T-like Structure Research Articles

Fast Hole Tunneling Times in Germanium Hut Wires Probed by Single-Shot Reflectometry.

Heavy holes confined in quantum dots are predicted to be promising candidates for the realization of spin qubits with long coherence times. Here we focus on such heavy-hole states confined in germanium hut wires. By tuning the growth density of the latter we can realize a T-like structure between two neighboring wires. Such a structure allows the realization of a charge sensor, which is electrostatically and tunnel coupled to a quantum dot, with charge-transfer signals as high as 0.3 e. By integrating the T-like structure into a radiofrequency reflectometry setup, single-shot measurements allowing the extraction of hole tunneling times are performed. The extracted tunneling times of less than 10 μs are attributed to the small effective mass of Ge heavy-hole states and pave the way toward projective spin readout measurements.

Fine Sampling of the R→T Quaternary-Structure Transition of a Tetrameric Hemoglobin.

Although the end points of the functional transitions of tetrameric hemoglobins (Hbs) have been well characterized, atomic-resolution data on R-T intermediate states are extremely limited. Herein, the X-ray structures of two independent tetramers of the fully ligated carbomonoxy form of Trematomus newnesi hemoglobin (Hb1Tn) within the same crystal are described. These structures show peculiar features in the heme pocket, EF corner, and tertiary/quaternary structure. Distal histidine side chains have a propensity to swing out of the heme pocket and thus allow compression of the EF corner. In this rotameric state, the distal His group does not interact with the CO ligand, consistent with FTIR spectra recorded in solution. At the quaternary-structure level, one tetramer is an intermediate R-T state, whereas the other assumes a T-like structure. Altogether, the structures of these tetramers provide the best available atomic-level picture of the R→T transition of vertebrate Hbs.

Radar Imaging Through a Building Corner

Through-wall imaging (TWI) requires dealing with targets embedded in a complex obscuring environment such as the walls of a building. This obscuring layout is often composed by many simple elements (possibly interacting) such as slabs, corners, and T-like structures. Most of the existing literature on TWI has focused on slab-like walls, which is reasonable when the targets are relatively far from corners. This paper instead concerns the TWI in the more challenging situation where the targets are in close proximity (inside and/or outside) of a building corner. The aim is to gain insight into how propagation through the corner impacts on the imaging problem. To keep the study simple, a preliminary analysis is presented for a 2-D geometry under the linearized Born approximation. First, the Green's function, as well as the kernel of the relevant scattering operator, is evaluated by using a high-frequency analytical approach based on the geometrical optics and the uniform theory of diffraction. This allows one to take into account the multipath propagation phenomena and provide thus an expression of the scattering operator more accurate than that viable under the assumption of a simple slab wall. Then, the imaging is achieved by solving the relevant linear inverse scattering problem with a regularizing truncated-singular-value-decomposition algorithm. The filtering introduced by the inversion procedure, which is dependent on the considered background scenario, is highlighted and linked to the achievable performance while imaging targets both internal and external with respect to the corner. Finally, reconstruction results obtained from synthetic data are reported to assess the approach.

Superoleophobic surfaces with short fluorinated chains?

Due to the extremely low surface tension of oils, the elaboration of superoleophobic surfaces implies the development of well-defined strategies to impede their tendency to spread. Usually, these strategies combine the presence of re-entrant structures (overhang, mushroom-like, T-like structures, etc.) with perfluorinated compounds containing long perfluorinated tails. Due to the toxicity (bioaccumulative potential) of long perfluorinated tails, our strategy is to develop superoleophobic properties with short perfluorinated tails (perfluorobutyl or F-butyl). However, almost all our strategies failed because of their much lower intrinsic oleophobicity. Here, we show the possibility to reach superoleophobic properties by electrodeposition of original fluorinated (F-butyl, F-hexyl and F-octyl) 3,4-ethylenedioxythiophene derivatives containing an amide connector. Surprisingly, if superhydrophobic properties are obtained whatever the fluorinated chain length, superoleophobic properties are only obtained with F-butyl tails (θsunflower oil = 150.0° and θhexadecane = 132.1°). Here, the fibers, only obtained with F-butyl chains, have a much higher tendency to repel oils than spherical particles. It seems that the presence of nanofibrils (re-entrant structures) and amide connectors allow the stabilization of the Cassie–Baxter state (fakir state) to be increased with oil droplets, which makes it possible to reach a state close to the Cassie–Baxter state with a polymer containing F-butyl tails. We think that the amide connectors increase the presence of the F-butyl tails at the extreme surface by reducing their mobility.

Allosteric action in real time: time-resolved crystallographic studies of a cooperative dimeric hemoglobin.

Protein allostery provides mechanisms for regulation of biological function at the molecular level. We present here an investigation of global, ligand-induced allosteric transition in a protein by time-resolved x-ray diffraction. The study provides a view of structural changes in single crystals of Scapharca dimeric hemoglobin as they proceed in real time, from 5 ns to 80 micros after ligand photodissociation. A tertiary intermediate structure forms rapidly (<5 ns) as the protein responds to the presence of an unliganded heme within each R-state protein subunit, with key structural changes observed in the heme groups, neighboring residues, and interface water molecules. This intermediate lays a foundation for the concerted tertiary and quaternary structural changes that occur on a microsecond time scale and are associated with the transition to a low-affinity T-state structure. Reversal of these changes shows a considerable lag as a T-like structure persists well after ligand rebinding, suggesting a slow T-to-R transition.

Carbamyl Phosphate Modifies the T Quaternary Structure of Aspartate Transcarbamylase, thereby Facilitating the Structural Transition Associated with Cooperativity

The allosteric enzyme aspartate transcarbamylase from Escherichia coli (ATCase) displays regulatory properties that involve various conformational changes, including a large quaternary structure rearrangement. This entails a major change in its solution X-ray scattering curve upon binding substrate analogues, thereby providing a direct means to monitor the amount of different quaternary structures present in solution. Scattering curves in the presence of variable concentrations of several such substrate analogues were recorded using an area detector. Data were analyzed by singular-value decomposition without any prior assumption as to the number of quaternary structure states. They can all be accounted for with only two states. The structural transition appears to be concerted, a conclusion whose validity has been extended to higher resolution and to other combinations of ligands than previously studied using a linear detector. The titration curve with the bisubstrate analogue N-(phosphonacetyl)-l-aspartate (PALA) alone is identical to that previously obtained, while that in the additional presence of carbamyl phosphate (CP) shows a clear R shift. However, the scattering pattern of the ATCase–CP complex is shown to be different from the T pattern of the unligated enzyme, though not a linear combination of the Tand R patterns. Therefore, we conclude that CP acts by modifying the quaternary structure of the unligated enzyme to a T-like structure, a structure more easily converted to the R conformation by subsequent addition of PALA.

Allosteric intermediates in hemoglobin. 1. Nanosecond time-resolved circular dichroism spectroscopy.

Time-resolved circular dichroism (TRCD) studies performed on photolyzed hemoglobin-CO complex (HbCO) probe room temperature protein relaxations in Hb, including the R --> T allosteric transition. TRCD spectroscopy of photolysis intermediates in the near-UV (250-400 nm) spectral region provides a diagnostic for T-like structure at the alpha 1 beta 2 interface via the effect of quaternary structure on the UV CD of aromatic residues. The TRCD of porphyrin-based transitions in the UV and Soret regions, reflecting transition-dipole couplings between hemes and aromatic residues over a radius wide enough to permit heme-interface and inter-dimer interactions, is modulated by the tertiary and quaternary structure of photolysis intermediates. In the allosteric core model of Hb cooperativity, Fe-CO bond breakage initiates a heme structural change, thought to be heme doming, that is transmitted to the alpha 1 beta 2 interface via the F helix. The TRCD results, analyzed in light of kinetic information from time-resolved absorption studies, suggest specific features for the mechanism by which the ensuing tertiary and quaternary conformational changes propagate through the protein. In particular, the UV-TRCD indicates that the alpha 1 beta 2 interface responds within several hundred nanoseconds to initial events at the heme by shifting from an R toward a T-like interface. The appearance of T-like character at the alpha 1 beta 2 interface tens of microseconds before the appearance of equilibrated T state deoxyHb indicates that the R --> T transition in photolyzed HbCO is a stepwise process, as previously suggested by time-resolved resonance Raman studies.

Enthalpy changes for inositol hexaphosphate binding to hemoglobins A and M iwate

Enthalpies of inositol hexaphosphate (IHP) binding to deoxy and carbonmonoxy (CO) HbA and HbM Iwate have been determined calorimetrically and compared as functions of pH. Values for deoxy HbA and for deoxy HbM Iwate are similar with CO HbM Iwate yielding slightly less heat of reaction. The results support the existence of both deoxy and CO HbM Iwate in T-like structures with only minor modifications occurring upon CO binding. For HbA observed heats of IHP binding have been corrected for heats of extraction of reacting protons from buffer. The resulting intrinsic IHP binding enthalpies show consistent values of −7 to −11 kcal/mol proton absorbed in binding. We suggest that a major driving force for organic phosphate binding is the exothermic protonation of histidine and/or a α-amino nitrogens induced by proximity of phosphate negative charges.