Crystal-to-crystal transformation from a 3D interpenetrated-type MOF {[Cu(BF(4))(2)(bpy)(H(2)O)(2)] (bpy)} (1) to a 2D square-grid-type [Cu(BF(4))(2)(bpy)(2)] (2) (bpy = 4,4'-bipyridine) was observed. It was derived from dehydration and confirmed by in situ FT-IR, TG, and elemental analysis. Moreover, we elucidate the novel expansion/shrinkage dynamic modulation of 2 triggered by clathrate formation with gas molecules.

The thermal transitions of confined polymers are important for the application of polymers in molecular scale devices and advanced nanotechnology. However, thermal transitions of ultrathin polymer assemblies confined in subnanometre spaces are poorly understood. In this study, we show that incorporation of polyethylene glycol (PEG) into nanochannels of porous coordination polymers (PCPs) enabled observation of thermal transitions of the chain assemblies by differential scanning calorimetry. The pore size and surface functionality of PCPs can be tailored to study the transition behaviour of confined polymers. The transition temperature of PEG in PCPs was determined by manipulating the pore size and the pore–polymer interactions. It is also striking that the transition temperature of the confined PEG decreased as the molecular weight of PEG increased.

Molecular dynamics simulations have been carried out in order to examine the mechanism of diffusion of small penetrants in amorphous polymer membranes. Diffusion processes of methane, water, and ethanol in poly(dimethylsiloxane) (PDMS) and in polyethylene (PE) were investigated. Pure liquid water and ethanol were also simulated. The insertion probabilities P(R) of hard-sphere atoms of radius R into the polymers and the liquids were calculated. The free volume fraction, P(0), of PDMS is large and the insertion probability of a finite size atom into PDMS is widely distributed compared with the other polymers and liquids. Simulations of 5 ns were performed for PDMS and in PE with a penetrant species, methane. The diffusion of methane in the polymer matrix exhibits anomalous (non-Einstein) behavior for time scales of 1 and 0.3 ns in PDMS and PE, respectively. Aggregates of water and ethanol are found to be formed in PDMS. Diffusion coefficients of water and ethanol in PDMS are reduced by more than 1 order of magnitude due to the aggregation. The calculated diffusion coefficients of the nonaggregated penetrants in PDMS and in the pure liquids agree well with the experimental values.

We recently identified the genes responsible for the serotype c-specific glucose side chain formation of rhamnose-glucose polysaccharide (RGP) in Streptococcus mutans. These genes were located downstream from the rgpA through rgpF locus that is involved in the synthesis of RGP. In the present study, the corresponding chromosomal regions were isolated from serotype e and f strains and characterized. The rgpA through rgpF homologs were well conserved among the three serotypes. By contrast, the regions downstream from the rgpF homolog differed considerably among the three serotypes. Replacement of these regions in the different serotype strains converted their serotypic phenotypes, suggesting that these regions participated in serotypespecific glucose side chain formation in each serotype strain. Based on the differences among the DNA sequences of these regions, a PCR method was developed to determine serotypes. S. mutans was isolated from 198 of 432 preschool children (3 to 4 years old). The serotypes of all but one S. mutans isolate were identified by serotyping PCR. Serotype c predominated (84.8%), serotype e was the next most common (13.3%), and serotype f occured rarely (1.9%) in Japanese preschool children. Caries experience in the group with a mixed infection by multiple serotypes of S. mutans was significantly higher than that in the group with a monoinfection by a single serotype.Streptococcus mutans strains are classified into three serotypes (c, e, and f), and the serologic specificity is defined by rhamnose-glucose polysaccharide (RGP) on the cell wall (6). We have characterized the genes involved in RGP synthesis in S. mutans Xc (serotype c) in the course of our previous studies. Four rml genes (rmlA through rmlD) are directly related to the synthesis of dTDP-L-rhamnose (12, 13), and the gluA gene encodes the enzyme producing UDP-D-glucose (18). The rgpG gene is implicated in the initiation of RGP synthesis by transfer of N-acetylglucosamine-1-phosphate to a lipid carrier (16). Furthermore, six other genes (rgpA through rgpF) required for RGP synthesis were identified in the region downstream from rmlD, and these genes are likely to be involved in the transport and assembly of RGP (11,19).The RGPs are composed of ␣1,2-and ␣1,3-linked rhamnan backbones with glucose side chains linked to alternate rhamnoses. Each serotype-specific polysaccharide has unique linkages of its glucose side chains (serotype c, ␣1,2-linkage; serotype e, ␤1,2-linkage; and serotype f, ␣1,3-linkage) (5, 10). Recently, we identified and characterized the genes required for glucose side chain formation of the serotype c-specific RGP (9). However, the loci responsible for the determination of the other serotypes have not yet been elucidated.In this study, we identified the loci involved in the glucose side chain formation of RGP in serotypes e and f of S. mutans and confirmed that these regions determine serotype specificities. Furthermore, we designed three pairs of primers from specific DNA sequences within each serotype determinan...

Atomistic computer-simulation evidences are presented for the possible existence of one-dimensional silicon nanostructures: the square, pentagonal, and hexagonal single-walled silicon nanotubes (SWSNTs). The local geometric structure of the SWSNTs differs from the local tetrahedral structure of cubic diamond silicon, although the coordination number of atoms of the SWSNTs is still fourfold. Ab initio calculations show that the SWSNTs are locally stable in vacuum and have zero band gap, suggesting that the SWSNTs are possibly metals rather than wide-gap semiconductors.L ow-dimensional nanostructures are known to have properties that can be markedly different from their bulk counterparts. For example, as shown in a recent experiment (1), Ho and coworkers demonstrated atom-by-atom evolution of electronic band structure of 1D gold chain. For semiconductor nanostructures, when the carriers (electrons and holes) are confined to dimensions less than their de Bröglie wavelength (typically a few nanometers), quantum-mechanical size effects can emerge. The carrier confinement at the nanoscale can be in 0D (quantum dots), 1D (quantum wires), or 2D (quantum wells) (2). Indeed, the current interest in fabricating low-dimensional semiconductor nanostructures, coined as band-structure engineering, has largely relied on novel quantum-mechanical size effects.The crystalline structure of 3D bulk silicon is cubic diamond, similar to that of carbon diamond. However, unlike the carbon counterpart (3), a 1D single-walled silicon nanotube (SWSNT) has not been found in nature yet, largely because silicon prefers sp 3 bonds rather than sp 2 bonds (4). Indeed, silicon has been viewed as nontubular solid rather than tubular solid, similar to carbon and boron nitride. The cubic diamond silicon is known as a semiconductor with an energy band gap of 1.17 eV (1 eV ϭ 1.602 ϫ 10 Ϫ19 J). At high pressures, however, the cubic-diamondstructured silicon can undergo a phase transformation to highly coordinated (sixfold or above) metallic phases such as the ␤-tin and hexagonal closed-packed structures (5). To date, experimentally produced 1D-like nanostructures of silicon [e.g., porous silicon (6) and silicon nanowires (7-12)] all assume either the cubic diamond crystalline structure or the local structure of amorphous silicon. A commonly reported electronic property of the 1D silicon nanostructures is that they all have wider band gap than the cubic diamond silicon. Here we present atomistic computer-simulation evidences of three thinnest SWSNTs: the square, pentagonal, and hexagonal silicon nanotubes. The local geometric structure of these SWSNTs differs from that of cubic diamond silicon and the surface-passivated 1D silicon nanowires. It also differs from that of the carbon-nanotube-like ''hypothetical'' SWSNTs (4, 13-15), because the latter types of SWSNTs are composed of both sp 3 and sp 2 bonds. Moreover, ab initio quantum-mechanical calculations show that the present SWSNTs exhibit an entirely opposite trend in the band-gap change compared wi...

Zeolitic imidazolate framework-8 (ZIF-8) has a “gate-opening” framework with narrow pore apertures that swing open by reorientation of 2-methylimidazolate (MeIM) linkers enforced by guest adsorption. The present study aimed to employ free energy analysis to provide insight into the mechanism of the adsorption-induced structural transition that results from the reorientation of the MeIM linkers. We combined experimental Ar adsorption at cryogenic temperatures with grand canonical Monte Carlo simulations to determine the free energy profiles as functions of the rotational angle of the MeIM linker (θIM) and bulk gas pressure. We also estimated the energy fluctuation of the system, which is crucial to discussing the structural transition from a metastable state. The results from the free energy analysis, for example, at 91 K, suggest the following conclusions: A gradual reorientation of the MeIM linkers up to θIM = 10.5° occurs with increasing gas pressure that is followed by a spontaneous structural transition to θIM = 25.5° during the adsorption process (gate opening), and then, during the desorption process, an equilibrium structural transition occurs with the opposite reorientation of the MeIM linkers from θIM = 25.5° to θIM = 10.5° (gate closing).

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Establishing new energy-saving systems for gas separation using porous materials is indispensable for ensuring a sustainable future. Herein, we show that ELM-11 ([Cu(BF 4) 2 (4,4′bipyridine) 2 ] n), a member of flexible metal-organic frameworks (MOFs), exhibits rapid responsiveness to a gas feed and an 'intrinsic thermal management' capability originating from a structural deformation upon gas adsorption (gate-opening). These two characteristics are suitable for developing a pressure vacuum swing adsorption (PVSA) system with rapid operations. A combined experimental and theoretical study reveals that ELM-11 enables the high-throughput separation of CO 2 from a CO 2 /CH 4 gas mixture through adiabatic operations, which are extreme conditions in rapid pressure vacuum swing adsorption. We also propose an operational solution to the 'slipping-off' problem, which is that the flexible MOFs cannot adsorb target molecules when the partial pressure of the target gas decreases below the gate-opening pressure. Furthermore, the superiority of our proposed system over conventional systems is demonstrated.