-degenerate (DD) model is close to those infe...     

A QM/MM study of the racemization of vinylglycolate catalyzed by mandelate racemase enzyme.

The experimentally postulated mechanism for the interconversion between (S)-vinylglycolate and (R)-vinylglycolate catalyzed by mandelate racemase enzyme consists of a two-step quite symmetric process through a dianionic enolic intermediate that is formed after the abstraction of the alpha-proton of vinylglycolate by a basic enzymatic residue and is then reprotonated by another residue. The challenging problem behind this reaction is how the enzyme manages to stabilize such an intermediate, that is, how it lowers enough the high pK(a) of the alpha-proton for the reaction to take place. The QM/MM simulations performed in this paper indicate that catalysis is based on the stabilization of the negative charge developed on the substrate along the reaction. We have identified three different reaction mechanisms starting from different quasi-degenerate structures of the substrate-enzyme complex. In two of them the stabilizing role is done by means of a catalytic proton transfer that avoids the formation of a dianionic intermediate, and they involve six steps instead of the two experimentally proposed. On the contrary, the third mechanism passes through a dianionic species stabilized by the concerted approach of a protonated enzymatic residue during the proton abstraction. The potential energy barriers theoretically found along these mechanisms are qualitatively in good agreement with the experimental free energy barriers determined for racemization of vinylglycolate and mandelate. The theoretical study of the effect of the mutation of Glu317 by Gln317 in the kinetics of the reaction reveals the important role in the catalysis of the hydrogen bond formed by Glu317 in the native enzyme, as only one of the mechanisms, the slower one, is able to produce the racemization in the active site of the mutant. However, we have found that this hydrogen bond is not an LBHB within our model.     

On Formality of a Class of Compact Homogeneous Spaces

We prove the formality property of any homogeneous space G/G  generated by an automorphism of finite order of a compact simple Lie group G.</sup>     

城域WDM环网及其应用

文章阐述了城域WDM环网的节点结构、保护方式、波长转换器、子速率复用和光网络管理等,给出了WDM环网在城域应用时应考虑的一些因素,并分析了运营商所关心的系统成本问题。     

Pauli exchange effects in the elastic scattering of O + O

The real optical potential for ¹⁶O+16</sup>O system is calculated within a generalized version of the double-folding model with the Pauli knock-on exchange effects between the projectile nucleons and the target nucleons taken into account from first principle. The elastic scattering data at E_lab=350 MeV supplemented by the new measurement at larger angles seem to be the first case in heavy-ion scattering where one can test the reliability of different theoretical heavy-ion optical potentials. Predictions are made for the elastic scattering of 16</sup>O+16</sup>O at laboratory energies of 240–480 MeV to illustrare the energy dependence of the rainbow structure which has been clearly observed in experiment at 350 MeV.</sup>     

Random walk near the surface

The random walk of a particle on a three-dimensional semi-infinite lattice is considered. In order to study the effect of the surface on the random walk, it is assumed that the velocity of the particle depends on the distance to the surface. Moreover it is assumed that at any point the particle may be absorbed with a certain probability. The probability of the return of the particle to the starting point and the average time of eventual return are calculated. The dependence of these quantities on the distance to the surface, the probability of absorption and the properties of the surface is discussed. The method of generating functions is used.     

Tunneling dynamics and spatial correlations of long chain growth in solid-state photochlorination of ethylene at low temperatures

A novel method is described for preparing reactive mixed crystals by vapor deposition in argon and subsequent evaporation of the argon. The kinetics of photochlorination chain reactions of ethylene-chlorine mixtures prepared by this method have been studied by dynamic UV and IR spectroscopy. Activationless chain growth occurs with mean length 260±70 in the temperature range 17–45 K. Chain lengths greater than 100 are observed up to 60–70% conversion to products. Comparison of the experimental data with computer simulations shows that the tunneling reaction is due to formation of a close-packed reaction complex with high amplitude zero-point vibrations which lower the vibrationally adiabatic barrier to reaction. The commensurability of reactant and product crystal lattices results in linear propagation of long chains without accumulation of deformation strain. Spatial correlations caused by nonlinear propagation are considered.