Unexpected Stereospecific Rearrangement-Addition Reaction of Trisubstituted Gibberellin Epoxides with Trimethylaluminium

The gibberellins (GAs) are diterpenoid plant growth hormones which are essential to the normal growth and development of plants1. Over one hundred GAs have been identified to date and most of these compounds are present in nature only in minute quantities. Based on their carbon skeleta, the known gibberellins can be classified into two major groups, i.e., C20-gibberellins (C20-GAs, e.g. GA12, GA53) which contain the full twenty diterpenoid carbon atoms and C19-gibberellins (C19-GAs, e.…     

Simple thinking using complex math vs. complex thinking using simple math—A study using model eliciting activities to compare students' abilities in standardized tests to their modelling abilities

Traditional mathematics assessments often fail to identify students who can powerfully and effectively apply mathematics to real-world problems, and many students who excel on traditional assessments often struggle to implement their mathematical knowledge in real-world settings (Lesh & Sriraman, 2005a). This study employs multi-tier design-based research methodologies to explore this phenomenon from a models and modeling perspective. At the researcher level, a Model Eliciting Activity MEA) was developed as a means to measure student performance on a complex real-world task. Student performance data on this activity and on traditional pre- and post-tests were collected from approximately 200 students enrolled in a second semester calculus course in the Science and Engineering department of the University of Southern Denmark during the winter of 2005. The researchers then used the student solutions to the MEA to develop tools for capturing and assessing the strengths and weaknesses of the mathematical models present in these solutions. Performance on the MEA, pre- and post-test were then analyzed both quantitatively and qualitatively to identify trends in the subgroups corresponding to those described by lesh and Sriraman.     

Di­chloro­[N,N,N′,N′-tetrakis(2-pyridyl­methyl)­benzene-1,4-di­amine]­iron(II)

In the title complex, [FeCl₂(C₃₀H₂₈N₆)], the Fe atom is five-coordinated by two terminal chloride ligands and one end of the bis-tridentate ligand. The complexes display intermolecular C—H⃛π, π-stacking and C—H⃛X (X = N, Cl) interactions.     

Perception of Pitch and Roughness in Vocal Signals with Subharmonics

Pitch and roughness were rated according to the extent of amplitude modulation (AM) and frequency modulation (FM) of a subharmonic [fundamental frequency (F0)/2]. The objective was to determine the identification boundaries for pitch and roughness and to discover how both kinds of modulation affect these boundaries. Another objective was to judge the reliability between subjects when identifying subharmonic-related pitch and roughness. Three procedures were used: ABX comparisons, method of adjustment, and rating of roughness. Results indicated that the crossover point to the lower pitch (associated with the subharmonic) occurred between 10% and 30% modulation, depending on modulation type and F0. Subjects demonstrated highly variable perceptions of pitch and roughness, with poor intersubject reliability.     

Activation by oxidation and ligand exchange in a molecular manganese vanadium oxide water oxidation catalyst

Despite their technological importance for water splitting, the reaction mechanisms of most water oxidation catalysts (WOCs) are poorly understood. This paper combines theoretical and experimental methods to reveal mechanistic insights into the reactivity of the highly active molecular manganese vanadium oxide WOC [Mn₄V₄O₁₇(OAc)₃]³⁻ in aqueous acetonitrile solutions. Using density functional theory together with electrochemistry and IR-spectroscopy, we propose a sequential three-step activation mechanism including a one-electron oxidation of the catalyst from [Mn₂³⁺Mn₂⁴⁺] to [Mn³⁺Mn₃⁴⁺], acetate-to-water ligand exchange, and a second one-electron oxidation from [Mn³⁺Mn₃⁴⁺] to [Mn₄⁴⁺]. Analysis of several plausible ligand exchange pathways shows that nucleophilic attack of water molecules along the Jahn–Teller axis of the Mn³⁺ centers leads to significantly lower activation barriers compared with attack at Mn⁴⁺ centers. Deprotonation of one water ligand by the leaving acetate group leads to the formation of the activated species [Mn₄V₄O₁₇(OAc)₂(H₂O)(OH)]⁻ featuring one H₂O and one OH ligand. Redox potentials based on the computed intermediates are in excellent agreement with electrochemical measurements at various solvent compositions. This intricate interplay between redox chemistry and ligand exchange controls the formation of the catalytically active species. These results provide key reactivity information essential to further study bio-inspired molecular WOCs and solid-state manganese oxide catalysts.

Combined theoretical and experimental studies shed light on the initial steps of redox-activation of a molecular manganese vanadium oxide water oxidation catalyst.     

Unambiguous NMR Structural Determination of (+)-Catechin—Laccase Dimeric Reaction Products as Potential Markers of Grape and Wine Oxidation

(+)-Catechin—laccase oxidation dimeric standards were hemi-synthesized using laccase from Trametes versicolor in a water-ethanol solution at pH 3.6. Eight fractions corresponding to eight potential oxidation dimeric products were detected. The fractions profiles were compared with profiles obtained with two other oxidoreductases: polyphenoloxidase extracted from grapes and laccase from Botrytis cinerea. The profiles were very similar, although some minor differences suggested possible dissimilarities in the reactivity of these enzymes. Five fractions were then isolated and analyzed by 1D and 2D NMR spectroscopy. The addition of traces of cadmium nitrate in the samples solubilized in acetone-d₆ led to fully resolved NMR signals of phenolic protons, allowing the unambiguous structural determination of six reaction products, one of the fractions containing two enantiomers. These products can further be used as oxidation markers to investigate their presence and evolution in wine during winemaking and wine ageing.