Non-equilibrium oxidation states of zirconium during early stages of metal oxidation

The chemical state of Zr during the initial, self-limiting stage of oxidation on single crystal zirconium (0001), with oxide thickness on the order of 1 nm, was probed by synchrotron x-ray photoelectron spectroscopy. Quantitative analysis of the Zr 3d spectrum by the spectrum reconstruction method demonstrated the formation of Zr[superscript 1+], Zr[superscript 2+], and Zr[superscript 3+] as non-equilibrium oxidation states, in addition to Zr[superscript 4+] in the stoichiometric ZrO2. This finding resolves the long-debated question of whether it is possible to form any valence states between Zr[superscript 0] and Zr[superscript 4+] at the metal-oxide interface. The presence of local strong electric fields and the minimization of interfacial energy are assessed and demonstrated as mechanisms that can drive the formation of these non-equilibrium valence states of Zr.United States. Department of Energy. Office of Basic Energy Sciences(Contract No. DE-AC02-98CH10886

Thermal conductivity control by oxygen defect concentration modification in reducible oxides: The case of Pr0.1Ce0.9O2−δ thin films

We demonstrate the impact on thermal conductivity of varying the concentration of oxygen vacancies and reduced cations in Pr[subscript 0.1]Ce[subscript 0.9]O[subscript 2−δ] thin films prepared by pulsed laser deposition. The oxygen vacancy concentration is controlled by varying the oxygen partial pressure between 1 × 10[superscript −4] and 1 atm at 650  °C. Corresponding changes in the oxygen non-stoichiometry (δ) are monitored by detecting the lattice parameters of the films with high-resolution X-ray diffraction, while the thermal properties are characterized by time-domain thermoreflectance measurements. The films are shown to exhibit a variation in oxygen vacancy content, and in the Pr[superscript 3+]/Pr[superscript 4+] ratio, corresponding to changes in δ from 0.0027 to 0.0364, leading to a reduction in the thermal conductivity from k = 6.62 ± 0.61 to 3.82 ± 0.51 W/m-K, respectively. These values agree well with those predicted by the Callaway and von Baeyer model for thermal conductivity in the presence of point imperfections. These results demonstrate the capability of controlling thermal conductivity via control of anion and cation defect concentrations in a given reducible oxide.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (MRSEC Program, Award No. DMR-0819762

Neck linker docking coordinates the kinetics of kinesin's heads

Conventional kinesin is a two-headed homodimeric motor protein, which is able to walk along microtubules processively by hydrolyzing ATP. Its neck linkers, which connect the two motor domains and can undergo a docking/undocking transition, are widely believed to play the key role in the coordination of the chemical cycles of the two motor domains and, consequently, in force production and directional stepping. Although many experiments, often complemented with partial kinetic modeling of specific pathways, support this idea, the ultimate test of the viability of this hypothesis requires the construction of a complete kinetic model. Considering the two neck linkers as entropic springs that are allowed to dock to their head domains and incorporating only the few most relevant kinetic and structural properties of the individual heads, here we develop the first detailed, thermodynamically consistent model of kinesin that can (i) explain the cooperation of the heads (including their gating mechanisms) during walking and (ii) reproduce much of the available experimental data (speed, dwell time distribution, randomness, processivity, hydrolysis rate, etc.) under a wide range of conditions (nucleotide concentrations, loading force, neck linker length and composition, etc.). Besides revealing the mechanism by which kinesin operates, our model also makes it possible to look into the experimentally inaccessible details of the mechanochemical cycle and predict how certain changes in the protein affect its motion.Comment: to appear in the Biophysical Journa

Kinesin's backsteps under mechanical load

Kinesins move processively toward the plus end of microtubules by hydrolyzing ATP for each step. From an enzymatic perspective, the mechanism of mechanical motion coupled to the nucleotide chemistry is often well explained using a single-loop cyclic reaction. However, several difficulties arise in interpreting kinesin's backstepping within this framework, especially when external forces oppose the motion of kinesin. We review evidence, such as an ATP-independent stall force and a slower cycle time for backsteps, that has emerged to challenge the idea that kinesin backstepping is due to ATP synthesis, i.e., the reverse cycle of kinesin's forward-stepping chemomechanics. Supplementing the conventional single-loop chemomechanics with routes for ATP-hydrolyzing backward steps and nucleotide-free steps, especially under load, gives a better physical interpretation of the experimental data on backsteps.Comment: 5 figures and 2 table

Super-resolution far-field ghost imaging via compressive sampling

Much more image details can be resolved by improving the system's imaging resolution and enhancing the resolution beyond the system's Rayleigh diffraction limit is generally called super-resolution. By combining the sparse prior property of images with the ghost imaging method, we demonstrated experimentally that super-resolution imaging can be nonlocally achieved in the far field even without looking at the object. Physical explanation of super-resolution ghost imaging via compressive sampling and its potential applications are also discussed.Comment: 4pages,4figure

The relevance of neck linker docking in the motility of kinesin

Conventional kinesin is a motor protein, which is able to walk along a microtubule processively. The exact mechanism of the stepping motion and force generation of kinesin is still far from clear. In this paper we argue that neck linker docking is a crucial element of this mechanism, without which the experimentally observed dwell times of the steps could not be explained under a wide range of loading forces. We also show that the experimental data impose very strict constraints on the lengths of both the neck linker and its docking section, which are compatible with the known structure of kinesin.Comment: Accepted for publication in BioSystems as part of the proceedings of BIOCOMP 200

Kinesin Is an Evolutionarily Fine-Tuned Molecular Ratchet-and-Pawl Device of Decisively Locked Direction

Conventional kinesin is a dimeric motor protein that transports membranous organelles toward the plus-end of microtubules (MTs). Individual kinesin dimers show steadfast directionality and hundreds of consecutive steps, yetthe detailed physical mechanism remains unclear. Here we compute free energies for the entire dimer-MT system for all possible interacting configurations by taking full account of molecular details. Employing merely first principles and several measured binding and barrier energies, the system-level analysis reveals insurmountable energy gaps between configurations, asymmetric ground state caused by mechanically lifted configurational degeneracy, and forbidden transitions ensuring coordination between both motor domains for alternating catalysis. This wealth of physical effects converts a kinesin dimer into a molecular ratchet-and-pawl device, which determinedly locks the dimer's movement into the MT plus-end and ensures consecutive steps in hand-over-hand gait.Under a certain range of extreme loads, however, the ratchet-and-pawl device becomes defective but not entirely abolished to allow consecutive back-steps. This study yielded quantitative evidence that kinesin's multiple molecular properties have been evolutionarily adapted to fine-tune the ratchet-and-pawl device so as to ensure the motor's distinguished performance.Comment: 10 printed page