Order restricted classical inference of a Weibull multiple step-stress model.

In this paper, a multiple step-stress model is designed and analyzed when the data are Type-I censored. Lifetime distributions of the experimental units at each stress level are assumed to follow a two-parameter Weibull distribution. Further, distributions under each of the stress levels are connected through a tampered failure-rate based model. In a step-stress experiment, as the stress level increases, the load on the experimental units increases and hence the mean lifetime is expected to be shortened. Taking this into account, the aim of this paper is to develop the order restricted inference of the model parameters of a multiple step-stress model based on the frequentist approach. An extensive simulation study has been carried out and two real data sets have been analyzed for illustrative purposes.

Emergence of sulfur quantum dots: Unfolding their synthesis, properties, and applications.

Over the past few decades, the sphere of applied science has witnessed soaring demand in developing high performance, novel and sustainable materials due to ever-increasing population coupled with need for alternative-green-energy resources. Inevitably, sulfur research expands through the breadth of materials sciences including sustainable use of the by-products obtained from petroleum industry, preparation of biocompatible materials, and constructing energy harvesting devices, indispensable to our everyday lives. Congruous with popular heavy-metal free elemental quantum dots such as the carbon, silicon and phosphorus, emergence of sulfur quantum dots (SQDs) has drawn substantial attention due to their bright luminescence, infrequent to other sulfur allotropes. In this review article, we focus some of the pioneering advances on synthesis and characterizations of luminescent sulfur nanodots and their potential applications in bioimaging, fabrication of light emitting devices, sensing and catalysis. Finally, critical challenges along with future perspectives corresponding to this newly discovered research area have been discussed.

Three-Dimensional Segmentation and Reconstruction of Neuronal Nuclei in Confocal Microscopic Images.

The detailed architectural examination of the neuronal nuclei in any brain region, using confocal microscopy, requires quantification of fluorescent signals in three-dimensional stacks of confocal images. An essential prerequisite to any quantification is the segmentation of the nuclei which are typically tightly packed in the tissue, the extreme being the hippocampal dentate gyrus (DG), in which nuclei frequently appear to overlap due to limitations in microscope resolution. Segmentation in DG is a challenging task due to the presence of a significant amount of image artifacts and densely packed nuclei. Accordingly, we established an algorithm based on continuous boundary tracing criterion aiming to reconstruct the nucleus surface and to separate the adjacent nuclei. The presented algorithm neither uses a pre-built nucleus model, nor performs image thresholding, which makes it robust against variations in image intensity and poor contrast. Further, the reconstructed surface is used to study morphology and spatial arrangement of the nuclear interior. The presented method is generally dedicated to segmentation of crowded, overlapping objects in 3D space. In particular, it allows us to study quantitatively the architecture of the neuronal nucleus using confocal-microscopic approach.

Template-Directed Nonenzymatic Primer Extension Using 2-Methylimidazole-Activated Morpholino Derivatives of Guanosine and Cytidine.

Efforts to develop self-replicating nucleic acids have led to insights into the origin of life and have also suggested potential pathways to the design of artificial life forms based on non-natural nucleic acids. The template-directed nonenzymatic polymerization of activated ribonucleotide monomers is generally slow because of the relatively weak nucleophilicity of the primer 3'-hydroxyl. To circumvent this problem, several nucleic acids based on amino-sugar nucleotides have been studied, and as expected, the more-nucleophilic amine generally results in faster primer extension. Extending this logic, we have chosen to study morpholino nucleic acid (MoNA), because the secondary amine of the morpholino-nucleotides is expected to be highly nucleophilic. We describe the synthesis of 2-methylimidazole-activated MoNA monomers from their corresponding ribonucleoside 5'-monophosphates and the synthesis of an RNA primer with a terminal MoNA nucleotide. We show that the activated G and C MoNA monomers enable rapid and efficient extension of the morpholino-terminated primer on homopolymeric and mixed-sequenced RNA templates. Our results show that MoNA is a non-natural informational polymer that is worthy of further study as a candidate self-replicating material.

Boron Doped Carbon Dots with Unusually High Photoluminescence Quantum Yield for Ratiometric Intracellular pH Sensing.

Herein we report that boron doping in carbon dots results in increased photoluminescence (PL) quantum yield, which could be used for ratiometric intracellular pH sensing in cancer cell lines. Using a mixture of citric acid monohydrate, thiourea, and boric acid, microwave-assisted synthesis of boron doped blue emitting carbon dots (B-Cdots) with an average size of 3.5±1.0 nm was achieved. For B-Cdots, the maximum quantum yield (QY) was observed to be 25.8 % (11.1 % (w/w) H3 BO3 input concentration), whereas, the same was calculated to be 16.9 % and 11.4 % for Cdots (synthesized from citric acid monohydrate and thiourea only) and P-Cdots (phosphorus doped carbon dots; synthesized using citric acid monohydrate, thiourea and phosphoric acid) (11.1 % (w/w) H3 PO4 input concentration), respectively. The observed luminescence efficiencies as obtained from steady state and time-resolved photoluminescence measurements suggest an alternative emission mechanism due to boron/phosphorus doping in carbon dots. We furthermore demonstrated facile composite formation using B-Cdots and another carbon dots with orange emission in presence of polyvinyl alcohol (PVA), resulting in white light emission (0.31, 0.32; λex 380 nm). The white light emitting composite enabled ratiometric pH sensing in the aqueous medium and showed favorable uptake properties by cancerous cells for intracellular pH sensing as well.

Enhanced Nonenzymatic RNA Copying with 2-Aminoimidazole Activated Nucleotides.

Achieving efficient nonenzymatic replication of RNA is an important step toward the synthesis of self-replicating protocells that may mimic early forms of life. Despite recent progress, the nonenzymatic copying of templates containing mixed sequences remains slow and inefficient. Here we demonstrate that activating nucleotides with 2-aminoimidazole results in superior reaction kinetics and improved yields of primer extension reaction products. This new leaving group significantly accelerates monomer addition as well as trimer-assisted RNA primer extension, allowing efficient copying of a variety of short RNA templates with mixed sequences.

Effect of terminal 3'-hydroxymethyl modification of an RNA primer on nonenzymatic primer extension.

The significance of the precise position of the hydroxyl at the 3'-end of an RNA primer for nonenzymatic template-directed primer extension is not well understood. We show that an RNA primer terminating in 3'-hydroxymethyl-2',3'-dideoxy-guanosine has greatly diminished activity, suggesting that the spatial preorganization of the terminal sugar contributes significantly to the efficiency of primer extension.

Thiolated uridine substrates and templates improve the rate and fidelity of ribozyme-catalyzed RNA copying.

Ribozyme-catalyzed RNA polymerization is inefficient and error prone. Here we demonstrate that two alternative bases, 2-thio-uridine (s(2)U) and 2-thio-ribo-thymidine (s(2)T), improve the rate and fidelity of ribozyme catalyzed nucleotide addition as NTP substrates and as template bases. We also demonstrate the functionality of s(2)U and s(2)T-containing ribozymes.

Conducting Carbon Dot-Polypyrrole Nanocomposite for Sensitive Detection of Picric acid.

We report the conducting nature of carbon dots (Cdots) synthesized from citric acid and ethylene diamine. Chemically synthesized conducting nanocomposite consisting of Cdots and polypyrrole (PPy) is further reported, which showed higher electrical conductiviy in comparison to the components i.e., Cdots or PPy. The conductive film of the composite material was used for highly sensitive and selective detection of picric acid in water as well as in soil. To the best of our knowledge, this is the first report on the conductivity based sensing application of Cdot nanocomposite contrary to the traditional fluorescence based sensing approaches.

Replacing uridine with 2-thiouridine enhances the rate and fidelity of nonenzymatic RNA primer extension.

The nonenzymatic replication of RNA oligonucleotides is thought to have played a key role in the origin of life prior to the evolution of ribozyme-catalyzed RNA replication. Although the copying of oligo-C templates by 2-methylimidazole-activated G monomers can be quite efficient, the copying of mixed sequence templates, especially those containing A and U, is particularly slow and error-prone. The greater thermodynamic stability of the 2-thio-U(s(2)U):A base pair, relative to the canonical U:A base pair, suggests that replacing U with s(2)U might enhance the rate and fidelity of the nonenzymatic copying of RNA templates. Here we report that this single atom substitution in the activated monomer improves both the kinetics and the fidelity of nonenzymatic primer extension on mixed-sequence RNA templates. In addition, the mean lengths of primer extension products obtained with s(2)U is greater than those obtained with U, augmenting the potential for nonenzymatic replication of heritable function-rich sequences. We suggest that noncanonical nucleotides such as s(2)U may have played a role during the infancy of the RNA world by facilitating the nonenzymatic replication of genomic RNA oligonucleotides.

Synthesis and NMR assignment of the two diastereomers of 8,6'-cyclo-2',6'-dideoxyadenosine.

We herein present the first synthesis and characterization of the two C5' diastereomers of 8,6'-cyclo-2',6'-dideoxyadenosine. Starting from commercially available 2'-deoxyadenosine, the target cyclonucleosides were synthesized in 11 linear steps. Following a zinc-mediated cyclization reaction to form the seven-membered ring, the stereochemistry of the newly formed chiral center was established using two-dimensional NOESY NMR experiments.

2,3-Dicyclohexylsuccinimide as a directing/protecting group for the regioselective glycosylation or alkylation of purines.

Here we describe the synthesis and application of a novel 2,3-dicyclohexylsuccinimide (Cy2SI) protecting group towards regioselective purine glycosylation and alkylation reactions. This bulky protecting group promotes high regioselectivity during the glycosylation (as well as diastereoselectivity) or alkylation of purines using Hoffer's chlorosugar or tert-butyl bromoacetate, respectively. Cy2SI offers the additional synthetic advantage that other base-labile protecting groups, such as toluoyl esters, can be selectively removed in its presence without affecting the imide.

Synthesis and properties of DNA containing cyclonucleosides.

Here, we present efficient syntheses of the R and S diastereomers of 8,5'-cyclo-2'-deoxyadenosine and 6,5'-cyclo-2'-deoxyuridine. We incorporated these interesting nucleosides into DNA to study how the cyclo linkage affects the stability of duplex formation.