Aminos?uren

Ohne Zusammenfassung     

Finite size effects and spontaneously broken symmetries: the case of theO(4) model

Finite volume numerical simulations of scalar models with continuous symmetry face strong finite size effects in the broken phase due to the presence of light Goldstone states. In the region where the light Goldstone bosons dominate the dynamics of the system universal finite size scaling formulae are predicted by chiral perturbation theory. Introducing a finite external source one can determine infinite volume, zero external source physical quantities from finite volume observables. Here we apply this theoretically controlled approach to the 4 dimensionalO(4) scalar model. All of our numerical results are in excellent agreement with the predicted finite size scaling forms. We confirm earlier results at zero external source where the infinite volume limit was approximated by projecting the fields to the direction of the magnetization.     

An electron-retardation device to improve the resolution of a turner-type UV photoelectron spectrometer

A simple pre-retardation device without intermediate images that may be used to increase the resolution of Turner-type UV photoelectron spectrometers has been analyzed by means of electron trajectory calculations. A set of qualitative rules based on the calculations as well as performance tests of the actual device are presented. High-resolution spectra of naphthalene and anthracene are presented as an illustration and compared in detail with recent model calculations of Franck—Condon profiles by Ohno [9].     

Hormonal regulation of hepatic amino acid transport.

The transport of 2-aminoisobutyric acid (AIB) into liver tissue was increased by both insulin and glucagon. We have now shown that these hormones do not stimulate the same transport system. Glucagon, possibly via cAMP, increased the hepatic uptake of AIB by a mechanism which resembled system A. This glucagon-sensitive system could be monitored by the use of the model amino acid MeAIB. In contrast, the insulin-stimulated system exhibited little or no affinity for MeAIB and will be referred to as system B. On the basis of other reports that the hepatic transport of AIB is almost entirely Na+ dependent and the present finding that the uptake of 2-aminobicyclo [2,2]heptane-2-carboxylic acid (BCH) was not stimulated by either hormone, we conclude that system B is Na+ dependent. Furthermore, insulin added to the perfusate of livers from glucagon-pretreated donors suppressed the increase in AIB or MeAIB uptake. Depending upon the specificities of systems A and B, both of which are unknown for liver tissue, the insulin/glucagon ratio may alter the composition of the intracellular pool of amino acids.     

Growth and form of planetary seedlings: results from a microgravity aggregation experiment

The outcome of the first stage of planetary formation, which is characterized by ballistic agglomeration of preplanetary dust grains due to Brownian motion in the free molecular flow regime of the solar nebula, is still somewhat speculative. We performed a microgravity experiment flown onboard the space shuttle in which we simulated, for the first time, the onset of free preplanetary dust accumulation and revealed the structures and growth rates of the first dust agglomerates in the young solar system. We find that a thermally aggregating swarm of dust particles evolves very rapidly and forms unexpected open-structured agglomerates.     

Versatile wettability gradients prepared by chemical modification of polymer brushes on polymer foils

A method to create a wettability gradient by variation of the chemical functionality in a polymer brush is presented. A poly(N-methyl-vinylpyridinium) (QP4VP) brush was created on a poly(ethylene-alt-tetrafluoroethylene) (ETFE) foil by the grafting of 4-vinylpyridine and subsequent quaternization. The instability of QP4VP, a strong polyelectrolyte, in alkaline media was exploited to transform it to the neutral poly(vinyl(N-methyl-2-pyridone)) (PVMP), as confirmed with ATR-IR spectroscopy. The slow transformation resulted in a substantial, time-dependent decrease in wettability. A nearly linear gradient in water contact angle (CA) was created by immersion of a QP4VP brush modified sample into a sodium hydroxide solution, resulting in CAs ranging from 10° to 60°. The concurrent decrease in the number of charged functional groups along the gradient was characterized by loading an anionic dye into the polymer brush and measuring the UV transmittance of the sample. The versatility of the wettability gradient was demonstrated by exchanging the counterions of the N-methyl-vinylpyridinium groups, whereby a reversal of gradient direction was reproducibly achieved.     

A perimidine-derived non-Kekulé triplet diradical

6,9-Di(tert-butyl)-1-methyltetrazolo[1,5-a]perimidine (1) has been synthesized from naphthalene in seven steps. The EPR spectra, recorded after irradiation of 1 in a butyronitrile matrix at 77 K (lambda = 351 nm) and in Ar and Xe matrixes at 4.6 K (lambda > or = 345 nm), showed a six-line, high-field signal (Delta m(S) = +/- 1), centered at 3350 G in butyronitrile, along with a half-field signal (Delta m(S) = +/- 2), which is characteristic for triplets. Simulation of the observed EPR spectra gave values for the zero-field splitting parameters of |D/hc|/cm(-1) = 0.0105, |E/hc|/cm(-1) = 0.0014 in butyronitrile and |D/hc|/cm(-1) = 0.0107, |E/hc|/cm(-1) = 0.0016 in Ar. These EPR parameters are consistent with the diradical 5,8-di(tert-butyl)-2-(N-methylimino)perimidine-1,3-diyl ((3)2) as source of the EPR spectra. Linearity of the Curie-Weiss plot and UB3LYP and (14/14)CASPT2 calculations of the singlet-triplet energy difference (DeltaE(ST) approximately 8-10 kcal/mol) indicate that the triplet is the ground state of 2, as predicted for such a nondisjoint diradical.     

Tunneling rearrangement of 1-azulenylcarbene

1-Azulenylcarbene was synthesized by photolysis of 1-azulenyldiazomethane in argon or neon matrices at 3-10 K. The highly polar singlet carbene is only metastable and undergoes a tunneling rearrangement to 8-methylene-bicyclo[5.3.0]deca-1,3,5,6,9-pentaene. After substitution of the 4 and 8 positions with deuterium, the rearrangement is completely inhibited. This indicates a very large kinetic isotope effect, as expected for a tunneling reaction.