Pluronic L61 accelerates flip-flop and transbilayer doxorubicin permeation

It has recently been found that Pluronics (block copolymers of ethylene oxide, EO, and propylene oxide, PO) favor the permeability and accumulation of anthracycline antibiotics, for example doxorubicin (Dox), in tumor cells. In an effort to understand these results, the interaction of EO(2)/PO(32)/EO(2) (Pluronic L61) with unilamellar egg yolk vesicles (80-100 nm in diameter) was examined. A partition coefficient K(p)=[Pl](membrane)/[Pl](water)=45 was determined. This corresponds to adsorption of about 20 polymer molecules to the surface of each vesicle in a 20 microM polymer solution. Despite this rather weak adsorption, Pluronic has a substantial effect upon the transmembrane permeation rate of Dox and upon the phospholipid flip-flop rate within the bilayers. Thus, the Dox permeation rate increases threefold and the flip-flop rate increases sixfold in 20 microM Pluronic. The two rates increase linearly with the amount of adsorbed polymer. The obvious ability of Pluronics to increase the mobility of membrane components may have important biomedical consequences.     

Angle-resolved neutralization-reionization mass spectrometry

Neutralization-reionization mass spectra of 2-propenal, isomeric butenes, and isomeric n-hexenes have been found to depend significantly on the z-axis scattering angle of the neutralization event. As shown by Cooks for ion dissociations, increasing scattering angles generally favor products of higher activation-energy reactions. For isomeric butenes and n-hexenes, these reactions provide more definitive information for isomeric characterization.     

Energy dependence of π+ induced two- and four-nucleon removal

De-excitation γ-rays have been detected from nuclear states following positive pion induced two-nucleon removal from ¹²C and four-nucleon removal from ¹⁶O. Excitation functions for population of discrete states were obtained for positive pions of 75, 125, 150, 200, 225, 250 and 290 MeV. The pion-nucleon (3, 3) resonance is not reflected in the cross sections for either reaction. Implications of the observed energy dependence are discussed.     

Science Vocabulary Load of Selected Secondary Science Textbooks

It has been argued that many students view science as a collection of facts and terms to be memorized. Robert Yager (1983) analyzed secondary level science textbooks to determine vocabulary load and found that the amount of new vocabulary terms presented in such textbooks is higher than that recommended for junior high and high school foreign language courses. Three textbooks analyzed in that study were re-evaluated, taking into account limitations expressed in the earlier study. The new study also included analysis of an earth science textbook; the first study did not. The re-analysis produced a noticeable reduction in vocabulary load estimates with only one book, Modern Chemistry, still measuring above the recommended level for foreign language courses. It is argued that the amount of vocabulary load presented is still too high and may contribute to the problem of science avoidance by secondary students.     

New perspectives in competitive location theory

This paper gives new perspectives in competitive location theory by considering new norms in two-dimensional problems and by considering (for the first time) the competitive location problem on a graph. The results in competitive location theory are generalized by exploiting an isomorphism to the literature in voting theory and by developing new results for competitive location problems on a graph.     

A note on the stress singularity at a non-uniformly moving crack tip

The stress-singularity at a crack tip moving arbitrarily in an elastic plate under plane strain conditions is investigated. By formulating the wave-equations in a polar coordinate system attached to the crack-tip, it is found by an asymptotic analysis that the angle-dependence of the singularity is only dependent on the instantaneous cracktip velocity. This result is used to derive a relation between the dynamic stress-intensity factor and the energyrelease rate.