High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe

A decade of intense research on two-dimensional (2D) atomic crystals has revealed that their properties can differ greatly from those of the parent compound. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atmosphere. Carrier mobilities are found to exceed 103cm2V-1s-1and 104cm2V-1s-1at room and liquid-helium temperatures, respectively, allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayer's mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides and black phosphorus.EU, EPSRC. The Royal Societ

Kinetics of a Two-Component p-Hydroxyphenylacetate Hydroxylase Explain How Reduced Flavin Is Transferred from the Reductase to the Oxygenase

p-Hydroxyphenylacetate hydroxylase (HPAH) from 'Acinetobacter baumannii' catalyzes the hydroxylation of p-hydroxyphenylacetate (HPA) to form 3,4-dihydroxyphenylacetate (DHPA). HPAH is composed of two proteins:  a flavin mononucleotide (FMN) reductase (C₁) and an oxygenase (C₂). C₁ catalyzes the reduction of FMN by NADH to generate reduced FMN (FMNH-) for use by C₂ in the hydroxylation reaction. C₁ is unique among the flavin reductases in that the substrate HPA stimulates the rates of both the reduction of FMN and release of FMNH- from the enzyme. This study quantitatively shows the kinetics of how the C₁-bound FMN can be reduced and released to be used efficiently as the substrate for the C₂ reaction; additional FMN is not necessary. Reactions in which O₂ is rapidly mixed with solutions containing C₁-FMNH- and C₂ are very similar to those in which solutions containing O₂ are mixed with one containing the C₂-FMNH- complex. This suggests that in a mixture of the two proteins FMNH- binds more tightly to C₂ and has already been completely transferred to C₂ before it reacts with oxygen. Rate constants for the transfer of FMNH- from C₁ to C₂ were found to be 0.35 and ≥74 s⁻¹ in the absence and presence of HPA, respectively. The reduction of cytochrome c by FMNH- was also used to measure the dissociation rate of FMNH- from C₁. In the absence of HPA, FMNH- dissociates from C1 at 0.35 s⁻¹, while with HPA present it dissociates at 80 s⁻¹; these are the same rates as those for the transfer from C₁ to C₂. Therefore, the dissociation of FMNH- from C₁ is rate-limiting in the intermolecular transfer of FMNH- from C₁ to C₂, and this process is regulated by the presence of HPA. This regulation avoids the production of H₂O₂ in the absence of HPA. Our findings indicate that no protein-protein interactions between C₁ and C₂ are necessary for efficient transfer of FMNH- between the proteins; transfer can occur by a rapid-diffusion process, with the rate-limiting step being the release of FMNH- from C₁

The Reaction Kinetics of 3-Hydroxybenzoate 6-Hydroxylase from Rhodococcus jostii RHA1 Provide an Understanding of the para-Hydroxylation Enzyme Catalytic Cycle

3-Hydroxybenzoate 6-hydroxylase (3HB6H) from Rhodococcus jostii RHA1 is an NADH-specific flavoprotein monooxygenase that catalyzes the para-hydroxylation of 3-hydroxybenzoate (3HB) to form 2,5-dihydroxybenzoate (2,5-DHB). Based on results from stopped-flow spectrophotometry, the reduced enzyme-3HB complex reacts with oxygen to form a C4a-peroxy flavin with a rate constant of 1.13 ± 0.01 × 10(6) m(-1) s(-1) (pH 8.0, 4 °C). This intermediate is subsequently protonated to form a C4a-hydroperoxyflavin with a rate constant of 96 ± 3 s(-1). This step shows a solvent kinetic isotope effect of 1.7. Based on rapid-quench measurements, the hydroxylation occurs with a rate constant of 36 ± 2 s(-1). 3HB6H does not exhibit substrate inhibition on the flavin oxidation step, a common characteristic found in most ortho-hydroxylation enzymes. The apparent kcat at saturating concentrations of 3HB, NADH, and oxygen is 6.49 ± 0.02 s(-1). Pre-steady state and steady-state kinetic data were used to construct the catalytic cycle of the reaction. The data indicate that the steps of product release (11.7 s(-1)) and hydroxylation (36 ± 2 s(-1)) partially control the overall turnove

Stabilization of C4a-Hydroperoxyflavin in a Two-component Flavin-dependent Monooxygenase Is Achieved through Interactions at Flavin N5 and C4a Atoms

p-Hydroxyphenylacetate (HPA) 3-hydroxylase is a two-component flavin-dependent monooxygenase. Based on the crystal structure of the oxygenase component (C(2)), His-396 is 4.5 Å from the flavin C4a locus, whereas Ser-171 is 2.9 Å from the flavin N5 locus. We investigated the roles of these two residues in the stability of the C4a-hydroperoxy-FMN intermediate. The results indicated that the rate constant for C4a-hydroperoxy-FMN formation decreased ∼30-fold in H396N, 100-fold in H396A, and 300-fold in the H396V mutant, compared with the wild-type enzyme. Lesser effects of the mutations were found for the subsequent step of H(2)O(2) elimination. Studies on pH dependence showed that the rate constant of H(2)O(2) elimination in H396N and H396V increased when pH increased with pK(a) >9.6 and >9.7, respectively, similar to the wild-type enzyme (pK(a) >9.4). These data indicated that His-396 is important for the formation of the C4a-hydroperoxy-FMN intermediate but is not involved in H(2)O(2) elimination. Transient kinetics of the Ser-171 mutants with oxygen showed that the rate constants for the H(2)O(2) elimination in S171A and S171T were ∼1400-fold and 8-fold greater than the wild type, respectively. Studies on the pH dependence of S171A with oxygen showed that the rate constant of H(2)O(2) elimination increased with pH rise and exhibited an approximate pK(a) of 8.0. These results indicated that the interaction of the hydroxyl group side chain of Ser-171 and flavin N5 is required for the stabilization of C4a-hydroperoxy-FMN. The double mutant S171A/H396V reacted with oxygen to directly form the oxidized flavin without stabilizing the C4a-hydroperoxy-FMN intermediate, which confirmed the findings based on the single mutation that His-396 was important for formation and Ser-171 for stabilization of the C4a-hydroperoxy-FMN intermediate in C(2)

Anisotropic electrical resistance in mesoscopic LaAlO 3 /SrTiO 3 devices with individual domain walls

International audienceThe crystal structure of bulk SrTiO 3 (STO) transitions from cubic to tetragonal at around 105 K. Recent local scanning probe measurements of LaAlO 3 /SrTiO 3 (LAO/STO) interfaces indicated the existence of spatially inhomogeneous electrical current paths and electrostatic potential associated with the structural domain formation in the tetragonal phase of STO. Here we report a study of temperature dependent electronic transport in combination with the polarized light microscopy of structural domains in mesoscopic LAO/STO devices. By reducing the size of the conductive interface to be comparable to that of a single tetragonal domain of STO, the anisotropy of interfacial electron conduction in relationship to the domain wall and its direction was characterized between T = 10–300 K. It was found that the four-point resistance measured with current parallel to the domain wall is larger than the resistance measured perpendicular to the domain wall. This observation is qualitatively consistent with the current diverting effect from a more conductive domain wall within the sample. Among all the samples studied, the maximum resistance ratio found is at least 10 and could be as large as 10 5 at T = 10 K. This electronic anisotropy may have implications on other oxide hetero-interfaces and the further understanding of electronic/magnetic phenomena found in LAO/STO