Does liquid–liquid phase separation drive peptide folding?

Proline–arginine (PR) dipeptide repeats have been shown to undergo liquid–liquid phase separation and are an example of a growing number of intrinsically disordered proteins that can assemble into membraneless organelles. These structures have been posited as nucleation sites for pathogenic protein aggregation. As such, a better understanding of the effects that the increased local concentration and volumetric crowding within droplets have on peptide secondary structure is necessary. Herein we use Fourier transform infrared (FTIR) and two-dimensional infrared (2DIR) spectroscopy to show that formation of droplets by PR₂₀ accompanies changes in the amide-I spectra consistent with folding into poly-proline helical structures.

Two-dimensional infrared spectroscopy reveals folding of an intrinsically disordered peptide when sequestered into a model “membrane-less” organelle.     

RhCl_3/CH_3I催化醋酸甲酯羰基化制醋酐——Ⅱ.LiOAc存在下的动力学和机理

1.前言 在醋酸甲酯羰基化高活性均相铑催化体系中,CH_3I和锂化合物及含氮、膦有机物是最常用的助催剂。Schrod等首先对RhCl_3—CH_3I—有机碱—络合物体系催化醋酸甲酯羰基化反应动力学及红外光谱进行了研究,发现反应对醋酸甲酯为零级;CO的反应级数在压力1.5MPa以上也为零级;铑和CH_3I反应级数为一级;膦的反应级数随浓度增加由一级变为零级;Cr(CO)_6对反应速率影响不大,同时发现存在着中间反应物Rh(CO)_2I_2~-。根据这些实验事实,他们提出了类似于甲醇羰基化制醋酸的催化反应机理,认为速率控制步骤是CH_3I与Rh(CO)l-2~-的氧化加成,但他们没有报导有关锂化物助催剂的作用。Eastman公司的研究人员Polichnowski研究了RhCl_3—CH_3I—Lil体系催化醋酸甲酯羰基化动力学和原位红外光谱,也发现当反应总压在3.0MPa以上时,反应初速与CO压力无关,但RhCl_3,CH_3I和LiI等浓度     

Optically detected electron resonance in phosphorus-doped ZnTe

Samples of ZnTe showing near gap edge luminescence predominantly due to exciton recombination at shallow neutral acceptors and donor- acceptor pair recombination have been investigated using optically detected magnetic resonance (ODMR). Emission polarization changes at 2.318 eV were observed due to magnetic resonance of electrons at g_e = + 0.401 ± 0.004. The observations are consistent with the donor trapped electron resonance resulting from microwave induced changes in donor-acceptor pair photoluminescence.     

Determination of the strain distribution in a circular plate under uniform pressure for biaxial stress tests

A simple apparatus is described for generating a biaxial stress at the surface of a plastics testpiece in the form of a circular plate. A theory is developed for the calculation of the strain distribution in situations where the deflection of the centre of the plate is comparable with its thickness. Theoretical predictions are compared with strain values determined by a moiré fringe method and using strain gauges. These comparisons are made on two plates of different thickness loaded to pressure levels which produce similar maximum strains in the two plates but different deflection-to-thickness ratios. Conclusions are drawn on the applicability of the theory, and recommendations are made for the testing of plastics plates for failure studies.     

New high-energy luminescence bands from Co2+ in ZnSe

Three sharp absorption features in the energy range 2.36–2.55 eV have been detected in the transmission spectrum of Co-diffused ZnSe, and a number of luminescence transitions originating from the lowest of these states at 2.361 eV have been observed. Photoluminescence excitation spectra prove that these are high energy excited states of the Co²⁺_Zn impurity, a conclusion confirmed by comparison of measured and predicted luminescence energies. This represents the first identification of luminescence branching from a higher excited state of a transition metal ion in any semiconductor. The sharp, weakly phonon-coupled transitions involve either intra-impurity excitation or transitions from the impurity to localised states split off from a minimum in the conduction band. The implications of these observations for the mechanism of host-impurity energy transfer and for the nature of the excited state wavefunctions are discussed.     

A topological approach to evasiveness

The complexity of a digraph property is the number of entries of the vertex adjacency matrix of a digraph which must be examined in worst case to determine whether the graph has the property. Rivest and Vuillemin proved the result (conjectured by Aanderaa and Rosenberg) that every graph property that is monotone (preserved by addition of edges) and nontrivial (holds for some but not all graphs) has complexity Ω(v ²) wherev is the number of vertices. Karp conjectured that every such property is evasive, i.e., requires that every entry of the incidence matrix be examined. In this paper the truth of Karp’s conjecture is shown to follow from another conjecture concerning group actions on topological spaces. A special case of the conjecture is proved which is applied to prove Karp’s conjecture for the case of properties of graphs on a prime power number of vertices. Supported in part by an NSF postdoctoral fellowship Supported in part by NSF under grant No. MCS-8102248     

Proposed substitution of apolane-87 for squalane as a nonpolar reference phase in gas chromatography

Squalane is the standard, nonpolar reference phase used in schemes to characterize the solvent strength and selectivity of stationary phases in gas-liquid chromatography. A commentary, supported by additional experimental evidence, is provided to indicate a lack of suitability for this purpose. Primary objections to its use as a reference phase are its high volatility, compositional variations due to the presence of impurities, susceptibility to oxidative degradation, poor film forming and stabilizing properties, and retention properties that are strongly influenced by the character and treatment of the support used to prepare column packings. Because similar problems were not observed, or were much less significant, for the synthetic hydrocarbon Apolane-87, its use as a reference phase is to be preferred. Reference values for retention indexes, gas—liquid partition coefficients, and partial molal Gibbs free energies of solution for a series of solvent-selectivity test solutes are provided for both phases.     

L 1 ergodic behavior of non-negative kernels

Some results that have been obtained in the study of strongly and weakly ergodic behavior of non-homogeneous stochastic kernels are generalized to the case of non-negative kernels. The first generalization simply involves extending the definitions of weakly and strongly ergiodic behavior to the case of non-negative kernels and using the ergodic coefficient which was first defined for stochastic kernels by Dobrushin and extended to non-negative kernels by Blum and Reichaw. It happens that this straightforward extension excludes many cases of non-negative kernels which do exhibit a types of ergodic behavior. In order to study these cases a definition ofL ₁ weakly and strongly ergodic behavior is given in which normalizing by constants is allowed. Sufficient conditions for these types of ergodic behavior are given.