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N-Heterocyclic Carbene-Catalyzed Umpolung of Imines for the Enantioselective Synthesis of Dihydroquinoxalines
No full textN-heterocyclic carbene (NHC) organocatalysis is widely employed for the umpolung of aldehydes and recently to the umpolung of Michael acceptors and aldimines. Described herein is the NHC-organocatalyzed umpolung of aldimines for the enantioselective synthesis of nitrogen heterocycles. The bisimines generated from the condensation of 1,2-phenylenediamines and salicylaldehydes undergo intramolecular cyclization in the presence of a chiral NHC catalyst, resulting in the formation of dihydroquinoxalines in moderate to good yields and er values. Detailed DFT studies shed light on the role of -OH groups in stabilizing the initially generated aza-Breslow intermediates via intramolecular hydrogen bonds. Preliminary photophysical studies on the synthesized dihydroquinoxalines revealed that these molecules can be used for the sensing of various acids and bases
Role of Disulfide Bonds and Topological Frustration in the Kinetic Partitioning of Lysozyme Folding Pathways
No full textDisulfide bonds in proteins can strongly influence the folding pathways by constraining the conformational space. Lysozyme has four disulfide bonds and is widely studied for its antibacterial properties. Experiments on lysozyme infer that the protein folds through a fast and a slow pathway. However, the reasons for the kinetic partitioning in the folding pathways are not completely clear. Using a coarse-grained protein model and simulations, we show that two out of the four disulfide bonds, which are present in the alpha-domain of lysozyme, are responsible for the slow folding pathway. In this pathway, a kinetically trapped intermediate state, which is close to the native state, is populated. In this state, the orientations of a-helices present in the alpha-domain are misaligned relative to each other. The protein in this state has to partially unfold by breaking down the interhelical contacts between the misaligned helices to fold to the native state. However, the topological constraints due to the two disulfide bonds present in the alpha-domain make the protein less flexible, and it is trapped in this conformation for hundreds of milliseconds. On disabling these disulfide bonds, we find that the kinetically trapped intermediate state and the slow folding pathway disappear. Simulations mimicking the folding of protein without disulfide bonds under oxidative conditions show that the native disulfide bonds are formed as the protein folds, indicating that folding guides the formation of disulfide bonds. The sequence of formation of the disulfide bonds is Cys64-Cys80 -> Cys76-Cys94 -> Cys30-Cys115 -> Cys6-Cys127. Any disulfide bond that forms before its precursor in the sequence has to break and follow the sequence for the protein to fold. These results show that lysozyme also serves as a very good model system to probe the role of disulfide bonds and topological frustration in protein folding. The predictions from the simulations can be verified by single-molecule fluorescence resonance energy transfer or single-molecule pulling experiments, which can probe heterogeneity in the folding pathways
Parametric Optimization of Self-Switching Diode
No full textParametric optimization of a novel nano diode called self-switching device has been demonstrated using Silvaco TCAD simulator. The device exhibits non-linear characteristics analogous to a conventional diode without requiring any p-n junction. The cut-in voltage can be tuned by varying channel width of the device. The increase in channel doping concentration exhibits velocity saturation, hence, leading to the saturation of output current at higher voltages. The low subthreshold swing of about 100 mV/decade suggests potential utilization of SSDs in high on-off ratio, low power and low voltage electronic applications
Enhanced piezoelectric properties of Sr-(1.7)(Na0.5Bi0.5)(0.3)Bi4Ti5O18 ceramics in the system Sr(2-x)(Na0.5Bi0.5)(x)Bi4Ti5O18 (where 0 x 0.5)
No full textFine powders synthesized via sol-gel route were employed to fabricate Sr(2-x)(Na0.5Bi0.5)(x)Bi4Ti5O18 (SNBT, where x = 0, 0.1, 0.25, 0.3, 0.4, and 0.5) ceramics. The composition (x)-dependent structural changes associated with SNBT ceramics were analyzed using X-ray powder diffraction, transmission electron microscopy, and Raman spectroscopic techniques. Average grain size analyses carried out on the SNBT ceramics by scanning electron microscopy revealed an important role played by the dopants in inhibiting the grain growth. Dielectric constants and the Curie temperature of the ceramics were found to decrease and increase, respectively, with increase in x. The increase in Curie temperature with increase in x was attributed to the decrease in the tolerance factor. The specific composition (x = 0.3) of the SNBT ceramics exhibited improved piezo- and ferroelectric properties associated with a higher Curie temperature (569 K). The piezoelectric coefficient (d(33)) and the planar electromechanical coupling coefficient (k(p)) of SNBT(x = 0.3) were enhanced by 25% and 42%, respectively, as compared to the undoped ceramics
Multidecadal see-saw of the impact of ENSO on Indian and West African summer monsoon rainfall
No full textThe strength of the simultaneous linear relationship between El Nino/Southern Oscillation (ENSO) and Indian summer monsoon (ISM) precipitation show strong variations on a decadal timescale. While some studies attribute this to shift in the state of the climate and consequent teleconnection pattern, some other argue this as natural variability between two random time series. In this study, we show that the relationship between West African Summer Monsoon (WASM) precipitation with ENSO also experiences decadal timescale oscillation. While the ENSO-ISM relationship weakened during the past seven decades, ENSO-WASM relationship strengthened to above the 95% significance level. We explain this multi-decadal see-saw of strong-weak impact of ENSO on ISM and WASM through a common mechanism. ENSO impacts ISM and WASM rainfall by modulating the upper tropospheric temperature of subtropical Africa and South Asia. While the impact of ENSO on this temperature anomaly was strong and concentrated over the northwest of Indian region before the 1980, the anomalies are spatially discontinuous and weak after 1980. Moreover, a westward shift of the center of this anomaly after 1980 help strengthen the ENSO-WASM relationship. We also show a dramatic change in the relationship between Atlantic Nino and ENSO before and after the 1980s. While before 1980 ENSO did not have much impact on Atlantic Nino index-3 (ATL3), after 1980 El Nino (La Nina) is coincidental with negative (positive) ATL3 index. Since a negative (positive) ATL3 reduce (enhance) WASM by increased south-westerly moisture flux, the ENSO-WASM relationship strengthens after 1980. Our study suggests that the decadal variations of ENSO-ISM and ENSO-WASM relationship is physically linked and possibly could not be due to pure noise in the time series
Exploration of the stability of many-body localization in d > 1
No full textRecent work by De Roeck et al. Phys. Rev. B 95, 155129 (2017)] has argued that many-body localization is unstable in two and higher dimensions due to a thermalization avalanche triggered by rare regions of weak disorder. To examine these arguments, we construct several models of a finite ergodic bubble coupled to an Anderson insulator of noninteracting fermions. We first describe the ergodic region using a Gaussian orthogonal ensemble random matrix and perform an exact diagonalization study of small systems. The results are in excellent agreement with a refined theory of the thermalization avalanche that includes transient finite-size effects, lending strong support to the avalanche scenario. We then explore the limit of large system sizes by modeling the ergodic region via a Hubbard model with all-to-all random hopping: the combined system, consisting of the bubble and the insulator, can be reduced to an effective Anderson impurity problem. We find that the spectral function of a local operator in the ergodic region changes dramatically when coupling to a large number of localized fermionic states; this occurs even when the localized sites are weakly coupled to the bubble. In principle, for a given size of the ergodic region, this may arrest the avalanche. However, this back-action effect is suppressed and the avalanche can be recovered if the ergodic bubble is large enough. Thus, the main effect of the back-action is to renormalize the critical bubble size
Optimization of laminated composite structure considering uncertainty effects
No full textIn this paper, the most conservative Tsai-Wu failure envelopes are obtained for laminated composite considering material as well as ply angle uncertainty. The uncertainty analysis is performed using Monte Carlo simulation (MCS). The obtained failure envelopes are then used as the constraint functions to perform the minimum weight design optimization problem using particle swarm optimization (PSO). Results show increase in weight of the laminate from the deterministic results and it varies from 4% to 50% depending upon the stacking sequence and loading condition. Substantial effects of uncertainty on the failure envelope and optimal design are quantified
Message Passing Receivers for Generalized Media-Based Modulation Signals
No full textWe consider generalized media-based modulation (GMBM), where transmit antennas and radio frequency (RF) mirrors (which are parasitic elements placed near the transmit antennas as digitally controlled scatterers) are indexed simultaneously to convey information bits. We develop low complexity receiver algorithms for the detection of high-rate, large-dimension GMBM signals. Message passing-based detection is a promising low-complexity approach. The elements in a GMBM transmit vector are interdependent. Message passing with a multi-layering approach is proposed to take care of this dependency. The proposed multi-layering approach decouples the dependencies through the addition of new layers and constraints that account for the antenna activation and RF mirror activation patterns. The proposed message passing detection with multi-layering, termed as multi-layered message passing detection, is shown to outperform message passing detection without multi-layering and constraints. In addition to exploiting multi-layering, the channel hardening property of large MIMO channels is exploited for GMBM signal detection and channel estimation. The algorithm based on this approach is termed as multilayered channel hardening-exploiting message passing algorithm. The proposed algorithms scale well in complexity and achieve good bit error performance in high-rate, large-dimension GMBM systems
Vertically aligned tree-like carbon nanostructure as an electrode of the electrochemical capacitor
No full textThe thin film of a vertically aligned tree-like carbon nanostructure is synthesised to study its performance as a novel electrode material of the electrochemical capacitor. The individual constituent nanostructures of the film are multiwalled carbon nanotubes aligned perpendicular to the substrate with carbon films attached to it like branches. This unique nanostructured carbon thin film has a regular geometrical arrangement with a very high surface area due to the distinctive structural morphology along with a good contact with the conducting substrate on which it is directly deposited. This makes the material an attractive candidate as the electrode of an electrochemical capacitor. The performance of this nanostructured material has been studied in a symmetric two-electrode configuration. The material has shown an electrochemical double-layer capacitance-type behaviour, the characteristic of carbon-based electrodes, along with a good cyclic retentivity. The material has shown a specific capacitance of 0.55mFcm(-2) (3.7Fcm(-3)) at a current density of 0.88mAcm(-2), while the aligned carbon nanotube films of similar thickness has exhibited a specific capacitance of 0.08mFcm(-2) (0.66Fcm(-3)) for the same current density