Closed Ring Structures Research Articles

Photoswitching of dipole moments, charge-transfer and spectroscopic properties

The photoswitching of the dipole moment, absorption spectra and oxidation/reduction potentials of donor–acceptor-substituted dihydropyrene–[2,2]metacyclophanediene photochromic systems has been studied theoretically. Two types of basic structures can be formulated for the closed-ring dihydropyrene photoisomers: a conjugated betaine dye type and a merocyanine type structure, respectively. The thermodynamically less stable [2,2]metacyclophanediene forms have significantly larger dipole moments (>40 D) and longer wavelength absorption (CT transition in NIR region) in relation to the closed-ring dihydropyrene structures (<20 D and absorption in visible region). The reason for the strong changing of these properties is that the [2,2]metacyclophanediene fragment acts as a π-electron conjugation barrier which gives rise to a charge separation of the donor and acceptor substituents, respectively.

Carbon nanotube closed-ring structures

We study the structure and stability of closed-ring carbon nanotubes using a theoretical model based on the Brenner-Tersoff potential. Many metastable structures can be produced. We focus on two methods of generating such structures. In the first, a ring is formed by geometric folding and is then relaxed into minimum energy using a minimizing algorithm. Short tubes do not stay closed. Yet tubes longer than 18 nm are kinetically stable. The other method starts from a straight carbon nanotube and folds it adiabatically into a closed-ring structure. The two methods give strikingly different structures. The structures of the second method are more stable and exhibit two buckles, independent of the nanotube length. This result is in strict contradiction to an elastic shell model. We analyze the results for the failure of the elastic model.

Cross-linking and amyloid formation by N- and C-terminal cysteine derivatives of human apolipoprotein C-II.

We have investigated the effect of disulfide cross-linking on amyloid formation by human apolipoprotein (apo) C-II. Three derivatives of apoC-II were generated by inserting a cysteine residue on either the N-terminus (C(N)-apoC-II), C-terminus (C(C)-apoC-II), or both termini (C(N)C(C)-apoC-II). Under reducing conditions, all derivatives formed amyloid with a fibrous ribbon morphology similar to that of wild-type apoC-II. Under oxidizing conditions, C(N)- and C(N)C(C)-apoC-II formed a highly tangled network of fibrils, suggesting that the addition of an N-terminal cysteine to apoC-II promotes interfibril disulfide cross-links. Fibrils formed by C(C)-apoC-II under oxidizing conditions were closely packed but less tangled than fibrils formed by the C(N) and C(N)C(C) derivatives. The frequency of closed ring structures was more than doubled for C(C)-apoC-II compared to wild-type apoC-II. The kinetics of fibril formation by all cysteine derivatives was markedly enhanced under oxidizing conditions, suggesting that disulfide cross-linking promotes amyloid formation. Substoichiometric levels of preformed C(N)- and C(C)-apoC-II dimers accelerate amyloid formation by wild-type apoC-II. These data suggest that the N- and C-termini of apoC-II are close together in the amyloid fibril such that covalent cross-linking of either the N or C end of apoC-II promotes nucleation and the "seeding" of fibril growth.

Synthesis of Ultralong and High Percentage of Semiconducting Single-walled Carbon Nanotubes

Two main results are reported here for single-walled carbon nanotubes (SWNTs) synthesized from well-controlled isolated catalytic Fe2O3 nanoparticles. The first is patterned growth of ultralong SWNTs (lengths up to 0.6 millimeters) by using a mixed methane and ethylene carbon source in chemical vapor deposition (CVD). Interesting loop and closed ring structures similar to those of fullerene “crop circles” are observed on these as-grown ultralong tubes. Second, surveying the electronic properties of individual SWNTs by transport measurements reveals that approximately 70% of individual SWNTs grown from isolated nanoparticles are semiconductors exhibiting field effect transistor (FET) characteristics. The distribution of CVD-grown SWNT chirality is elucidated for the first time, and implications to array-based nanotube electronics and sensors are discussed.

Template and Guest Effects on the Self-Assembly of a Neutral and Homochiral Helix.

Coordination-driven assembly based on manganese(II) centers and flexible 1,3-bis(4-pyridyl)propane leads to the solid-state formation of electronically neutral, self-templated homochiral helices, closed ring structures, and racemic mixtures of helices depending upon the presence or absence of guests such as benzene and 1,2-diphenylethane.

Amorphization of C 60 fullerite by applying pressure in the GPa range using a scanning tunneling microscope: Observation of transitions and aging

Fullerene molecules of a few monolayers thick C 60(111) film on Au(111) supported on mica, have been exposed to a pressure of ≈ 20 GPa by using the tip of a scanning tunneling microscope (STM) operated at 40 nA and 4 mV. Subsequent imaging at 0.7 nA and − 11 mV shows that this pressure has induced a transformation of C 60 fullerite into an amorphous structure with a nearest neighbour distance of 0.28 nm. The time evolution of this structure within ≈ 90 min reveals formation of nanocrystalline domains, their transformation into square eight-membered closed ring structures, and the breaking-up of these rings initially zig-zag, and then finally straight chains. Assignment of pentagon and hexagon carbon rings to the observed arrangements of protrusions is a possible explanation. Pair distribution and angular distribution functions are calculated based on the position of the protrusions extracted from the images.

Characterization of fluorescent products from reaction of methyl linoleate hydroperoxides with adenine in the presence of Fe 2+ and ascorbic acid

The structures of fluorescent products formed in the reaction of methyl linoleate hydroperoxides with adenine, FeSO 4 and ascorbic acid were investigated to elucidate the mechanism of interaction. The fluorescent products consisted of at least four major components (I-IV), which could be separated by thin-layer chromatography and high-performance liquid chromatography. Both 2-octenal and 2,4-decadienal, degradation products of methyl linoleate hydroperoxides, reacted with adenine to produce a fluorescent product similar to one of the major compounds (II) formed in the reaction of methyl linoleate hydroperoxides. Spectroscopic data suggest that I and III are the same type of compounds, which have closed ring structures with α,β-unsaturated carbonyl groups between the amino group at the 6-position and the nitrogen at the 1-position of adenine. Component II has a closed ring structure at the same site as I and III, and the presence of an ether linkage was suggested. On the basis of these structures, the involvement of 3-nonenal, methyl 12-oxo-9-dodecenoate and 2-octenal was suggested in the interaction of the methyl linoleate hydroperoxides decomposition products and adenine or DNA in the presence of FeSO 4 and ascorbic acid.

Basal-lateral transport and transcellular flux of methyl α- d-glucoside across LLC-PK 1 renal epithelial cells

The characteristics of methyl α- d-glucoside transport by the LLC-PK 1 cell line are extended by a study of the interaction of this glucose analog with the basal-lateral membrane of these cells: (1) 1 mM methyl α- d-glucoside enters LLC-PK 1 cells across the basal-lateral membrane 10-times more slowly than when entering across the apical membrane; (2) neither 10 mM glucose nor 10 mM methyl α- d-glucoside affect the rate of methyl glucoside uptake at the basal lateral membrane; (3) 0.1 mM phlorizin in the apical hemichamber significantly decreases the rate at which methyl glucoside enter the cell when methyl glucoside is present in the basal-lateral hemichamber; (4) the methyl glucoside transcellular flux ratio, J a J b (apical to basal vs. basal to apical) is 15, whereas J a J b for d-mannitol equals 1; and (5) basal-lateral to apical fluxes ( J b) of mannitol consistently exceed those of methyl glucoside. These results demonstrate that methyl glucoside, unlike glucose, is accumulated within these cells to a high degree because of the limited ability of methyl glucoside to exit the cells by a carrier-mediated pathway. They also raise the important caveat for any studies with glucose (and other low-molecular-weight substrates) by showing that a monosaccharide presented to one surface of these cells will traverse the cell sheet (in part) by the intercellular route and will enter the cell at the unintended cell surface. The ability of the tight junctions of this intercellular route to discriminate between open-chain molecules, such as mannitol, vs. closed ring structures, like methyl glucoside, is also described.

The question of an open-ring form of cyclopropanone

The semi-empirical lcao-mo-scf theory has been applied to both the closed-ring and open-ring structures of cyclopropanone. Three main geometrical configurations have been considered, namely the (0,0), (0,90), and (90,90) forms, with the numbers in parentheses referring to the angles between the planes of the CH 2 group and the CCC plane. The ground state of the compound is found to be the (90,90) form, with a small value (62.5°) of the unique CCC angle. Thus the compound is best described as a closed-ring ketone, in agreement with recent experimental data. The open-ring form, which is found to be quite high in energy by these calculations, should probably not be considered as a major intermediate in the cycloaddition reactions of the compound. The interconversion of the two forms through a disrotatory motion involves a barrier, which occurs around the (30,30) form. Protonated cyclopropanone is also predicted to resist opening, as is the lowest triplet state of the compound ( 3 A 2), which loosely corresponds to an n → π * transition.

CYCLIC COMPOUNDS CONTAINING A CARBONYL GROUP; A MECHANISM FOR THE FORMATION OF PYRYLLIUM SALTS FROM 1,5-DIKETONES

In the presence of alkali, cyclohexanone and its homologues add to chalcones to form either semicyclic diketones or dicyclic keto-alcohols; the latter contain a carbonyl bridge, and the former can be converted into closed ring structures and dehydrated to form substances having a carbonyl bridge. In these dicyclic ketones the bridge is not removed by heating, in contrast to the behavior of certain other compounds having a similar ring system.A second mode of ring closure gives rise to pyryllium salts; the isolation of a methyl ether has made it possible to devise a plausible mechanism for this hitherto obscure reaction. Four varieties of salts are described, the perchlorates being obtained by a different procedure than that previously employed.