Sophisticated macromolecular structures by cationic ring-opening polymerizations

A combination of different types of cationic ring-opening polymerizations (CROPs) has been used to construct macromolecular structures containing polymer segments with different physicochemical properties. The cyclic monomers used are tetrahydrofuran (THF), N-tert-butyl aziridine (TBA), 2-methyl-1, 3-oxazoline (MeOX) and 1, 3-dioxolane (DXL). The macromolecular structures include different block and graft copolymers, telechelics and macromonomers, star- and comb-shaped polymers, polymer networks and interpenetrating polymer networks (IPNs).     

Excitation spectra of ferromagnets with crystal fields

We have investigated the ferromagnetic phase transition and excitation spectrum of systems, such as the rare earths, where the crystalline fields are significant in determining the magnetic properties. Using the full level scheme appropriate to a system ofJ-4 ions in a crystal field of cubic symmetry and assuming a Heisenberg interaction, we have calculated the full temperature- andq-dependence of the dynamic susceptibility within the RPA, both in the paramagnetic and ordered regimes. Variations in behavior, both with changes in crystal field parameter and with changes in magnetic interaction strength are discussed, and emphasis is placed on the limiting case which corresponds to current experiments in the light rare earths.     

Practical synthesis of a potent hepatitis C virus RNA replication inhibitor

A practical, efficient synthesis of 1, a hepatitis C virus RNA replication inhibitor, is described. Starting with the inexpensive diacetone glucose, the 12-step synthesis features a novel stereoselective rearrangement to prepare the key crystalline furanose diol intermediate. This is followed by a highly selective glycosidation to couple the C-2 branched furanose epoxide with deazapurine.     

Auditory temporal resolution in birds: discrimination of harmonic complexes

The ability of three species of birds to discriminate among selected harmonic complexes with fundamental frequencies varying from 50 to 1000 Hz was examined in behavioral experiments. The stimuli were synthetic harmonic complexes with waveform shapes altered by component phase selection, holding spectral and intensive information constant. Birds were able to discriminate between waveforms with randomly selected component phases and those with all components in cosine phase, as well as between positive and negative Schroeder-phase waveforms with harmonic periods as short as 1-2 ms. By contrast, human listeners are unable to make these discriminations at periods less than about 3-4 ms. Electrophysiological measures, including cochlear microphonic and compound action potential measurements to the same stimuli used in behavioral tests, showed differences between birds and gerbils paralleling, but not completely accounting for, the psychophysical differences observed between birds and humans. It appears from these data that birds can hear the fine temporal structure in complex waveforms over very short periods. These data show birds are capable of more precise temporal resolution for complex sounds than is observed in humans and perhaps other mammals. Physiological data further show that at least part of the mechanisms underlying this high temporal resolving power resides at the peripheral level of the avian auditory system.     

Exact thought in a demented time: Karl menger and his viennese mathematical colloquium

This column is a forum for discussion of mathematical communities throughout the world, and through all time. Our definition of “mathematical community” is the broadest. We include “schools” of mathematics, circles of correspondence, mathematical societies, student organizations, and informal communities of cardinality greater than one. What we say about the communities is just as unrestricted. We welcome contributions from mathematicians of all kinds and in all places, and also from scientists, historians, anthropologists, and others.     

Crystal Memory near Discontinuous Triacylglycerol Phase Transitions: Models,Metastable Regimes,and Critical Points

It is proposed that “crystal memory”, observed in a discontinuous solid-liquid phase transition of saturated triacylglycerol (TAG) molecules, is due to the coexistence of solid TAG crystalline phases and a liquid TAG phase, in a superheated metastable regime. Such a coexistence has been detected. Solid crystals can act as heterogeneous nuclei onto which molecules can condense as the temperature is lowered. We outlined a mathematical model, with a single phase transition, that shows how the time-temperature observations can be explained, makes predictions, and relates them to recent experimental data. A modified Vogel-Fulcher-Tammann (VFT) equation is used to predict time-temperature relations for the observation of “crystal memory” and to show boundaries beyond which “crystal memory” is not observed. A plot of the lifetime of a metastable state versus temperature, using the modified VFT equation, agrees with recent time-temperature data. The model can be falsified through its predictions: the model possesses a critical point and we outline a procedure describing how it could be observed by changing the hydrocarbon chain length. We make predictions about how thermodynamic functions will change as the critical point is reached and as the system enters a crossover regime. The model predicts that the phenomenon of “crystal memory” will not be observed unless the system is cooled from a superheated metastable regime associated with a discontinuous phase transition.