The role of London forces in defining noncentrosymmetric order of high dipole moment–high hyperpolarizability chromophores in electrically poled polymeric thin films

Graphs of second harmonic generation coefficients and electro-optic coefficients (measured by ellipsometry, attenuated total reflection, and two-slit interference modulation) as a function of chromophore number density (chromophore loading) are experimentally observed to exhibit maxima for polymers containing chromophores characterized by large dipole moments and polarizabilities. Modified London theory is used to demonstrated that this behavior can be attributed to the competition of chromophore-applied electric field and chromophore–chromophore electrostatic interactions. The comparison of theoretical and experimental data explains why the promise of exceptional macroscopic second-order optical nonlinearity predicted for organic materials has not been realized and suggests routes for circumventing current limitations to large optical nonlinearity. The results also suggest extensions of measurement and theoretical methods to achieve an improved understanding of intermolecular interactions in condensed phase materials including materials prepared by sequential synthesis and block copolymer methods.

Ptolemaiida, a new order of Mammalia--with description of the cranium of Ptolemaia grangeri.

All records of the exotic mammalian family Ptolemaiidae are known from 182 m of section in the lower to middle parts of the upper Eocene and lower Oligocene Jebel Qatrani Formation, Fayum Depression, Egypt. Previous tentative assignments of ptolemaiid affinity have suggested that these animals are allied with the primitive suborder Pantolesta (currently placed in the order Cimolesta). Though perhaps ultimately derived from an unknown member of that group, the likelihood that ptolemaiids constitute a distinct group is considered, and analysis of all known materials of Ptolemaia, Qarunavus, and Cleopatrodon demonstrates that these genera belong in their own order, the Ptolemaiida, described here. The morphologically unique dentition and only known ptolemaiid cranium, that of Ptolemaia grangeri, is described. Although Qarunavus and Cleopatrodon show some similarities in primitive characters to European merialine Paroxyclaenidae (suborder Pantolesta), their affinities clearly lie with Ptolemaia and the Ptolemaiida.

The two single-strand cleavages at each end of Tn10 occur in a specific order during transposition.

During Tn10 transposition, the element is excised from the donor site by double-strand cleavages at the two transposon ends. Double-strand cleavage is a central step in the nonreplicative transposition reaction of many transposons in both prokaryotes and eukaryotes. Evidence is presented to show that the Tn10 double-strand cut is made by an ordered, sequential cleavage of the two strands. The transferred strand is cut first, and then the nontransferred strand is cleaved. The single-strand nicked intermediate is seen to accumulate when Mn2+ is substituted for Mg2+ in the reaction or when certain mutant transposases are used. The fact that the transferred strand is cleaved before the non-transferred strand implies that the order of strand cleavages is not the determining factor that precludes a replicative mechanism of transposition.

Mechanisms underlying the sensitivity of songbird forebrain neurons to temporal order.

Neurons in the songbird forebrain area HVc (hyperstriatum ventrale pars caudale or high vocal center) are sensitive to the temporal structure of the bird's own song and are capable of integrating auditory information over a period of several hundred milliseconds. Extracellular studies have shown that the responses of some HVc neurons depend on the combination and temporal order of syllables from the bird's own song, but little is known about the mechanisms underlying these response properties. To investigate these mechanisms, we recorded intracellular responses to a set of auditory stimuli designed to assess the degree of dependence of the responses on temporal context. This report provides evidence that HVc neurons encode information about temporal structure by using a variety of mechanisms including syllable-specific inhibition, excitatory postsynaptic potentials with a range of different time courses, and burst-firing nonlinearity. The data suggest that the sensitivity of HVc neurons to temporal combinations of syllables results from the interactions of several cells and does not arise in a single step from afferent inputs alone.