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941.
Given a metric d on a finite set X, a realization of d is a weighted graph
with
such that for all
the length of any shortest path in G between x and y equals d(x,y). In this paper we consider two special kinds of realizations,
optimal realizations and hereditarily optimal realizations, and their relationship with the so-called tight span. In particular,
we present an infinite family of metrics {dk}k≥1, and—using a new characterization for when the so-called underlying graph of a metric is an optimal realization that we also
present—we prove that dk has (as a function of k) exponentially many optimal realizations with distinct degree sequences. We then show that this family
of metrics provides counter-examples to a conjecture made by Dress in 1984 concerning the relationship between optimal realizations
and the tight span, and a negative reply to a question posed by Althofer in 1988 on the relationship between optimal and hereditarily
optimal realizations. 相似文献
942.
Foster SR Pearce A Blake AJ Welham MJ Dowden J 《Chemical communications (Cambridge, England)》2007,(24):2512-2514
A novel octavalent, resorcin[4]arene derived, cross-linker designed to overcome some of the limitations of commercially available reagents is significantly more efficient for covalent stabilisation of protein-protein interactions. 相似文献
943.
Guigo N Mija A Vincent L Sbirrazzuoli N 《Physical chemistry chemical physics : PCCP》2007,9(39):5359-5366
The complete curing of furfuryl alcohol (FA), was studied by chemorheological analysis and model-free kinetics under isothermal and non-isothermal modes. Polymerization of FA under acidic catalysis involves complex reactions, with several steps (such as condensations and Diels-Alder cycloadditions). To account for the polymerization complexity, kinetic analysis of DSC data was performed with a model-free isoconversional method. The obtained E(alpha)-dependencies were closely-correlated with the variation of complex viscosity during curing. Linear condensations are predominant during the early curing stage and are followed by two distinct stages of branching cycloadditions. Gelation and vitrification, identified by rheometric measurements, were associated with a decrease of the overall reaction rate that becomes controlled by diffusion of small oligomers. Before vitrification, the rate of crosslinking is limited by the mobility of longer polymer chains and diffusion encounters a large energy barrier due to the cooperative nature of the motions, leading to higher E(alpha) values. 相似文献
944.
Reynolds NP Soragni A Rabe M Verdes D Liverani E Handschin S Riek R Seeger S 《Journal of the American Chemical Society》2011,133(48):19366-19375
Parkinson's disease is a common progressive neurodegenerative condition, characterized by the deposition of amyloid fibrils as Lewy bodies in the substantia nigra of affected individuals. These insoluble aggregates predominantly consist of the protein α-synuclein. There is increasing evidence suggesting that the aggregation of α-synuclein is influenced by lipid membranes and, vice versa, the membrane integrity is severely affected by the presence of bound aggregates. Here, using the surface-sensitive imaging technique supercritical angle fluorescence microscopy and F?rster resonance energy transfer, we report the direct observation of α-synuclein aggregation on supported lipid bilayers. Both the wild-type and the two mutant forms of α-synuclein studied, namely, the familiar variant A53T and the designed highly toxic variant E57K, were found to follow the same mechanism of polymerization and membrane damage. This mechanism involved the extraction of lipids from the bilayer and their clustering around growing α-synuclein aggregates. Despite all three isoforms following the same pathway, the extent of aggregation and their effect on the bilayers was seen to be variant and concentration dependent. Both A53T and E57K formed cross-β-sheet aggregates and damaged the membrane at submicromolar concentrations. The wild-type also formed aggregates in this range; however, the extent of membrane disruption was greatly reduced. The process of membrane damage could resemble part of the yet poorly understood cellular toxicity phenomenon in vivo. 相似文献
945.
Vezzaro A Boschetti A Dell'Anna R Canteri R Dimauro M Ramina A Ferasin M Giulivo C Ruperti B 《Analytical and bioanalytical chemistry》2011,399(7):2571-2582
Volatile organic compounds of extra virgin olive oils obtained from the local Italian cultivar Grignano were measured by proton
transfer reaction–mass spectrometry (PTR-MS). Oils were extracted by olives harvested at different ripening stages across
veraison, performing each extraction step and the whole extraction process in nitrogen atmosphere to observe the changes in the volatile
profiles of the oils. Principal component analysis carried out on the full spectral signature of the PTR-MS measurements showed
that the stage of ripening has a stronger effect on the global definition of volatile profiles than the use of nitrogen during
oil extraction. The fingerprint-like chemical information provided by the spectra were used to construct a heat map, which
allowed the dynamical representation of the multivariate nature of mass evolution during the ripening process. This provided
the first evidence that some groups of volatile organic compounds displayed a time course of regulation with coordinated increasing
or decreasing trends in association with specific stages of fruit ripening. 相似文献
946.
Evangelista JS Evangelista JJ Evangelista IL Nojosa DM Nascimento NR Souza MH Alves RS Martins AM Moraes ME Monteiro HS 《Natural product communications》2011,6(6):871-874
Crotalus durissus cascavella is a snake native of northeastern Brazil. The aim of the study was to investigate the effects of C. d. cascavella venom on rat mean arterial pressure and vascular reactivity in the mesenteric vascular bed. The venom evoked a dose-dependent decrease in mean arterial pressure, cardiac and respiratory frequency with increased plasma nitrite levels. L-NAME (10 mg/kg) blunted both the hypotension and increased nitrite production observed after the venom administration. To investigate the effects of C. d. cascavella in resistance vessels, the vascular mesenteric bed was studied, and the results suggested that the hypotensive effect of the venom is not dependent on a direct vasodilatory activity. In conclusion, C. d. cascavella venom presented indirect hypotensive effects with the involvement of nitric oxide. 相似文献
947.
Ma YC Wang XQ Hou FF Ma J Luo M Lu S Jin P Terevsky N Chen A Xu I Pate AV Gorecki D 《Natural product communications》2011,6(5):645-650
A simple, sensitive and reliable reversed phase Rapid Resolution Liquid Chromatography (RRLC) method was developed and validated for six biologically active compounds (salidroside, tyrosol, rosarin, rosavin, rosin and rosiridin) in Rhodiola rosea L. roots and powder extracts. The method uses a Phenomenex C18 (2)-HST column at 40 degrees C with a neutral gradient system mobile phase (H20 and acetonitrile), a flow rate of 1.0 mL/min, and UV detection wavelengths set at 205 and 254 nm, simultaneously. Baseline separation of the six active compounds was achieved within 8 minutes. The average percentages of rosavins (rosarin, rosavin, and rosin) in authentic R. rosea roots and root powder extracts were quantitatively determined and a characteristic R. rosea roots RRLC profile was established. The RRLC method is accurate and sensitive; in addition, it effectively increases the sample analysis throughput compared with conventional HPLC. 相似文献
948.
Ocimum basilicum L., popularly known as sweet basil, is a Lamiaceae species whose essential oil is mainly composed of monoterpenes, sesquiterpenes and phenylpropanoids. The contents of these compounds can be affected by abiotic and biotic factors such as infections caused by viruses. The main goal of this research was an investigation of the effects of viral infection on the essential oil profile of common basil. Seeds of O. basilicum L. cv. Genovese were sowed and kept in a greenhouse. Plants presenting two pairs of leaves above the cotyledons were inoculated with an unidentified virus isolated from a field plant showing chlorotic yellow spots and foliar deformation. Essential oils of healthy and infected plants were extracted by hydrodistillation and analyzed by GCMS. Changes in essential oil composition due to viral infection were observed. Methyleugenol and p-cresol,2,6-di-tert-butyl were the main constituents. However, methyleugenol contents were significantly decreased in infected plants. 相似文献
949.
950.