13 research outputs found
Enhancing surface heat transfer by carbon nanofins: towards an alternative to nanofluids?
Background: Nanofluids are suspensions of nanoparticles and fibers which have recently attracted much attention because of their superior thermal properties. Nevertheless, it was proven that, due to modest dispersion of nanoparticles, such high expectations often remain unmet. In this article, by introducing the notion of nanofin, a possible solution is envisioned, where nanostructures with high aspect-ratio are sparsely attached to a solid surface (to avoid a significant disturbance on the fluid dynamic structures), and act as efficient thermal bridges within the boundary layer. As a result, particles are only needed in a small region of the fluid, while dispersion can be controlled in advance through design and manufacturing processes. Results: Toward the end of implementing the above idea, we focus on single carbon nanotubes to enhance heat transfer between a surface and a fluid in contact with it. First, we investigate the thermal conductivity of the latter nanostructures by means of classical non-equilibrium molecular dynamics simulations. Next, thermal conductance at the interface between a single wall carbon nanotube (nanofin) and water molecules is assessed by means of both steady-state and transient numerical experiments. Conclusions: Numerical evidences suggest a pretty favorable thermal boundary conductance (order of 107 W·m-2·K-1) which makes carbon nanotubes potential candidates for constructing nanofinned surface
Reaction Dynamics of ATP Hydrolysis Catalyzed by PâGlycoprotein
P-glycoprotein
(P-gp) is a member of the ABC transporter family
that confers drug resistance to many tumors by catalyzing their efflux,
and it is a major component of drugâdrug interactions. P-gp
couples drug efflux with ATP hydrolysis by coordinating conformational
changes in the drug binding sites with the hydrolysis of ATP and release
of ADP. To understand the relative rates of the chemical step for
hydrolysis and the conformational changes that follow it, we exploited
isotope exchange methods to determine the extent to which the ATP
hydrolysis step is reversible. With Îł<sup>18</sup>O<sub>4</sub>-labeled ATP, no positional isotope exchange is detectable at the
bridging ÎČ-phosphorusâOâÎł-phosphorus bond.
Furthermore, the phosphate derived from hydrolysis includes a constant
ratio of three <sup>18</sup>O/two <sup>18</sup>O/one <sup>18</sup>O that reflects the isotopic composition of the starting ATP in multiple
experiments. Thus, H<sub>2</sub>O-exchange with HPO<sub>4</sub><sup>2â</sup> (P<sub>i</sub>) was negligible, suggesting that a
[P-gp·ADP·P<sub>i</sub>] is not long-lived. This further
demonstrates that the hydrolysis is essentially irreversible in the
active site. These mechanistic details of ATP hydrolysis are consistent
with a very fast conformational change immediately following, or concomitant
with, hydrolysis of the Îł-phosphate linkage that ensures a high
commitment to catalysis in both drug-free and drug-bound states