Bibliothek

Notizen: Time delays, dwell times, resonant state lifetimes, and electron sheet densities associated with tunneling through quantum well structures are analyzed from a scattering theory (S-matrix) viewpoint. Some of the results differ from intuitively motivated expressions which have appeared in the resonant tunneling literature. It is shown that the sheet density is given by a formula similar to the Tsu–Esaki formula [Appl. Phys. Lett. 22, 562 (1973)] for current density. Sheet density and dwell time are related to Hermitian matrices which are expressed in terms of the S-matrix and in terms of partial widths associated with resonant states.

Notizen: Based on a closed surface of triangles fitted to atomic coordinates determined crystallographically, Brune and Kim [Proc. Natl. Acad. Sci. USA 90, 3835–3839 (1993)] proposed a boundary-element Stokes-flow technique for ab initio computation of a translational diffusion coefficient and the rotational diffusion tensor Dr of globular proteins. They applied their approach to atomic coordinates for a tetragonal structure of hen egg-white lysozyme, and reported that computed values of a translational diffusion coefficient and Dr=tr(Dr)/3 agreed well with experiment. After establishing the identity between the infinite-dilution tracer diffusion coefficient of the protein macroion (D+ for lysozyme cation) and the "translational diffusion coefficient" computed by Brune and Kim, we adopt a somewhat different computational approach and show how convergence of D+ and Dr for tetragonal lysozyme depends on two computational parameters characterizing the fidelity of the geometric approximation to the protein surface and two others characterizing the accuracy of the Stokes-flow computations. We then compute D+ and Dr for lysozyme using atomic coordinates for the triclinic crystal structure, three structures determined by nuclear magnetic resonance spectroscopy in the liquid phase (presumably corresponding more closely to in vivo structures), the solvated tetragonal structure (with 108 water molecules) considered by Brune and Kim, and a "dry" version of the same structure. These computations show that D+ and Dr computed for all of the dry crystal structures are in excellent agreement with those for the liquid-phase conformations. Values of D+ and Dr computed for the solvated structure are lower, consistent with the larger volume and area of the corresponding polyhedral surface. We also show that several choices of the origin of the force system [discussed by Brenner, J. Colloid Interface Sci. 23, 407–436 (1967)] give rise to nearly identical translational diffusion coefficients. Finally, we show how to estimate the thickness of the "solvation shell" contributing to the hydrodynamic resistance of the protein cation, and use the binary Nernst–Hartley equation to then estimate the effective cation charge at the two pH values at which the binary diffusion coefficient has been accurately measured in recent interferometric experiments. © 2002 American Institute of Physics.

Notizen: Silicon nitride (Si3N4) and SiAlONs can be self-toughened through the growth of elongated β-Si3N4/β-SiAlON grains in sintering. α-SiAlONs usually retain an equiaxed grain morphology and have a higher hardness but lower toughness than β-SiAlONs. The present work has demonstrated that elongated alpha-SiAlON grains can also be developed through pressureless sintering. alpha-SiAlONs with high-aspect-ratio grains in the calcium SiAlON system have exhibited significant grain debonding and pull-out effects during fracture, which offers promise for in-situ-toughened α-SiAlON ceramics.

Notizen: Microstructural development in the interface region of α-Al2O3 bilayer composites has been systematically investigated in terms of the sintering additive CaO–SiO2, residual impurity level in the starting powders (particularly MgO), and sintering conditions. The interfacial microstructure is strongly related to relative CaO–SiO2 doping levels in the two constituting layers and to residual impurities in the starting powders. The presence of high levels of impurities in the starting powder can substantially modify the features of CaO–SiO2-Al2O3 liquid at the interface region, thereby strongly influencing α-Al2O3 grain growth across the interface. Three grain growth modes in the interface region thus have been identified for different combinations of impurity level and CaO–SiO2 dopant in the α-Al2O3 bilayer. This provides an important mechanism for controlling two-dimensional structures in coatings, films, and layered ceramic materials for various engineering applications.

Notizen: A gap junction is composed of two hemichannels and possesses a relatively large pore size (∼ 10–15 Å), allowing passage of ions and molecules up to 1 kDa. Here, we report that connexin hemichannels and gap junctions in the guinea pig cochlea had significant charge selectivity among permeating molecules. In coincubation with anionic and cationic fluorescent dyes, hemichannel permeability in isolated cochlear supporting cells showed significant charge selectivity; 31% of cells had only cationic dye influx and 6% of cells had only anionic dye influx. Charge-selective influx contrary to dye size was also found, indicating charge as a dominant determinant in permeability. The cell–cell gap junctional permeability was consistent with hemichannel permeability and also showed strong charge selectivity; the permeation of anionic dyes was slower than that of cationic probes in the cochlear sensory epithelium. With a combination of immunofluorescent staining for connexin26 (Cx26) and Cx30, which are the predominant connexin isoforms in the cochlea, Cx26 was demonstrated to correlate with anionic permeability. The data indicated that cochlear gap junctions have strong charge selectivity in molecular permeability and metabolic communication. Cx26 mutation may induce specific, irreparable impairment in intercellular signalling and energy and nutrient supplies in the cochlea, causing cell degeneration and hearing loss, given that many important cell-signalling and nutrient and energy molecules (e.g. IP3, ATP, cAMP and cGMP) are anions.

Notizen: Background:  Breath odour is a complaint encountered worldwide, often linked to microbial overload in the oral cavity. This double blind, crossover, randomised study assessed the efficacy of several antiseptic mouthrinses or slurry vs. a control solution in the prevention of morning bad breath during an experimental period of 7 days without mechanical plaque control. Methods: Sixteen dental students with a healthy periodontium abolished, after a thorough professional cleaning, all means of mechanical plaque control during five experimental periods of 7 days, interleaved by washout periods of at least 3 weeks. During each experimental period, as the only oral hygiene measure, the students rinsed twice a day with one of the following formulations (in a randomised order): a 0.2% chlorhexidine-alcohol mouthrinse (CHX-Alc), a 0.05% CHX + 0.05% cetylpyridinium chloride + 0.14% zinc lactate mouthrinse (CHX-CPC-Zn), an amine fluoride/stannous fluoride (125 ppm F–/125 ppm F–) containing mouthrinse (AmF/SnF2Mr), a slurry of a tooth paste (AmF/SnF2Sl) containing amine fluoride (350 ppm F–) and stannous fluoride (1050 ppm F–) and a placebo solution (placebo). At days 0, 3 and 7, morning breath was scored via VSC level measurements of the mouth air, and organoleptic ratings of the mouth air and tongue coating. At the same visits both the degree of gingival inflammation and the de novo plaque formation were rated. At the end of each period a questionnaire for subjective ratings was completed and microbiological samples were taken from the tongue dorsum, the saliva and the supragingival plaque for anaerobic and aerobic culturing. Results: Although oral hygiene during the experimental periods was limited to rinsing, bad breath parameters systematically improved (P 〈 0.001) with the three mouthrinses (CHX-Alc, CHX-CPC-Zn, AmF/SnF2Mr), with a superiority of the CHX-CPC-Zn solution when only VSC values were considered (P 〈 0.003). The AmF/SnF2 slurry and the placebo solution showed only minor changes with time. The three mouthrinses reduced significantly (P 〈 0.001) the bacterial load (aerobic & anaerobic) in the saliva (≥ 0.5 log reduction with a superiority (P 〈 0.005) for the CHX-Alc when compared to the two others). Changes in the bacterial load on the tongue dorsum could only be detected for the CHX-Alc solution (0.5 log). The antibacterial effect of the placebo solution and the slurry were negligible. The composition of microflora on the other hand did not reveal significant changes. The de novo supragingival plaque formation was significantly (P 〈 0.05) inhibited by the three mouthrinses with a slight superiority for the CHX-Alc solution. The degree of gingival inflammation at day 7 remained low (〈 0.16) for all products. The CHX-Alc solution scored significantly worse for the subjective evaluation (questionnaires) concerning taste and sensitivity of tongue).Conclusions:  The results of this study demonstrate that morning breath odour can be successfully reduced by the sole twice daily use of CHX-Alc, CHX-CPC-Zn or AmF/SnF2Mr mouthrinses, which all significantly reduced the bacterial load in the saliva and retarded the de novo plaque formation.

Notizen: The molecular genetics of indole-3-acetic (IAA) synthesis and regulation in Azospirillum brasilense was investigated in this study. Tn5 mutagenesis was performed and five mutants with decreased IAA production were isolated. Five Tn5-inserted genes from these mutants were cloned and sequenced. Four genes were reported for the first time to be involved in IAA production, namely, atrA, ftsA, omaA and aldA that code for GntR-family transcriptional regulator, iron-binding protein component of ABC-type Fe3+ transport system, outer membrane protein, and aldehyde dehydrogenase, respectively. In addition, two genes atrB and atrC, with predicted proteins that showed high homology to aminotransferases, were cloned from the downstream of atrA in this bacterium. Studies also showed that complementation of atrA, ftsA and omaA were able to restore the IAA production of the corresponding IAA− mutants. Comparison of Fe3+ concentrations in culture supernatants of the wild-type strain, the ftsA mutant and the complemented strain revealed that the iron-uptake ability of the ftsA mutant was highly reduced. This result also points to the necessity of iron as a metal ion in IAA synthesis. Statistical analysis showed no significant difference in the IAA accumulated in cells between the omaA mutant and the wild-type strain, suggesting the omaA might not affect IAA secretion but be involved in IAA production in other unknown ways.