Role of key enzymes in the production of docosahexaenoic acid (DHA) by Thraustochytrium sp. T01.

Docosahexaenoic acid (DHA) is an essential dietary supplement that is highly coveted due to its benefits for human health. Extensive research has been conducted for the sustainable commercial production of DHA by various strains in thraustochytrid family due to the accumulation of higher lipid content in the cells. The current study is focused on improving DHA production by investigating various key enzymes like glucose-6-phosphate dehydrogenase (G6PDH), malic enzyme (ME), and ATP-citrate lyase (ACL) involved in DHA production using Thraustochytrium sp. T01. The growth of this strain was compared in batch and fed-batch mode. The fed-batch yielded better Dry cell weight (40 g L-1), lipid (27.75 g L-1 or 693 mg g-1 of DCW), and DHA contents (11.10 g L-1 or 277 mg g-1 of DCW). G6PDH activity increased 4-fold during the glucose fed-batch, but ME and ACL did not increase significantly. Furthermore, a study was conducted to determine the effects of organic acids (pyruvate and malate) on key enzyme activities. The addition of pyruvate increased the lipid content by 1.35-fold, and ACL activity by 10-fold as compared with control (without added organic acids). Malate addition into the culture media increased DHA content 1.4-fold, and ME activity increased 14-fold compared with control.

Dirac surface plasmons in photoexcited bismuth telluride nanowires: optical pump-terahertz probe spectroscopy.

Collective excitation of Dirac plasmons in graphene and topological insulators has opened new possibilities of tunable plasmonic materials ranging from THz to mid-infrared regions. Using time resolved Optical Pump-Terahertz Probe (OPTP) spectroscopy, we demonstrate the presence of plasmonic oscillations in bismuth telluride nanowires (Bi2Te3 NWs) after photoexcitation using an 800 nm pump pulse. In the frequency domain, the differential conductivity (Δσ = σpump on-σpump off) spectrum shows a Lorentzian response where the resonance frequency (ωp), attributed to surface plasmon oscillations, shifts with photogenerated carrier density (n) as . This dependence establishes the absorption of THz radiation by the Dirac surface plasmon oscillations of the charge carriers in the Topological Surface States (TSS) of Bi2Te3 NWs. Moreover, we obtain a modulation depth, tunable by pump fluence, of ∼40% over the spectral range of 0.5 to 2.5 THz. In addition, the time evolution of Δσ(t) represents a long relaxation channel lasting for more than 50 ps. We model the decay dynamics of Δσ(t) using coupled second order rate equations, highlighting the contributions from surface recombination as well as from trap mediated relaxation channels of the photoinjected carriers.

Doping controlled Fano resonance in bilayer 1T'-ReS2: Raman experiments and first-principles theoretical analysis.

In the bilayer ReS2 channel of a field-effect transistor (FET), we demonstrate using Raman spectroscopy that electron doping (n) results in softening of frequency and broadening of linewidth for the in-plane vibrational modes, leaving the out-of-plane vibrational modes unaffected. The largest change is observed for the in-plane Raman mode at ∼151 cm-1, which also shows doping induced Fano resonance with the Fano parameter 1/q = -0.17 at a doping concentration of ∼3.7 × 1013 cm-2. A quantitative understanding of our results is provided by first-principles density functional theory (DFT), showing that the electron-phonon coupling (EPC) of in-plane modes is stronger than that of out-of-plane modes, and its variation with doping is independent of the layer stacking. The origin of large EPC is traced to 1T to 1T' structural phase transition of ReS2 involving in-plane displacement of atoms whose instability is driven by the nested Fermi surface of the 1T structure. Results are compared with those of the isostructural trilayer ReSe2.

Symmetry induced phonon renormalization in few layers of 2H-MoTe2 transistors: Raman and first-principles studies.

Understanding of electron-phonon coupling (EPC) in two-dimensional (2D) materials manifesting as phonon renormalization is essential to their possible applications in nanoelectronics. Here we report in situ Raman measurements of electrochemically top-gated 2, 3 and 7 layered 2H-MoTe2 channel based field-effect transistors. While the [Formula: see text] and B2g phonon modes exhibit frequency softening and linewidth broadening with hole doping concentration (p) up to ∼2.3 × 1013/cm2, A1g shows relatively small frequency hardening and linewidth sharpening. The dependence of frequency renormalization of the [Formula: see text] mode on the number of layers in these 2D crystals confirms that hole doping occurs primarily in the top two layers, in agreement with recent predictions. We present first-principles density functional theory analysis of bilayer MoTe2 that qualitatively captures our observations, and explain that a relatively stronger coupling of holes with [Formula: see text] or B2g modes as compared with the A1g mode originates from the in-plane orbital character and symmetry of the states at valence band maximum. The contrast between the manifestation of EPC in monolayer MoS2 and those observed here in a few-layered MoTe2 demonstrates the role of the symmetry of phonons and electronic states in determining the EPC in these isostructural systems.

Pressure-induced isostructural electronic topological transitions in 2H-MoTe2: x-ray diffraction and first-principles study.

Synchrotron x-ray diffraction measurements on powder 2H-MoTe2 (P63/mmc) up to ∼46 GPa have been performed along with first-principles based density functional theoretical analysis to probe the isostructural transition in low pressure regime and two electronic topological transitions (ETT) of Lifshitz-type in high pressure regime. The low pressure isostructural transition at ∼7 GPa is associated with the lattice parameter ratio c/a anomaly and the change in the compressibility of individual layers. The pressure dependence of the volume by linearizing the Birch-Murnaghan equation of state as a function of Eulerian strain shows a clear change of the bulk modulus at the ETT pressure of ∼20 GPa. The minimum of c/a ratio around 32 GPa is associated with the change in topology of electron pockets marked as second ETT of Lifshitz-type. We do not observe any structural transition up to the maximum applied pressure of ∼46 GPa under quasi-hydrostatic condition.

Thickness dependent transition from the 1T' to Weyl semimetal phase in ultrathin MoTe2: electrical transport, noise and Raman studies.

Bulk 1T'-MoTe2 shows a structural phase transition from the 1T' to Weyl semimetallic (WSM) Td phase at ∼240 K. This phase transition and transport properties in the two phases have not been investigated on ultra-thin crystals. Here we report electrical transport, 1/f noise and Raman studies on ultra-thin 1T'-MoTe2 (∼5 to 16 nm thick) field-effect transistor (FETs) devices as a function of temperature. The electrical resistivities for a thickness of 16 nm and 11 nm show maxima at temperatures of 208 K and 178 K, respectively, making a transition from the semiconducting to semi-metallic phase, hitherto not observed in bulk samples. Raman frequencies and linewidths for an 11 nm thick crystal show a change around 178 K, attributed to the additional contribution to the phonon self-energy due to the enhanced electron-phonon interaction in the WSM phase. Furthermore, the resistivity at low temperature shows an upturn below 20 K along with the maximum in the power spectral density of the low frequency 1/f noise. The latter rules out the metal-insulator transition (MIT) being responsible for the upturn of resistivity below 20 K. The low temperature resistivity follows ρ∝ 1/T, changing to ρ∝T with increasing temperature supports electron-electron interaction physics at electron-hole symmetric Weyl nodes below 20 K. These observations will pave the way to unravel the properties of the WSM state in layered ultra-thin van der Waals materials.

Pressure-induced Lifshitz and structural transitions in NbAs and TaAs: experiments and theory.

High pressure Raman, resistivity and synchrotron x-ray diffraction studies on Weyl semimetals NbAs and TaAs have been carried out along with density functional theoretical (DFT) analysis to explain pressure induced structural and electronic topological phase transitions. The frequencies of first order Raman modes harden with increasing pressure, exhibiting a slope change at [Formula: see text] GPa for NbAs and [Formula: see text] GPa for TaAs. The resistivities of NbAs and TaAs exhibit a minimum at pressures close to these transition pressures and also a change in the bulk modulus is observed. Our first-principles calculations reveal that the transition is associated with an electronic Lifshitz transition at [Formula: see text] for NbAs while it is a structural phase transition from body centered tetragonal to hexagonal phase at [Formula: see text] for TaAs. Further, our DFT calculations show a structural phase transition at 24 GPa from body centered tetragonal phase to hexagonal phase.

Back-Bonding Signature with High Pressure: Raman Studies on Silver Nitroprusside.

In centrosymmetric molecules, like An+[M(CN)6]n- (where A is alkali metal cation), normally all stretching vibrations of cyanide (CN-) shift to high frequency in response to nonhydrostatic pressure, whereas, in non-centrosymmetric molecules in which one axial CN ligand is replaced by NO ligand, one observes unusual softening of only equatorial CN stretching modes. This effect is pronounced when A+ is replaced by Ag+ with difference in coordination ability of latter, resulting in expression of characteristic signature of back-bonding. One can correlate this uneven stretching of cyanide to Poisson-like effect, where the axial Fe-N, Fe-C, and C-N stretching modes harden but the equatorial C-N stretching modes soften due to expansion at the equatorial plane. Thus, the present study is focused on results of non-hydrostatic high-pressure Raman measurements on silver nitroprusside up to 11.5 GPa, for not only observing characteristic signature of "back-bonding" interaction, rarely featured in literature, but also for generating reversible flexible structures akin to noncovalent interaction.

Pressure-dependent semiconductor to semimetal and Lifshitz transitions in 2H-MoTe2: Raman and first-principles studies.

High pressure Raman spectroscopy of bulk 2H-MoTe2 up to ∼29 GPa is shown to reveal two phase transitions (at ∼6 and 16.5 GPa), which are analyzed using first-principles density functional theoretical calculations. The transition at 6 GPa is marked by changes in the pressure coefficients of A 1g and [Formula: see text] Raman mode frequencies as well as in their relative intensity. Our calculations show that this is an isostructural semiconductor to a semimetal transition. The transition at ∼16.5 GPa is identified with the changes in linewidths of the Raman modes as well as in the pressure coefficients of their frequencies. Our theoretical analysis clearly shows that the structure remains the same up to 30 GPa. However, the topology of the Fermi-surface evolves as a function of pressure, and abrupt appearance of electron and hole pockets at [Formula: see text] GPa marks a Lifshitz transition.

Pressure-induced phase transition in Bi2Se3 at 3 GPa: electronic topological transition or not?

In recent years, a low pressure transition around P3 GPa exhibited by the A2B3-type 3D topological insulators is attributed to an electronic topological transition (ETT) for which there is no direct evidence either from theory or experiments. We address this phase transition and other transitions at higher pressure in bismuth selenide (Bi2Se3) using Raman spectroscopy at pressure up to 26.2 GPa. We see clear Raman signatures of an isostructural phase transition at P2.4 GPa followed by structural transitions at ∼ 10 GPa and 16 GPa. First-principles calculations reveal anomalously sharp changes in the structural parameters like the internal angle of the rhombohedral unit cell with a minimum in the c/a ratio near P3 GPa. While our calculations reveal the associated anomalies in vibrational frequencies and electronic bandgap, the calculated Z2 invariant and Dirac conical surface electronic structure remain unchanged, showing that there is no change in the electronic topology at the lowest pressure transition.

Phonon anomalies, orbital-ordering and electronic raman scattering in iron-pnictide Ca(Fe0.97Co0.03)2As2: temperature-dependent Raman study.

We report inelastic light scattering studies on Ca(Fe0.97Co0.03)2As2 in a wide spectral range of 120-5200 cm( - 1) from 5 to 300 K, covering the tetragonal to orthorhombic structural transition as well as magnetic transition at Tsm ~ 160 K. The mode frequencies of two first-order Raman modes B1g and Eg, both involving the displacement of Fe atoms, show a sharp increase below Tsm. Concomitantly, the linewidths of all the first-order Raman modes show anomalous broadening below Tsm, attributed to strong spin-phonon coupling. The high frequency modes observed between 400 and 1200 cm( - 1) are attributed to electronic Raman scattering involving the crystal field levels of d-orbitals of Fe(2+). The splitting between xz and yz d-orbital levels is shown to be ~25 meV, which increases as temperature decreases below Tsm. A broad Raman band observed at ~3200 cm( - 1) is assigned to two-magnon excitation of the itinerant Fe 3d antiferromagnet.

Sharp Raman anomalies and broken adiabaticity at a pressure induced transition from band to topological insulator in Sb2Se3.

The nontrivial electronic topology of a topological insulator is thus far known to display signatures in a robust metallic state at the surface. Here, we establish vibrational anomalies in Raman spectra of the bulk that signify changes in electronic topology: an E(g)(2) phonon softens unusually and its linewidth exhibits an asymmetric peak at the pressure induced electronic topological transition (ETT) in Sb(2)Se(3) crystal. Our first-principles calculations confirm the electronic transition from band to topological insulating state with reversal of parity of electronic bands passing through a metallic state at the ETT, but do not capture the phonon anomalies which involve breakdown of adiabatic approximation due to strongly coupled dynamics of phonons and electrons. Treating this within a four-band model of topological insulators, we elucidate how nonadiabatic renormalization of phonons constitutes readily measurable bulk signatures of an ETT, which will facilitate efforts to develop topological insulators by modifying a band insulator.

Transient receptor potential vanilloid type 1-dependent regulation of liver-related neurons in the paraventricular nucleus of the hypothalamus diminished in the type 1 diabetic mouse.

The paraventricular nucleus (PVN) of the hypothalamus controls the autonomic neural output to the liver, thereby participating in the regulation of hepatic glucose production (HGP); nevertheless, mechanisms controlling the activity of liver-related PVN neurons are not known. Transient receptor potential vanilloid type 1 (TRPV1) is involved in glucose homeostasis and colocalizes with liver-related PVN neurons; however, the functional role of TRPV1 regarding liver-related PVN neurons has to be elucidated. A retrograde viral tracer was used to identify liver-related neurons within the brain-liver circuit in control, type 1 diabetic, and insulin-treated mice. Our data indicate that TRPV1 regulates liver-related PVN neurons. This TRPV1-dependent excitation diminished in type 1 diabetic mice. In vivo and in vitro insulin restored TRPV1 activity in a phosphatidylinositol 3-kinase/protein kinase C-dependent manner and stimulated TRPV1 receptor trafficking to the plasma membrane. There was no difference in total TRPV1 protein expression; however, increased phosphorylation of TRPV1 receptors was observed in type 1 diabetic mice. Our data demonstrate that TRPV1 plays a pivotal role in the regulation of liver-related PVN neurons. Moreover, TRPV1-dependent excitation of liver-related PVN neurons diminishes in type 1 diabetes, thus indicating that the brain-liver autonomic circuitry is altered in type 1 diabetes and may contribute to the autonomic dysfunction of HGP.

Functional plasticity of central TRPV1 receptors in brainstem dorsal vagal complex circuits of streptozotocin-treated hyperglycemic mice.

Emerging data indicate that central neurons participate in diabetic processes by modulating autonomic output from neurons in the dorsal motor nucleus of the vagus (DMV). We tested the hypothesis that synaptic modulation by transient receptor potential vanilloid type 1 (TRPV1) receptors is reduced in the DMV in slices from a murine model of type 1 diabetes. The TRPV1 agonist capsaicin robustly enhanced glutamate release onto DMV neurons by acting at preterminal receptors in slices from intact mice, but failed to do so in slices from diabetic mice. TRPV1 receptor protein expression in the vagal complex was unaltered. Brief insulin preapplication restored TRPV1-dependent modulation of glutamate release in a PKC- and PI3K-dependent manner. The restorative effect of insulin was prevented by brefeldin A, suggesting that insulin induced TRPV1 receptor trafficking to the terminal membrane. Central vagal circuits critical to the autonomic regulation of metabolism undergo insulin-dependent synaptic plasticity involving TRPV1 receptor modulation in diabetic mice after several days of chronic hyperglycemia.

Cannabinoid-mediated inhibition of recurrent excitatory circuitry in the dentate gyrus in a mouse model of temporal lobe epilepsy.

Temporal lobe epilepsy (TLE) is a neurological condition associated with neuron loss, axon sprouting, and hippocampal sclerosis, which results in modified synaptic circuitry. Cannabinoids appear to be anti-convulsive in patients and animal models of TLE, but the mechanisms of this effect are not known. A pilocarpine-induced status epilepticus mouse model of TLE was used to study the effect of cannabinoid agonists on recurrent excitatory circuits of the dentate gyrus using electrophysiological recordings in hippocampal slices isolated from control mice and mice with TLE. Cannabinoid agonists WIN 55,212-2, anandamide (AEA), or 2-arachydonoylglycerol (2-AG) reduced the frequency of spontaneous and tetrodotoxin-resistant excitatory postsynaptic currents (EPSCs) in mice with TLE, but not in controls. WIN 55,212-2 also reduced the frequency of EPSCs evoked by glutamate-photolysis activation of other granule cells in epileptic mice. Secondary population discharges evoked after antidromic electrical stimulation of mossy fibers in the hilus were also attenuated by cannabinoid agonists. Agonist effects were blocked by the cannabinoid type 1 receptor (CB1R) antagonist AM251. No change in glutamate release was observed in slices from mice that did not undergo status epilepticus. Western blot analysis suggested an up-regulation of CB1R in the dentate gyrus of animals with TLE. These findings indicate that activation of CB1R present on nerve terminals can suppress recurrent excitation in the dentate gyrus of mice with TLE. This suggests a mechanism for the anti-convulsive role of cannabinoids aimed at modulating receptors on synaptic terminals expressed de novo after epileptogenesis.

Effects of TRPV1 activation on synaptic excitation in the dentate gyrus of a mouse model of temporal lobe epilepsy.

Temporal lobe epilepsy (TLE) is a condition characterized by an imbalance between excitation and inhibition in the temporal lobe. Hallmarks of this change are axon sprouting and accompanying synaptic reorganization in the temporal lobe. Synthetic and endogenous cannabinoids have variable therapeutic potential in treating intractable temporal lobe epilepsy, in part because cannabinoid ligands can bind multiple receptor types. This study utilized in vitro electrophysiological methods to examine the effect of transient receptor potential vanilloid type 1 (TRPV1) activation in dentate gyrus granule cells in a murine model of TLE. Capsaicin, a selective TRPV1 agonist had no measurable effect on overall synaptic input to granule cells in control animals, but significantly enhanced spontaneous and miniature EPSC frequency in mice with TLE. Exogenous application of anandamide, an endogenous cannabinoid that acts at both TRPV1 and cannabinoid type 1 receptors (CB1R), also enhanced glutamate release in the presence of a CB1R antagonist. Anandamide reduced the EPSC frequency when TRPV1 were blocked with capsazepine. Western blot analysis of TRPV1 receptor indicated protein expression was significantly greater in the dentate gyrus of mice with TLE compared with control mice. This study indicates that a prominent cannabinoid agonist can increase excitatory circuit activity in the synaptically reorganized dentate gyrus of mice with TLE by activating TRPV1 receptors, and suggests caution in designing anticonvulsant therapy based on modulating the endocannabinoid system.

Raman evidence for orbiton-mediated multiphonon scattering in multiferroic TbMnO3.

Temperature-dependent Raman spectra of TbMnO(3) from 5 to 300 K in the spectral range of 200-1525 cm(-1) show five first-order Raman allowed modes and two high frequency modes. The intensity ratio of the high frequency Raman band to the corresponding first-order Raman mode is nearly constant and high (∼0.6) at all temperatures, suggesting an orbiton-phonon mixed nature of the high frequency mode. One of the first-order phonon modes shows anomalous softening below T(N) (∼46 K), suggesting a strong spin-phonon coupling.

Temperature-dependent Raman study of a CeFeAsO(0.9)F(0.1) superconductor: crystal field excitations, phonons and their coupling.

We report temperature-dependent Raman spectra of CeFeAsO(0.9)F(0.1) from 4 to 300 K in the spectral range of 60-1800 cm(-1) and interpret them using estimates of phonon frequencies obtained from first-principles density functional calculations. We find evidence for strong coupling between the phonons and crystal field excitations; in particular the Ce(3 + ) crystal field excitation at 432 cm(-1) couples strongly with the E(g) oxygen vibration at 389 cm(-1). Below the superconducting transition temperature, the phonon mode near 280 cm(-1) shows softening, signaling its coupling with the superconducting gap. The ratio of the superconducting gap to T(c), thus estimated to be ~10, suggests CeFeAsO(0.9)F(0.1) to be a strong coupling superconductor. In addition, two high frequency modes observed at 1342 and 1600 cm(-1) are attributed to electronic Raman scattering from the (x(2)-y(2)) to xz /yz d-orbitals of Fe.

Double-walled carbon nanotubes under hydrostatic pressure: Raman experiments and simulations.

We have used Raman spectroscopy to study the behavior of double-walled carbon nanotubes (DWNT) under hydrostatic pressure. We find that the rate of change of the tangential mode frequency with pressure is higher for the sample with traces of polymer compared to the pristine sample. We have performed classical molecular dynamics simulations to study the collapse of single (SWNT) and double-walled carbon nanotube bundles under hydrostatic pressure. The collapse pressure (pc) was found to vary as 1/R3, where R is the SWNT radius or the DWNT effective radius. The bundles showed approximately 30% hysteresis and the hexagonally close packed lattice was completely restored on decompression. The pc of a DWNT bundle was found to be close to the sum of its values for the inner and the outer tubes considered separately as SWNT bundles, demonstrating that the inner tube supports the outer tube and that the effective bending stiffness of DWNT, D(DWNT) - 2D(SWNT).

Irreversible pressure-induced transformation of boron nitride nanotubes.

We have used Raman spectroscopy to study the behavior of multi-walled boron nitride nanotubes and hexagonal boron nitride crystals under high pressure. While boron nitride nanotubes show an irreversible transformation at about 12 GPa, hexagonal boron nitride exhibits a reversible phase transition at 13 GPa. We also present molecular dynamics simulations which suggest that the irreversibility of the pressure-induced transformation in boron nitride nanotubes is due to the polar nature of the bonds between boron and nitrogen.