2 research outputs found
Dust release from cold ring particles as a mechanism of spoke formation in Saturn's rings
Spokes in Saturn's rings are radially-extended structures consisting of dust
grains. Although spacecraft and space telescope observations have revealed
various detailed features of the spokes and their time variation, their
formation mechanism is still under debate. Previous models examined charging
mechanisms to attempt at explaining dust release from cm-sized ring particles;
however, the attempt has been unsuccessful, because the electrostatic force
caused by such charging mechanisms is much weaker than the cohesive force
acting on dust grains at ordinary conditions in the ring environment. Here we
propose a novel model for the formation of the spokes, where the temperature
dependence of cohesion plays an essential role. Ring particles with a
temperature below 60K adsorb an O2 ring atmosphere, which facilitates release
of dust grains from them by a reduction in the cohesive force between the
grains and the particles on the morning ansa. Then, intense electrostatic
forces sufficient to overcome the cohesive force are generated on the surface
of ring particles and the released dust grains form the structure of spokes.
Our model explains observational features of the spokes including their
longitudinal location, lifetime, radial expansion velocity, and seasonality.Comment: 37 pages, 7 figure
Enhancing the cellular uptake of PyāIm polyamides through next-generation aryl turns
Pyrroleāimidazole (PyāIm) hairpin polyamides are a class of programmable, sequence-specific DNA binding oligomers capable of disrupting proteināDNA interactions and modulating gene expression in living cells. Methods to control the cellular uptake and nuclear localization of these compounds are essential to their application as molecular probes or therapeutic agents. Here, we explore modifications of the hairpin Ī³-aminobutyric acid turn unit as a means to enhance cellular uptake and biological activity. Remarkably, introduction of a simple aryl group at the turn potentiates the biological effects of a polyamide targeting the sequence 5ā²-WGWWCW-3ā² (Wā=āA/T) by up to two orders of magnitude. Confocal microscopy and quantitative flow cytometry analysis suggest this enhanced potency is due to increased nuclear uptake. Finally, we explore the generality of this approach and find that aryl-turn modifications enhance the uptake of all polyamides tested, while having a variable effect on the upper limit of polyamide nuclear accumulation. Overall this provides a step forward for controlling the intracellular concentration of PyāIm polyamides that will prove valuable for future applications in which biological potency is essential