A structure-permeability relationship of ultrathin nanoporous silicon membrane: a comparison with the nuclear envelope

We report on a simple, quantitative relationship between structure and permeability of a novel ultrathin nanoporous membrane based on nanocrystalline silicon. Large permeability of the free-standing nanomembrane to Ru(NH3)63+, O2, or 1,1'-ferrocenedimethanol, which was able to be measured for the first time by employing scanning electrochemical microscopy, is proportional to the density (67 mum-2) and average radius (5.6 nm) of nanopores. As solution electrolyte concentration decreases down to 0.01 M, the nanopores are selectively "closed" against Fe(CN)64- because of electrostatic repulsion against negative charges at the pore wall. Permeability of the silicon nanomembrane was compared to permeability of the nuclear envelope to find that the channel diameter of the nuclear pore complex that perforates the nuclear envelope is much larger than the average diameter of the silicon nanopores and concomitantly a hypothetical diameter of 10 nm.     

Chiral recognition in noncovalent bonding interactions between helicenes: right-handed helix favors right-handed helix over left-handed helix

Our studies of helicenes are summarized in regard to chiral recognition phenomenon in noncovalent bonding interactions. The interactions between helical molecules show a tendency for pairs of the same configuration of the helicenes to form more stable complexes than pairs of enantiomeric helicenes. The observations are made in charge transfer complexation, crystallization, homocoupling reaction, layer structure formation, self-aggregation, and double helix formation. The interactions between a helicene and a right-handed helical polymer, double strand DNA, are also described.     

Synthesis and structure of built-up organic macromolecules containing helicene

Built-up macromolecules are acyclic molecules with molecular weights of several thousand daltons, which are synthesized by connecting small molecular units using stepwise methods. The chemical study of built-up macromolecules reveals some noteworthy properties that are different from those of conventional biological and synthetic macromolecules. A characteristic feature of built-up organic macromolecules is that their structures and properties are discontinuous at a certain molecular weight. For such macromolecules, variation in the small molecular units and the formation of cyclic structures substantially affect the structure and properties. The built-up organic macromolecules obtained by connecting helicenes with amide, acetylene, and amine groups are discussed in this paper. Some chiral built-up macromolecules are linked by covalent bonds, and the effects of linking on the structure are compared.     

Ion core structure in (C(2)n+ and (CS2)n- (n=3-10) studied by infrared photodissociation spectroscopy

Infrared photodissociation spectra of (CS(2))(n) (+) and (CS(2))(n) (-) with n=3-10 are measured in the 1100-2000 cm(-1) region. All the (CS(2))(n) (+) clusters exhibit three bands at approximately 1410, approximately 1490, and approximately 1540 cm(-1). The intensity of the 1540 cm(-1) band relative to those of the other bands increases with increasing the cluster size, indicating that the band at 1540 cm(-1) is assignable to the antisymmetric CS stretching vibration of solvent CS(2) molecules in the clusters. On the basis of density functional theory calculations, the 1410 and 1490 cm(-1) bands of (CS(2))(n) (+) are assigned to CS stretching vibrations of the C(2)S(4) (+) cation core with a C(2) form. The (CS(2))(n) (-) clusters show two bands at around 1215 and 1530 cm(-1). Similar to the case of cation clusters, the latter band is ascribed to the antisymmetric CS stretching vibration of solvent CS(2) molecules. Vibrational frequency analysis of CS(2) (-) and C(2)S(4) (-) suggests that the 1215 cm(-1) band is attributed to the antisymmetric CS stretching vibration of the CS(2) (-) anion core with a C(2v) structure.     

Conformation of L-tyrosine studied by fluorescence-detected UV-UV and IR-UV double-resonance spectroscopy

The laser-induced fluorescence spectrum of jet-cooled L-tyrosine exhibits more than 20 vibronic bands in the 35450-35750 cm(-1) region. We attribute these bands to eight conformers by using results of UV-UV hole-burning spectroscopy. These isomers are classified into four groups; each group consists of two rotational isomers that have a similar side-chain conformation but different orientations of the phenolic OH. The splitting of band origins of rotational isomers is 31, 21, 5, and 0 cm(-1) for these groups. IR-UV spectra suggest that conformers belonging to two of the four groups have an intramolecular OH...N hydrogen bond between the COOH and NH2 groups. By comparing experimental and theoretical results of L-tyrosine with those of L-phenylalanine, we propose probable conformers of L-tyrosine.     

A study of ADMET polyethylene with 21-carbon branches on every 15th compared to every 19th carbon: What a difference four extra backbone methylenes make

Precision polyethylenes with 21-carbon alkyl branches precisely spaced on every either 15th or 19th carbon along the polymer backbone lead to the formation of two kinds of lamellae, yielding different thicknesses during the crystallization process. Thinner lamellae originate from side-chain crystallization, whereas thicker lamellae are formed by cocrystallization of the branch and the main chain. Side-chain crystallization (separate from main chain crystallization) is favored when the branch is placed on every 15th carbon. Cocrystallization (side chain with main chain) is favored with the branch on every 19th carbon. Both form stable hexagonal crystal units. A branch spacing separation of just four carbons along the main chain makes a remarkable difference in crystallization behavior. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3090–3096     

"O-acyl isopeptide method" for peptide synthesis: synthesis of forty kinds of "O-acyl isodipeptide unit" Boc-Ser/Thr(Fmoc-Xaa)-OH

The O-acyl isopeptide method has recently received attention as an efficient synthetic method for peptides. Herein, forty kinds of "O-acyl isodipeptide unit" Boc-Ser/Thr(Fmoc-Xaa)-OH (1-40) were effectively synthesized in two-steps without epimerization. The O-acyl isodipeptide units are important building blocks to enable the routine use of the O-acyl isopeptide method.     

Effect of impurity atoms on α2/γ lamellar interfacial misfit in Ti–Al alloy: a systematic first principles study

The effect of transition metal solutes on the lattice parameters of γ-TiAl and α₂-Ti₃Al were studied by first principles calculations to find suitable elements for controlling the α₂/γ interfacial misfit in lamellar Ti–Al alloys. Better agreement was found between the calculated and experimental phase and site preferences of impurity atoms than in a previous first principles study. The calculated lattice parameters suggest that elements in groups 6–11 of the 4th period (Cr, Mn, Fe, Co, Ni and Cu) are effective for increasing the misfit, leading to increasing density of misfit dislocation and, in turn, higher yield strength and ductility. This effect is caused by the change in the lattice parameter of the γ-TiAl phase rather than those of α₂-Ti₃Al phase. This prediction agrees qualitatively with experimental data from a previous study although the effects of temperature are not taken into account. Further improvements should be possible by considering those effects. Nevertheless, the results highlight the effects of impurity addition on interfacial misfit at a level which cannot be achieved by classical concepts such as atomic size in a hard sphere model. The results will also be valuable in further more quantitative predictions and in understanding the effects of temperature, including off-stoichiometry, thermal expansion and vibration entropy.     

Organocatalytic tail-to-tail dimerization of olefin: umpolung of methyl methacrylate mediated by N-heterocyclic carbene

Highly selective tail-to-tail dimerization of methyl methacrylate has been realized by an N-heterocyclic carbene catalyst, giving dimethyl 2,5-dimethyl-2-hexenedioate with an E/Z ratio of 95:5 in 86% isolated yield. The umpolung mechanism is proposed on the basis of interception of the intermediates using ESI-MS analyses and deuterium-labeling experiments.