Simplified estimation of multivariate duration models with unobserved heterogeneity

A simple structure is suggested for modelling unobserved heterogeneity in multivariate duration models which avoids the “curse of dimensionality” and numerical integration of the likelihood function.     

Polyampholytes

Polyampholytes are charged polymers with both positively and negatively charged groups. The conformation of these polymers in solutions strongly depends on the distribution of charged monomers along the polymer backbone and their environment. In this review we summarize the current level of understanding of such amphoteric polymers in solutions and their interactions with surfaces and polyelectrolytes. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3513–3538, 2004     

Effect of substitution of Ce on superconducting properties of Bi1.7Pb0.3Sr2Ca2−x Ce x Cu3O10+δ system

We have investigated the superconducting properties of the Bi_1.7 Pb_0.3Sr₂Ca_2−xCe _x Cu₃O_10+δ system with x=0.00, 0.02, 0.04, 0.08 and 0.1 by X-ray diffraction and magnetic susceptibility. The substitution of Ce for Ca has been found to drastically change the superconducting properties of the system. X-ray diffraction studies on these compounds indicate decrease in the c-parameter with increased substitution of Ce at Ca site and volume fraction of high T _c (2 : 2 : 2 : 3) phase decreases and low T _c phase increases. The magnetic susceptibility of this compound shows that the diamagnetic on set superconducting transition temperature (onset) varies from 109 K to 51 K for x=0.00, 0.02, 0.04, 0.08 and 0.1. These results suggest the possible existence of Ce in a tetravalent state rather than a trivalent state in this system; that is, Ca²⁺ → Ce⁴⁺ replacement changes the hole carrier concentration. Hole filling is the cause of lowering T _c of the system.     

Total synthesis and evaluation of cytostatin, its C10-C11 diastereomers, and additional key analogues: impact on PP2A inhibition

The total synthesis of cytostatin, an antitumor agent belonging to the fostriecin family of natural products, is described in full detail. The convergent approach relied on a key epoxide-opening reaction to join the two stereotriad units and a single-step late-stage stereoselective installation of the sensitive (Z,Z,E)-triene through a beta-chelation-controlled nucleophilic addition. The synthetic route provided rapid access to the C4-C6 stereoisomers of the cytostatin lactone, which were prepared and used to define the C4-C6 relative stereochemistry of the natural product. In addition to the natural product, each of the C10-C11 diastereomers of cytostatin was divergently prepared (11 steps from key convergence step) by this route and used to unequivocally confirm the relative and absolute stereochemistry of cytostatin. Each of the cytostatin diastereomers exhibited a reduced activity toward inhibition of PP2A (>100-fold), demonstrating the importance of the presence and stereochemistry of the C10-methyl and C11-hydroxy groups for potent PP2A inhibition. Extensions of the studies provided dephosphocytostatin, sulfocytostatin (a key analogue related to the natural product sultriecin), 11-deshydroxycytostatin, and an analogue lacking the entire C12-C18 (Z,Z,E)-triene segment, which were used to define the magnitude of the C9-phosphate (>4000-fold), C11-alcohol (250-fold), and triene (220-fold) contribution to PP2A inhibition. A model of cytostatin bound to the active site of PP2A is presented, compared to that of fostriecin, which is also presented in detail for the first time, and used to provide insights into the role of the key substituents. Notably, the alpha,beta unsaturated lactone of cytostatin, like that of fostriecin, is projected to serve as a key electrophile, providing a covalent adduct with Cys269 unique to PP2A, contributing to its potency (> or =200-fold for fostriecin) and accounting for its selectivity.     

New riches in carbaporphyrin chemistry: silver and gold organometallic complexes of benzocarbaporphyrins

The NH-N-NH-N core of the porphyrin system represents one of the best studied and most versatile platforms for coordination chemistry. However, the replacement of one or more of the interior nitrogens with carbon atoms would be expected to diminish the ability of these systems to form metallo derivatives considerably. Despite this expectation, carbaporphyrinoid systems have been shown to form stable organometallic derivatives. Although azuliporphyrins and benziporphyrins act as dianionic ligands, benzocarbaporphyrins are trianionic ligands. Treatment of five different meso unsubstituted benzocarbaporphyrins and two different meso tetraarylbenzocarbaporphyrins with excess silver(I) acetate afforded 65-97% yields of the corresponding silver(III) organometallic derivatives. The insertion of silver metal was confirmed by mass spectrometry and X-ray crystallography. The UV-vis spectra showed a strong Soret band at wavelengths between 437 and 451 nm, together with a series of Q-type bands at longer wavelengths. The new metallo carbaporphyrins demonstrate the presence of a strong diatropic ring current in their proton NMR spectra, and carbon-13 NMR spectroscopy indicates that the derivatives retain a plane of symmetry. The reaction of meso tetraaryl carbaporphyrins with gold(III) acetate afforded the related gold(III) complexes, and these also showed strongly porphyrin-like aromatic characteristics. The UV-vis spectra for the gold complexes again showed a strong Soret band between 437-439 nm, but a secondary band near 400 nm is somewhat intensified for the gold species compared to the spectra for the related silver(III) meso tetrasubstituted carbaporphyrins. The ring currents observed for the gold(III) complexes by proton NMR spectroscopy were comparable to those of the silver(III) derivatives, implying that both series have similar macrocyclic conformations. Cyclic voltammetry was performed on two different carbaporphyrins, their silver(III) derivatives, and a gold(III) complex. The silver complexes display a reversible cathodic wave that is assigned to the Ag(III/II) couple. However, the gold porphyrinoid gave a value for the reductive wave that could be due to a gold(III/II) couple or a ligand-based process.     

Enhanced elasticity and soft glassy rheology of a smectic in a random porous environment

We report studies of the frequency-dependent shear modulus, G(*)(omega) = G(')(omega) + iG(')(omega), of the liquid crystal octylcyanobiphenyl (8CB) confined in a colloidal aerosil gel. With the onset of smectic order, G' grows approximately linearly with decreasing temperature, reaching values that exceed by more than 3 orders of magnitude the values for pure 8CB. The modulus at low temperatures possesses a power-law component, G(*)(omega) approximately omega(alpha), with exponent alpha that approaches zero with increasing gel density. The amplitude of G' and its variation with temperature and gel density indicate that the low temperature response is dominated by a dense population of defects in the smectic. In contrast, when the 8CB is isotropic or nematic, the modulus is controlled by the elastic behavior of the colloidal gel.     

High-harmonic inverse-free-electron-laser interaction at 800 nm

We present the first direct observation of a higher-order inverse-free-electron-laser (IFEL) interaction. Interaction at the fourth, fifth, and sixth harmonics is observed from an IFEL operating at 800 nm. The harmonic spacing, relative harmonic strength, and transverse beam overlap of the interaction are all in good agreement with tracking simulations.     

Dynamic scaling approach to glass formation

Experimental data for the temperature dependence of relaxation times are used to argue that the dynamic scaling form, with relaxation time diverging at the critical temperature T(c) as (T-T(c))(-nuz), is superior to the classical Vogel form. This observation leads us to propose that glass formation can be described by a simple mean-field limit of a phase transition. The order parameter is the fraction of all space that has sufficient free volume to allow substantial motion, and grows logarithmically above T(c). Diffusion of this free volume creates random walk clusters that have cooperatively rearranged. We show that the distribution of cooperatively moving clusters must have a Fisher exponent tau=2. Dynamic scaling predicts a power law for the relaxation modulus G(t) approximately t(-2/z), where z is the dynamic critical exponent relating the relaxation time of a cluster to its size. Andrade creep, universally observed for all glass-forming materials, suggests z=6. Experimental data on the temperature dependence of viscosity and relaxation time of glass-forming liquids suggest that the exponent nu describing the correlation length divergence in this simple scaling picture is not always universal. Polymers appear to universally have nuz=9 (making nu=3 / 2). However, other glass-formers have unphysically large values of nuz, suggesting that the availability of free volume is a necessary, but not sufficient, condition for motion in these liquids. Such considerations lead us to assert that nuz=9 is in fact universal for all glass- forming liquids, but an energetic barrier to motion must also be overcome for strong glasses.     

Magnetic resonance imaging of an infected urethral diverticulum: A case report

We present a case report of a urethral diverticulum where magnetic resonance imaging suggested infected contents of the urethral diverticulum besides providing superb detail of periurethral anatomy. The critical clinical question was answered.