Droplet States in the XXZ Heisenberg Chain

We consider the ground states of the ferromagnetic XXZ chain with spin up boundary conditions. The ground state of this model, restricted to a sector with a fixed number of down spins, describes a droplet of down spins in an environment of up spins. We find the exact energy and the states that describe these droplets in the limit of an infinite number of down spins. We prove that there is a gap in the spectrum above the droplet states. As the XXZ Hamiltonian has a gap above the fully magnetized ground states as well, this means that the droplet states (for sufficiently large droplets) form an isolated band. The width of this band tends to zero in the limit of infinitely large droplets. We also prove the analogous results for finite chains with periodic boundary conditions and for the infinite chain. Received: 5 September 2000 / Accepted: 8 December 2000     

Alignment of guest-host liquid crystals with polarized laser light

Surface-mediated alignment of nematic liquid crystals with polarized laser light was reported recently [1]. In this communication we describe the alignment of a guest-host liquid crystal medium with polarized laser light. Liquid crystals in the illuminated region orient perpendicular to the direction of the laser polarization and remain aligned in the absence of laser radiation. The liquid crystals can be reoriented again by subsequent illumination. The kinetic feature of this surface-mediated liquid crystal orientation is characterized by the presence of coexisting liquid crystal regions of directors pointing away from the initial alignment.     

A Survey of Orbitrap All Ion Fragmentation Analysis Assessed by an R MetaboList Package to Study Small-Molecule Metabolites

Leukemia cell and melanoma tumor tissue extracts were studied for small (mostly m/z?<?250) polar metabolites by LC-ESI-HRMSⁿ analysis powered by a hybrid Quadrupole-Orbitrap. MS data were simultaneously acquired in fast polarity switching mode operating in MS¹ and MS/MS (All Ion Fragmentation, AIF) full-scan analyses at high mass resolution. Positive metabolite assignments were achieved by AIF analysis considering at least two characteristic transitions. Targeted metabolite profiling was achieved by the relative quantification of 18 metabolites through spiking of their respective deuterated counterparts. Manual data processing of MS¹ and AIF scans were compared for the accurate determination of natural metabolites and their deuterated analogs by chromatographic alignment and peak area integration. Evaluation of manual and automated (MetaboList R package) AIF data processing yielded comparable results. The versatility of AIF analysis also enabled the untargeted metabolite profiling of leukemia and melanoma samples in which 22 and 53 compounds were, respectively, identified outside those studied by labeling. The main limitation of this method was that low abundance metabolites with scan rates below 8 scans/peak could not be accurately quantified by AIF analysis. The combination of AIF analysis with MetaboList R package represents an opportunity to move towards automated, faster, and more global metabolomics approaches supported by an entirely flexible open source data processing platform freely available from Comprehensive R Archive Network (CRAN, https://CRAN.R-project.org/package=MetaboList).     

Conformational changes in a photosensory LOV domain monitored by time-resolved NMR spectroscopy

Phototropins are light-activated kinases from plants that utilize light-oxygen-voltage (LOV) domains as blue light photosensors. Illumination of these domains leads to the formation of a covalent linkage between the protein and an internally bound flavin chromophore, destabilizing the surrounding protein and displacing an alpha-helix from its surface. Here we use a combination of spectroscopic tools to monitor the kinetic processes that spontaneously occur in the dark as the protein returns to the noncovalent ground state. Using time-resolved two-dimensional (2D) NMR methods, we measured the rate of this process at over 100 independent sites throughout the protein, establishing that regeneration of the dark state occurs cooperatively within a 1.6-fold range of observed rates. These data agree with other spectroscopic measurements of the kinetics of protein/FMN bond cleavage and global conformational changes, consistent with these processes experiencing a common rate-limiting step. Arrhenius analyses of the temperature dependence of these rates suggest that the transition state visited during this regeneration has higher energy than the denatured form of this protein domain despite the fact that there is no global unfolding of the domain during this process.     

Convenient access to glutamic acid side chain homologues compatible with solid phase peptide synthesis

[reaction: see text] Preparation of several side chain length variants of glutamic acid is achieved via olefin cross metathesis of allyl glycine derivatives. The products are suitably protected for direct use in Fmoc solid-phase peptide synthesis, as demonstrated by successful synthesis of test sequences.     

Skeletal rearrangement for water loss from molecular protonated ions of t-butoxycyclohexane

Isotopic labelling and chemical substitution support the proposition that the skeletal rearrangement for water loss from molecular protonated ions of t-butoxycyclohexane involves competition between three reaction pathways. The principal reaction pathway (83%) involves migration of the t-butyl group to the 2-(6-) position of the cyclohexyl ring with reciprocal hydrogen transfer. A second reaction pathway (12%) involves ring contraction followed by reciprocal exchange of the t-butyl group with the 2-(5-) hydrogen atom of the nascent cyclopentyl ring. The third reaction pathway (5%) involves rearrangement of a proton-bound complex to permit ipso attack by isobutene. Stereospecific substitutions indicate that the principal reaction pathway is susceptible to 1,3-diaxial interactions.     

Visco-elastic and adhesive properties of adsorbed polyelectrolyte multilayers determined in situ with QCM-D and AFM measurements

The build-up of multilayers constructed from polyallylamine hydrochloride (PAH) and polyacrylic acid (PAA) under different pH conditions was continuously monitored using the quartz crystal microbalance with dissipation. The adsorbed amount of polymer as well as the amount of coupled water was determined. Furthermore, from dissipation measurements, it was possible to determine the visco-elastic properties of the adsorbed multilayer. These properties were highly dependent on the polyelectrolyte present in the outermost layer. The multilayer was far more rigid and elastic with PAA as the outermost layer. Furthermore, a link has been established between the conformability or rigidity of a multilayer covered surface and the adhesion between such surfaces. Adhesion measurements using the atomic force microscope showed a greater pull-off force when the more viscous PAH was present in the outermost layer.     

Crystallographic shear structures derived from CrO2: An electron microscopic study

We have established by electron microscopy and electron diffraction the existence of several crystallographic shear phases in the family of chromium oxides Cr_nO_2n?1, where n = 4 and 6. As in the systems Ti_nO_2n?1 and V_nO_2n?1, the phases are based on crystallographic shear on the planes {121}. A further series of phases found in this study show a superlattice along $\text{g}\text{?}{}_{210}$ and bear similarities to the M_mO_2m?2 phases found by Gibb and Anderson in the Ga₂O₃TiO₂ system.