The perpetual fragility of creeping hillslopes

Soil creeps imperceptibly but relentlessly downhill, shaping landscapes and the human and ecological communities that live within them. What causes this granular material to ‘flow’ at angles well below repose? The unchallenged dogma is churning of soil by (bio)physical disturbances. Here we experimentally render slow creep dynamics down to micron scale, in a laboratory hillslope where disturbances can be tuned. Surprisingly, we find that even an undisturbed sandpile creeps indefinitely, with rates and styles comparable to natural hillslopes. Creep progressively slows as the initially fragile pile relaxes into a lower energy state. This slowing can be enhanced or reversed with different imposed disturbances. Our observations suggest a new model for soil as a creeping glass, wherein environmental disturbances maintain soil in a perpetually fragile state.

Ligand-dependent Linkage of the ATP Site to Inhibition Gate Closure in the KATP Channel

Major advances have been made on the inhibition gate and ATP site of the Kir6.2 subunit of the KATP channel, but little is known about conformational coupling between the two. ATP site mutations dramatically disrupt ATP-dependent gating without effect on ligand-independent gating, observed as interconversions between active burst and inactive interburst conformations in the absence of ATP. This suggests that linkage between site and gate is conditionally dependent on ATP occupancy. We studied all substitutions at position 334 of the ATP site in Kir6.2ΔC26 that express in Xenopus oocytes. All substitutions disrupted ATP-dependent gating by 10-fold or more. Only positive-charged arginine or lysine at 334, however, slowed ligand-independent gating from the burst, and this was in some but not all patches. Moreover, the polycationic peptide protamine reversed the slowed gating from the burst of 334R mutant channels, and speeded the slow gating from the burst of wild-type SUR1/Kir6.2 in the absence of ATP. Our results support a two-step ligand-dependent linkage mechanism for Kir6.2 channels in which ATP-occupied sites function to electrostatically dissociate COOH-terminal domains from the membrane, then as in all Kir channels, free COOH-terminal domains and inner M2 helices transit to a lower energy state for gate closure.

Hydroxyapatite-Chitosan Hybrid Nano-Materials

Self-assembled nano-materials are currently an area of research with high throughput due to the opportunities it provides to fields ranging from semiconductor engineering to gene delivery. There is also considerable interest in nano-particulate systems that attain a lower energy state by self-assembly through favorable and repeated surface interactions as they mimic those commonly found in natural biological systems. This work presents a simple route to first synthesise a highly stable suspension of nano-hydroxyapatite (~40nm) with chitosan and subsequently self-assemble the suspended nano-hydroxyapatite particulates onto a substrate.