Bending Two-Dimensional Materials To Control Charge Localization and Fermi-Level Shift

High-performance electronics requires the fine control of semiconductor conductivity. In atomically thin two-dimensional (2D) materials, traditional doping technique for controlling carrier concentration and carrier type may cause crystal damage and significant mobility reduction. Contact engineering for tuning carrier injection and extraction and carrier type may suffer from strong Fermi-level pinning. Here, using first-principles calculations, we predict that mechanical bending, as a unique attribute of thin 2D materials, can be used to control conductivity and Fermi-level shift. We find that bending can control the charge localization of top valence bands in both MoS₂ and phosphorene nanoribbons. The donor-like in-gap edge-states of armchair MoS₂ ribbon and their associated Fermi-level pinning can be removed by bending. A bending-controllable new in-gap state and accompanying direct–indirect gap transition are predicted in armchair phosphorene nanoribbon. We demonstrate that such emergent bending effects are realizable. The bending stiffness as well as the effective thickness of 2D materials are also derived from first principles. Our results are of fundamental and technological relevance and open new routes for designing functional 2D materials for applications in which flexuosity is essential

Voltage-Dependent Regulation of Complex II Energized Mitochondrial Oxygen Flux

<div><p>Oxygen consumption by isolated mitochondria is generally measured during state 4 respiration (no ATP production) or state 3 (maximal ATP production at high ADP availability). However, mitochondria <i>in vivo</i> do not function at either extreme. Here we used ADP recycling methodology to assess muscle mitochondrial function over intermediate clamped ADP concentrations. In so doing, we uncovered a previously unrecognized biphasic respiratory pattern wherein O₂ flux on the complex II substrate, succinate, initially increased and peaked over low clamped ADP concentrations then decreased markedly at higher clamped concentrations. Mechanistic studies revealed no evidence that the observed changes in O₂ flux were due to altered opening or function of the mitochondrial permeability transition pore or to changes in reactive oxygen. Based on metabolite and functional metabolic data, we propose a multifactorial mechanism that consists of coordinate changes that follow from reduced membrane potential (as the ADP concentration in increased). These changes include altered directional electron flow, altered NADH/NAD⁺ redox cycling, metabolite exit, and OAA inhibition of succinate dehydrogenase. In summary, we report a previously unrecognized pattern for complex II energized O₂ flux. Moreover, our findings suggest that the ADP recycling approach might be more widely adapted for mitochondrial studies.</p></div

Mitochondria respiring on succinate are not irreversibly depolarized.

<p>O₂ flux in mitochondria respiring as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0154982#pone.0154982.g002" target="_blank">Fig 2B</a> on 5 mM succinate could be rescued by addition of 5 mM pyruvate (panel A), 5 μM rotenone (panel B), or 2 mM ascorbate + 0.5 mM N,N,N',N'-Tetramethyl-p-phenylenediamine dihydrochloride (TMPD) to provide electrons directly to complex IV (panel C); indicating that the functional status of the organelles was preserved. O₂ flux could not be rescued by adding additional succinate to a final concentration of 10 mM (panel D). Solid lines indicate O₂ flux prior to addition of the compound indicated. Dotted lines indicate O₂ flux after addition. Each panel is representative of 2–3 repetitions.</p

The 2-deoxyglucose (2DOG) energy clamp.

<p>(A) Graphic depiction. Saturating amounts of 2-deoxyglucose (2DOG) and hexokinase (HK) recycle ATP back to ADP by rapidly and irreversibly converting 2DOG into 2-deoxyglucose phosphate (2DOGP). The ADP concentration is clamped at levels determined by the amount of ADP added. IMM = inner mitochondrial membrane. (B and C) Oxygraph tracings of inner membrane potential (inversely related to electrode potential shown on y-axis after calculation using the Nernst equation) vs. time obtained by incubating mouse hindlimb muscle mitochondria (mito), 0.05 mg/ml, fueled by 5 mM succinate (panel B) or 5 mM succinate plus 5 μM rotenone (panel C). ADP was added in incremental amounts to generate the final recycling nucleotide phosphate concentrations shown. After each addition, plateau values were reached, consistent with recycling at a steady [ADP].</p

O₂ flux in mitochondria isolated from different tissues as a function of clamped [ADP].

<p>Mitochondria were energized by 5 mM succinate and incubated as described in the legend to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0154982#pone.0154982.g002" target="_blank">Fig 2</a> in the presence of HK and 2DOG (data represent mean ± SE, n = 3).</p

Mitochondrial functional parameters as a function of clamped [ADP] (mean ± SEM).

<p>ADP was added in incremental amounts in the presence of hexokinase and excess 2-deoxyglucose in order to clamp [ADP] and ΔΨ. ADP additions were made at approximately 90 sec intervals and completed well before depletion of chamber oxygen tension. (A) O₂ flux in hindlimb muscle mitochondria respiring on 5 mM glutamate plus 1 mM malate (n = 3). (B-F) Parameters assessed in a total of 33 preparations of hindlimb skeletal muscle mitochondria respiring on 5 mM succinate with (open circles) or without (closed circles) 5 μM rotenone (number of isolates studied for each parameter are indicated in panels). (B) O₂ flux. (C) Plateau values of ΔΨ recorded simultaneously with O₂ flux. (D) ATP production measured as 2-deoxyglucose phosphate accumulation in multiwell plates over 20 min. (E) NADH fluorescence. (F) H₂O₂ production. * p < 0.05, ** p < 0.01, † p < 0.001 by t-test corrected for multiple comparisons by the Holm-Sidak method compared to succinate alone.</p

O₂ flux in in mitochondria energized with 5 mM succinate in the absence of rotenone as affected by ADP clamped at the concentrations indicated.

<p>In these experiments ADP was maintained, rather than titrated, at the same concentration throughout the incubation period. A) [ADP] = 8 μM. B) [ADP] = 32 μM. C) [ADP] = 128 μM. Mitochondria (mito), 0.05 mg/ml, and ADP were added at the times indicated (arrows). O₂ concentration (blue) and O₂ flux (red) are depicted graphically. X-axis depicts time relative to the addition of mitochondria (time 0). These experiments are representative of three replicates. Data show actual Oxygraph tracings with axis labels added for clarity.</p

Metabolite accumulation determined by NMR spectroscopy in mitochondria incubated for 20 min in the absence (0 ADP) or presence of low (6 μM) or high (32 μM) ADP.

<p>Mitochondria were energized by 10 mM uniformly labeled [¹³C]succinate in the presence of HK and 2DOG. (A) Representative 2D ¹³C/¹H HMQC spectra for malate and fumarate and HSQC spectra for oxaloacetate (OAA) of the medium after mitochondrial incubation in the presence of various ADP concentrations as indicated by spectral color. One-dimensional slices through the cross-peaks as indicated by the dotted line are also shown. (B-D) Content of indicated metabolite: malate, fumarate, or OAA within mitochondria (nmol/mg mitochondrial protein) or external (μM in medium) at end of incubation. (E-G) Metabolite ratios of internal mitochondrial to external medium from data of panels B-D. The ratios are expressed as mitochondrial content (nmol/mg) to external concentration since it is difficult to be sure of mitochondrial volumes. ND, not determined. Data represent mean ± SEM, n = 6. * p < 0.05, ** p < 0.01, or *** p < 0.001 by repeated measures ANOVA and multiple comparisons by the Holm-Sidak method or rank test for non-parametric data. (H) Ratio of total (internal plus external) OAA to total malate (mean ± SEM, n = 6, † p = 0.001 by 2 tailed, paired t-test).</p

Mechanistic regulation of respiration by succinate-fueled mitochondria at differing clamped ADP levels.

<p>(A) [ADP] = 0 μM, (B) [ADP] = 6 μM, (C) [ADP] = 32 μM. O₂ flux is proportional to forward electron transport (green arrows) leading to downstream O₂ consumption. O₂ flux is highest at [ADP] = 6 μM, lowest at [ADP] = 32 μM, and intermediate at [ADP] = 0 μM in accord with the data of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0154982#pone.0154982.g002" target="_blank">Fig 2B</a>. The extent of O₂ flux is regulated by the net effects of membrane potential (blue charge symbols), oxaloacetate (OAA) inhibition of succinate dehydrogenase (SDH), the extent of reverse electron transport (red arrows), the extent of reduction of the NADH/NAD⁺ redox couple, and metabolite exit. The changes in these factors are supported by our metabolite and functional data as detailed in the text. Thickness of arrows or lines and size of text depict magnitude of effects or concentrations. C I and C II depict complexes 1 and 2. Note that OAA cannot undergo further metabolism in the absence of a source of acetyl CoA (and evidenced in that we were unable to detect ¹³C-citrate).</p

O₂ flux versus [ADP] in mitochondria energized with 5 mM succinate alone or 5 mM succinate plus 1 mM malate.

<p>Mitochondria were incubated as described in the legend to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0154982#pone.0154982.g002" target="_blank">Fig 2</a> in the presence of HK and 2DOG. Data are representative of triplicate experiments.</p