Absolute negative conductivity and spontaneous current generation in semiconductor superlattices with hot electrons

We study transport through a semiconductor superlattice with an electric field parallel to and a magnetic field perpendicular to the growth axis. Using a semiclassical balance equation model with elastic and inelastic scattering, we find that (1) the current-voltage characteristic becomes multistable in a large magnetic field and (2) "hot" electrons display novel features in their current-voltage characteristics, including absolute negative conductivity and a spontaneous dc current at zero bias. We discuss experimental situations providing hot electrons to observe these effects.     

Economic implications of poor access to antenatal care in rural and remote Western Australian Aboriginal communities: An individual sampling model of pregnancy

Background

Australian Aboriginal women attend antenatal care less frequently and experience poorer pregnancy outcomes than non-Aboriginal women. Improving access to antenatal care is recognised as a means to improve pregnancy outcomes for mother and baby.

Objective

To estimate the costs of inadequate antenatal care and provide baseline measurements and implications for policy that targets improving access to care in rural and remote Western Australian (WA) Aboriginal communities.

Methods

An individual sampling model of pregnancy was developed that simulated hypothetical women with pregnancy events and outcomes observed in the WA Aboriginal population. Weekly pregnancy events were modelled via logistic regression according to maternal characteristics, events during pregnancy and current gestation, with adequate and inadequate care (?4 and <4 antenatal visits) to reflect differences in outcomes reported in the literature. The pregnancy model simulated clinical management including antenatal visits, hospitalisations, and transfers to tertiary care.

Results

The mean cost of pregnancy was AUD$8985, with a large difference depending on access to antenatal care (AUD$7635 and AUD$10,216 for adequate and inadequate care respectively). The main difference in costs resulted from neonatal care (AUD$1021 vs AUD$3205 for adequate and inadequate care respectively). In a rural community with 150 births per year, up to AUD$123,082 may be spent to improve access to care at no extra cost to the total current expenditure (AUD$1,347,733).

Conclusions

The large difference in pregnancy costs between those receiving adequate and inadequate care demonstrates that additional expenditure on improving access to antenatal care may be cost-effective and should be further investigated.     

Urea-amide preferential interactions in water: quantitative comparison of model compound data with biopolymer results using water accessible surface areas

Two fundamentally different thermodynamic approaches are in use to interpret or predict the effects of urea on biopolymer processes: one is a synthesis of transfer free energies obtained from measurements of the effects of urea on the solubilities of small, model compounds; the other is an analysis of preferential interactions of urea with a range of folded and unfolded biopolymer surfaces. Here, we compare the predictions of these two approaches for the contribution of urea-amide (peptide) interactions to destabilization of folded proteins by urea. For these comparisons, we develop independent thermodynamic analyses of osmometric and solubility data characterizing interactions of a model compound with urea (or any other solute) and apply them to all five model compounds (glycine, alanine, diglycine, glycylalanine, and triglycine) where both isopiestic distillation (ID) and solubility data in aqueous urea solutions are available. We use model-independent expressions to calculate mu ex 23, the derivative of the "excess" chemical potential of solute "2" (either a model compound or a biopolymer) with respect to the molality of solute "3" (urea). Analyses of ID data for these systems reveal significant dependences of mu ex 23 on both m2 and m3, which must be taken into account in making comparisons with values of mu ex 23 obtained from solubility studies or from analyses of urea-biopolymer preferential interactions. Values of mu ex 23 calculated from model compound ID data at low m2 and m3 are directly proportional to the amount of polar amide (N, O) surface area, and not to any other type of surface. The proportionality constant in this limit, mu ex 23 /(RT x ASA) = (1.0 +/- 0.1) x 10(-3) A(-2), is very similar to that previously obtained by analysis of urea-biopolymer preferential interactions ((1.4 +/- 0.3) x 10(-3) A(-2)). This level of agreement for amide surface in the low concentration limit, as well as the absence of any significant preferential interaction of urea with Gly and Ala, reinforces the conclusion that the primary preferential interaction of urea with protein surface is a favorable interaction (resulting in local accumulation of urea) at polar amide surface, located mostly on the peptide backbone. However, mu ex 23 for interactions of urea with these model amides is found from both ID and solubility data to be urea concentration-dependent, in contrast to the urea concentration independence of the analogous quantity for protein unfolding.     

Is There No End to the Total Syntheses of Strychnine? Lessons Learned in Strategy and Tactics in Total Synthesis

From the 19th century to the present, the complex indole alkaloid strychnine has engaged the chemical community. In this Review, we examine why strychnine has been and remains today an important target for directed synthesis efforts. A selection of the diverse syntheses of strychnine is discussed with the aim of identifying their influence on the evolution of the strategy and tactics of organic synthesis.     

New benchmark for chemical nomenclature software

We propose a new, robust benchmark, called Percentage Round Tripping of Canonical Isomeric SMILES (%RTCS), for assessing the ability of chemical nomenclature software to convert chemical structures to names and chemical names to structures. The benchmark is based on a string comparison between canonical isomeric SMILES generated from the original structure and the resultant structure from round tripping. Using the latest version of the OpenEye chemical nomenclature toolkit, Lexichem v2.1.0, we report %RTCS values of over 92% on average for a variety of challenging compound collections.     

Palladium(II)-catalyzed enantioselective synthesis of 2-vinyl oxygen heterocycles

2-Vinylchromanes (1), 2-vinyl-1,4-benzodioxanes (2), and 2,3-dihydro-2-vinyl-2H-1,4-benzoxazines (3) can be prepared in high yields (90-98%) and excellent enantiomeric purities (87-98% ee) by [COP-OAc](2)-catalyzed cyclization of phenolic (E)-allylic trichloroacetimidate precursors. Deuterium-labeling and computational experiments are consistent with these cyclization reactions taking place by an anti-oxypalladation/syn-deoxypalladation mechanism. 2-Vinylchromanes can also be prepared in good yields and high enantiomeric purities from analogous (E)-allylic acetate precursors, which constitutes the first report that acetate is a competent leaving group in COP-catalyzed enantioselective S(N)2' substitution reactions.     

Chemoinformatics-based classification of prohibited substances employed for doping in sport

Representative molecules from 10 classes of prohibited substances were taken from the World Anti-Doping Agency (WADA) list, augmented by molecules from corresponding activity classes found in the MDDR database. Together with some explicitly allowed compounds, these formed a set of 5245 molecules. Five types of fingerprints were calculated for these substances. The random forest classification method was used to predict membership of each prohibited class on the basis of each type of fingerprint, using 5-fold cross-validation. We also used a k-nearest neighbors (kNN) approach, which worked well for the smallest values of k. The most successful classifiers are based on Unity 2D fingerprints and give very similar Matthews correlation coefficients of 0.836 (kNN) and 0.829 (random forest). The kNN classifiers tend to give a higher recall of positives at the expense of lower precision. A na?ve Bayesian classifier, however, lies much further toward the extreme of high recall and low precision. Our results suggest that it will be possible to produce a reliable and quantitative assignment of membership or otherwise of each class of prohibited substances. This should aid the fight against the use of bioactive novel compounds as doping agents, while also protecting athletes against unjust disqualification.     

ChemInform Abstract: Experimental and Theoretical Observations of Aromaticity in Heterocyclic XAl3‐ (X: Si,Ge, Sn,Pb) Systems.

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