Mechanically-Driven Vase–Kite Conformational Switch in Cavitand Cross-Linked Polyurethanes

The eligibility of tetraquinoxaline cavitands (QxCav) as molecular grippers relies on their unique conformational mobility between a closed (vase) and an open (kite) form, triggered in solution by conventional stimuli like pH, temperature and ion concentration. In the present paper, the mechanochemical conformational switching of ad hoc functionalized QxCav covalently embedded in an elastomeric polydimethylsiloxane and in a more rigid polyurethane matrix is investigated. The rigid polymer matrix is more effective in converting mechanical force into a conformational switch at the molecular level, provided that all four quinoxaline wings are covalently connected to the polymer.     

Analysis of the thermogenesis mechanism of natural rubber under high speed strain

This work highlights the relationship of crosslink density, entanglement points and various sulfide crosslinks with the thermogenesis properties of natural rubber (NR). The impact of cross‐link and entanglement on thermogenesis properties was evaluated by heat build‐up test, swelling behavior, statistical thermodynamic calculation, and classic viscoelastic theory. It was found that cross‐link and entanglement points have “pinning” effect to the rubber chain, thus remarkably restricting the motion of the rubber chain and reducing thermogenesis. Besides, the effects of various sulfide crosslinks and cross‐link length on thermogenesis were compared and discussed varying the sulfur vulcanization system. It was found that the semi‐effective vulcanization system using N‐cyclohexyl‐2‐benzothiazolesulfenamide (CZ) and 2‐Mercaptobenzothiazole (M) has the lowest thermogenesis (bottom temperature rise is 7.5°C, middle is 18.7°C), which on account of combined short crosslink length with high rigid rubber chain (crosslink network dominated by mono‐ and disulfides). As a result, the deformation degree of the rubber chain during curl up‐extension and the thermogenesis are further reduced. Finally, a combination of natural film coagulation and semi‐effective vulcanization system was used to prepare a low thermogenesis NR, in which bottom and middle temperature rise were only 5.0°C and 14.1°C, respectively.     

Electronic structure and reactivity of S—S dications

The electronic structure and reactivity of some S—S dications were studied at the MP2/6-31G* level of theory. The results obtained indicate a stepwise electrophilic addition of disulfonium dication moiety to the double C=C bond to be the preferable mechanism.     

Translocation of biomolecules through solid‐state nanopores: Theory meets experiments

The interest in polynucleotide translocation through nanopores has moved from purely biological to the need of realizing nanobiotechnological applications related to personalized genome sequencing. Polynucleotide translocation is a process in which biomolecules, like DNA or RNA, are electrophoretically driven through a narrow pore and their passage can be monitored by the change in the ionic current through the pore. Such a translocation process, which will be described here offers a very promising technology aiming at ultra‐fast low‐cost sequencing of DNA, though its realization is still confronted with challenges and drawbacks. In this review, we present the main aspects involved in the polynucleotide translocation through solid‐state nanopores by discussing the most relevant experimental, theoretical, and computational approaches and the way these can supplement each other. The discussion will expose the goals that have been reached so far, the open questions, and contains an outlook to the future of nanopore sequencing. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 985–1011, 2011     

Computational study on the conformational preferences in nateglinide

Ab initio and semi‐empirical calculations were performed on the monomers, dimers and tetramers of the antidiabetic drug nateglinide to understand the conformational preferences and to explore their possible relation with polymorphism. The reported crystal structure of bis(nateglinide) hydronium chloride shows one asymmetric unit consisting of four different conformations of the drug nateglinide. The Becke, three‐parameter, Lee–Yang–Parr /6‐31+G(d,p) optimizations indicate that these conformers are energetically quite comparable and the differences disappear in gas phase. Our analysis shows that Φ (phi) torsion angle of this phenylalanine derivative is responsible for the observed differences in stability among the nateglinide conformations. Four different polymorphs of nateglinide (B, H, S and X2) were reported but the structural differences are not available. This quantum chemical study on the dimers of nateglinide helps in proposing the structures of polymorphs. As per the quantum chemical analysis, the dimer N‐44 is the structure of the stable polymorph, whereas, the dimers N‐AA, N‐CC and N‐AC are almost isoenergetic, thus proposed to be the structures of metastable state. The dimerization and tetramerization energies are estimated to be about ?9.0 and ?38.67 kcal/mol, respectively. The extra stability in tetrameric state compared with the dimeric form is attributed to additional hydrophobic and van der Waals interactions. Copyright © 2011 John Wiley & Sons, Ltd.     

Conformational and structural studies of n‐propylamine from temperature dependent Raman and far infrared spectra of xenon solutions and ab initio calculations

The Raman and infrared spectra (4000 to 50 cm^–1) of the gas, liquid or solution, and solid have been recorded of n‐propylamine, CH₃CH₂CH₂NH₂. Variable temperature (−60 to −100 °C) studies of the Raman (1175 to 625 cm^–1) and far infrared (600 to 10 cm^–1) spectra dissolved in liquid xenon were carried out. From these data, the five possible conformers were identified and their relative stabilities obtained with enthalpy difference relative to trans–trans (Tt) for trans–gauche (Tg) of 79 ± 9 cm^–1 (0.9 ± 0.1 kJ/mol); for Gg of 91 ± 26 cm^–1 (1.08 ± 0.3 kJ/mol); for Gg′ of 135 ± 21 cm^–1 (1.61 ± 0.2 kJ/mol); for Gt of 143 ± 11 cm^–1 (1.71 ± 0.1 kJ/mol). The percentage of the five conformers is estimated to be 18% for the Tt, 24 ± 1% for Tg, 23 ± 3% for Gg, 18 ± 1% for Gg′ and 18 ± 1% for Gt at ambient temperature. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug‐cc‐pVTZ from both second‐order Møller–Plesset (MP2, full) and density functional theory calculations by the Becke, three‐parameter, Lee–Yang–Parr method. Vibrational assignments were provided for the observed bands for all five conformers, which are supported by MP2(full)/6‐31G(d) ab initio calculations to predict harmonic force constants, wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. Estimated r₀ structural parameters were obtained from adjusted MP2(full)/6‐311+G(d,p) calculations. The results are discussed and compared with the corresponding properties of some related molecules. Copyright © 2012 John Wiley & Sons, Ltd.     

Single long-polymer translocation through a long pore

This paper theoretically studies the free energy and conformational entropy of a long polymer threading a long nanopore (n₀/N \ge 0.1) on external electric field. The polymer expanded model is built in this paper, that is, a single long polymer chain with N monomers (each of size a) threading a pore with n₀ monomers can be regarded as polymer with N+n_{0} monomers translocating a 2-dimension hole embedded in membrane. A theoretical approach is presented which explicitly takes into account the nucleation theory. Our calculations imply that, the structure of polymer changes more acutely than other situation, while its leading monomer reaches the second vacuum and its end monomer escapes the first vacuum. And it is also shown that the length scale of polymer and pore play a very important role for polymer translocation dynamics. The present model predicts that the translocation time depends on the chemical potential gradient and the property of the solvent on sides of pore to some extent.     

Measurement of 1H-1H residual dipolar coupling constants for structural studies of medium size molecules.

Residual dipolar coupling constants (RDCs) are being increasingly applied to elucidate the configuration and conformation of small organic molecules, peptides and oligosaccharides. In this paper we describe a set of robust 1D NMR methods for accurate and precise measurement of proton-proton RDCs of small and medium size molecules. The performance of these techniques is not impeded by the presence of overlapping and broad (1)H multiplets that are typically observed for such molecules in weakly aligned media. The use of these techniques provides access to a large pool of proton-proton RDCs opening new avenues for the solution structure elucidation of medium size molecules by NMR. The techniques are illustrated on the determination of the alignment tensor of the reducing monosaccharide ring of cellobiose and the determination of the relative configuration of sodium cholate.