Solvophobically Driven Complexation of Adamantyl Mannoside with β-Cyclodextrin in Water and Structured Organic Solvents

The effects of solvent and temperature on the complexation of adamantyl mannoside with β-cyclodextrin and 6-O-monotosyl-6-deoxy-β-cyclodextrin were explored experimentally and by means of molecular dynamics simulations. Efficient binding was observed only in hydrogen-bonded solvents, which indicated solvophobically driven complexation. The stability of the inclusion complex was considerably higher in aqueous media. A pronounced temperature dependence of Δ_rH^○ and Δ_rS^○, resulting in perfect enthalpy–entropy compensation, was observed in water. The complexation thermodynamics was in line with classical rationale for the hydrophobic effect at lower temperatures and the nonclassical explanation at higher temperatures. This finding linked cyclodextrin complexation thermodynamics with insights regarding the effect of temperature on the hydration water structure. The complexation enthalpies and entropies were weakly dependent on temperature in organic media. The signs of Δ_rH^○ and Δ_rS^○ were in accordance with the nonclassical hydrophobic (solvophobic) effect. The structures of the optimized product corresponded to those deduced spectroscopically, and the calculated and experimentally obtained values of Δ_rG^○ were in very good agreement. This investigation clearly demonstrated that solvophobically driven formation of cyclodextrin complexes could be anticipated in structured solvents in general. However, unlike in water, adamantane and the host cavity behaved solely as structure breakers in the organic media explored so far.     

Student interpretations of the terms in first-order ordinary differential equations in modelling contexts

A study of first-year undergraduate students′ interpretational difficulties with first-order ordinary differential equations (ODEs) in modelling contexts was conducted using a diagnostic quiz, exam questions and follow-up interviews. These investigations indicate that when thinking about such ODEs, many students muddle thinking about the function that gives the quantity to be determined and the equation for the quantity's rate of change, and at least some seem unaware of the need for unit consistency in the terms of an ODE. It appears that shifting from amount-type thinking to rates-of-change-type thinking is difficult for many students. Suggestions for pedagogical change based on our results are made.     

A new framework for implicit restarting of the Krylov–Schur algorithm

This paper introduces a new framework for implicit restarting of the Krylov–Schur algorithm. It is shown that restarting with arbitrary polynomial filter is possible by reassigning some of the eigenvalues of the Rayleigh quotient through a rank‐one correction, implemented using only the elementary transformations (translation and similarity) of the Krylov decomposition. This framework includes the implicitly restarted Arnoldi (IRA) algorithm and the Krylov–Schur algorithm with implicit harmonic restart as special cases. Further, it reveals that the IRA algorithm can be turned into an eigenvalue assignment method. Copyright © 2014 John Wiley & Sons, Ltd.     

Killing Magnetic Curves in Sol Space

We determine magnetic curves corresponding to the Killing magnetic fields in the 3-dimensional homogenous Riemannian space Sol₃.     

1′-Acetylferrocene amino acid esters and amides. A simple model for parallel β-helical peptides

In this study we present a synthesis and conformational analysis of 1′-acetylferrocene amino acid derivatives of type Ac–Fn–CO–AA–Y (Fn=ferrocene-1,1-diyl; AA=Gly, Ala or Val; Y=OMe or NHMe) as a simple model for parallel β-helical peptides. Derivatives with only one amino acid adopt a reduced number of total conformations and allow a more exact analysis of intramolecular hydrogen bonds (IHB) close to the ferrocene unit. Conformational analysis of these bioconjugates was performed by a combination of spectroscopic techniques (IR, NMR and CD) and corroborated by solution-phase DFT calculations. The investigation of ester conjugates 1–3 indicates the coexistence of non-bonded (an open forms) and hydrogen bonded NH_a group forming a 7-membered ring (γ-turn). The amide derivatives 4–6 with an additional NH_b hydrogen bond donor are mostly constituted of conformers with a 10-membered ring (β-turn) as a single IHB pattern or the β-turn accompanied by a 7-membered ring (γ-turn) containing NH_a group. The exchange of the amino acid side-chain does not significantly affect the conformational properties and IHB pattern of the studied conjugates 1–6.     

Synthesis of new amidino-substituted 2-aminothiophenoles: mild basic ring opening of benzothiazole

The efficient synthesis of new amidino, N-isopropylamidino, 2-imidazolinyl substituted benzothiazoles and 2-aminothiophenoles by the Pinner reaction is described. The novel ring opening of benzothiazole with ammonia and ethylenediamine was found, and a plausible reaction mechanism proposed. The ring opening with ethylenediamine is selective and applicable to compounds bearing hydrolytically and amonolytically unstable substituents. Different amidino-substituted 2-aminothiophenoles were isolated in zwitterionic and disulfide form and their structures were determined by X-ray crystal structure analysis.     

Substituent effects on carbon-13 chemical shifts in 2,6-disubstituted adamantanes

Carbon-13 NMR spectral data for a series of symmetrical 2,6-disubstituted adamantanes (O?, CH₂?, CH₃, OH, OCOCH₃) are presented. The substituent effects on ¹³C chemical shifts are additive, except for carbons 2 and 6 in 2,6-adamantanedione. The non-additivity of the substituent effect in the diketone can be interpreted in terms of a through-space interaction of the carbonyl π-electrons; the additivity in the other derivatives indicates that there is no appreciable interaction between the substituents.     

Carbon-13 chemical shifts of inverted carbon atoms

The inverted carbons in 2,4-methano-2,4-dehydroadamantane are shielded by 14.3 ppm relative to the corresponding carbons in 2,4-methanoadamantane. This was explained by the balance of the inverted carbon hybridization deshielding, the shielding effect of the cyclopropane rings formed, and the change in the influence of the neighbouring atoms.