Organosoluble star polymers from a cyclodextrin core

Well‐defined star polymers were synthesized with a combination of the core‐first method and atom transfer radical polymerization. The control of the architecture of the macroinitiator based on β‐cyclodextrin bearing functional bromide groups was determined by ¹³C NMR, fast atom bombardment mass spectrometry, and elemental analysis. In a second step, the polymerization of the tert‐butyl acrylate monomer was optimized to avoid a star–star coupling reaction and allowed the synthesis of a well‐defined organosoluble polymer star. The determination of the macromolecular dimensions of these new star polymers by size exclusion chromatography/light scattering was in agreement with the structure of armed star polymers in a large range of predicted molecular weights. This article describes a new approach to polyelectrolyte star polymers by postmodification of poly(tert‐butyl acrylate) by acrylic arm hydrolysis in a water‐soluble system. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5186–5194, 2005     

Simplicial groups that are models for algebraic K-theory

In this paper, we present and compare some simplicial groups, functorially associated to a ring R, whose homotopy groups are Quillens K-groups of R. The first such simplicial group is the group (NQPR), where is the loop space construction of Clemens Berger, applied to the simplicial set NQPR (the nerve of Quillens category QPR). The second is a subgroup GR of the simplicial group (NQPR). This second group is compared to Kans construction [12] of a loop group for a connected simplicial set, and shown to be isomorphic to it as a simplicial group. Other simplicial groups that are models for algebraic K-theory are also presented; in particular, the subgroup G(s.PR) of (s.PR); here, s.PR is Waldhausens simplicial set [25], [26]. We initially give an exposition of Bergers construction in general; then, we present the construction of GR and a summary of Kans construction. Next, we point out that GR is an infinite loop object in the category of simplicial groups, and draw some corollaries. We then compare directly the homotopy groups thus constructed with the classical K-theory in degrees 0 and 1. The final section compares various models.     

A quantum-chemical study of the structure, vibrations and SiH bond properties of disilylamine, NH(SiH3)2

Quantum-chemical calculations at HF, MP2 and B3LYP levels with 6-31G* and 6-311G** basis sets are reported for disilylamine, NH(SiH3)2. The equilibrium structure is found to vary with both level and basis set, all but one of the structures exhibiting a small lack of planarity of the HNSi2 system. The barrier to inversion, however, is found to be very low, at most 38 cm(-1). Vibration frequencies and intensities are calculated. The frequencies are scaled, where possible, either using updated infrared data or with the aid of factors transferred from N(CH3)(SiH3)2. Unobserved frequencies due to the v(s)NSi2, deltaNSi2 and delta(perpendicular)NH modes are predicted near 610, 210 and 360 cm(-1), respectively. The lower silyl torsion lies below 40 cm(-1). The appearance of a single broad vSiH band in gas-phase samples of both NH(SiH3)2 and NH(SiH3)(SiD3) is suggestive of signal averaging due to internal rotation. The frequencies v(is)SiH, infrared intensities and Raman scattering activities of the bands due to an isolated SiH bond in an otherwise deuterated species are calculated and correlated with the torsional angle of this bond and with the Mulliken charge on the hydrogen atom. The strength of the bond is a minimum, and the infrared intensity and Raman scattering activity are maxima, when the bond direction is roughly orthogonal to the skeletal plane. A major part of the frequency and intensity variations is attributed to n(p)(N)-sigma*(Si-H)) hyperconjugation which, NBO calculations show, reaches a maximum for this conformation. However, systematic smaller variations are found for SiH bonds lying in the skeletal plane, which reflect the proximity of the other silyl group and only partly correlate with Mulliken charge. vSiH-vSiH interaction force constants, f', are calculated for pairs of SiH bonds in different silyl groups and compared with the corresponding dipole-dipole potential energy, the latter calculated using a classical treatment of the interaction between point dipoles arising from delta mu/delta r for the SiH bonds involved. The gradient of the correlation is very close to that expected from the theory, but a negative intercept indicates the presence of additional factors.     

Biosensor immunoassay of ivermectin in bovine milk

A rapid and sensitive biosensor immunoassay was developed for determination of ivermectin residues in bovine milk. A detection limit of 16.2 ng/mL was achieved. A Biacore optical biosensor based on surface plasmon resonance was used, and a range of extraction techniques was investigated. In the final assay procedure, ivermectin was extracted with acetonitrile followed by C8 solid-phase extraction cleanup. It was proven experimentally that 2 methods of milk storage, freezing or addition of mercury-containing compounds as preservatives, could be used without considerable change in detected concentrations (samples were fortified with ivermectin after storage). The average values for milk samples spiked at 100 and 50 ng/mL concentrations were 102.6 and 51.5 ng/mL, respectively. Extraction and analysis of 20 milk samples were performed within a single working day.     

Allosteric control through mechanical tension

Conformational changes of proteins modulate their function. In allosteric control, the conformational change is induced by the binding of a signaling molecule. Here we insert a "molecular spring" on the enzyme guanylate kinase, to control the conformation of this protein. The stiffness of the spring can be varied externally, which allows one to exert a controlled mechanical tension between the two points on the protein's surface where the spring is attached. We show that by applying and releasing the tension we can reversibly turn the enzyme off and on.     

A Monte Carlo study of isomers and structural evolution in benzene-cyclohexane clusters: (C6H6)(C6H12)n, n=3-7, 12

Monte Carlo simulated annealing strategies, carried out on four different potential energy surfaces, are applied to benzene-cyclohexane clusters, BCn, n=3-7, 12, to identify low-energy isomers and to trace the evolution of structures as a function of cluster size. Initial structures are first heated to ensure randomization, and subsequent annealing yields optimized rigid, low-energy clusters. Five major structural isomers are identified for BC3: one assumes the form of a symmetric, modified sandwich; the remaining four lack general symmetry, assuming distorted tetrahedral arrangements. For BC4 and larger clusters, the number of low-temperature isomers is large. It is, nevertheless, feasible to classify isomers into groups based on structural similarities. The evolution of BCn structures as a function of cluster size is observed to follow one of two primary paths: The first maximizes benzene-cyclohexane interactions and places benzene in or near the BCn cluster center; the competing path maximizes cyclohexane-cyclohexane interactions and distances benzene from the cluster's center of mass. Results for BC3 and BC4 are discussed with reference to experimental results and models previously applied to interpret benzene-argon cluster spectra.     

Propriétés électriques des gels aqueux d'amphiphile

Summary This paper deals with the calculation of electrical conductivity in soap-water-systems. Two models are discussed and compared with experimental data.

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Zusammenfassung In der vorliegenden Arbeit werden zwei Modelle zur Berechnung der Leitf?higkeit in Seifen-Wasser-Mischungen beschrieben und mit experimentellen Daten verglichen.

     

PHOTOMOVEMENT, PIGMENTATION AND UV-B SENSITIVITY IN PLANARIA

Four differently pigmented species of planarians were examined for UV-B (Λ= 320 280 nm) tolerance and ability to delect and avoid UV-B-exposed areas. It was found that lighter-pigmented species are more UV sensitive and that movement out of the UV-B is directly related to UV sensitivity. Three species showed positioning preference corresponding to their pigmentation; the darker the species, the less avoidance of UV-B. The white (unpigmented) species showed exceptional and very sensitive behavior.