Computational NMR spectroscopy of organoarsenicals and the natural polyarsenic compound arsenicin A

The (1)H and (13)C NMR chemical shifts and coupling constants of a series of organoarsenic compounds were calculated with DFT methods and compared with available experimental spectra. We show that non-relativistic methods successfully model the NMR spectra of these molecules; relativistic spin-orbit effects are small but appreciable for (13)C shifts, and their inclusion is beneficial. Application of the same methods of calculation to the intriguing natural polyarsenic compound arsenicin A allowed several viable alternative structures to be ruled out and thereby confirmed the previously suggested adamantane-like structure of arsenicin A. These results not only reinforce the known predictive power of DFT NMR calculations, but also open the way for the investigation of other naturally occurring molecules with unusual structures outside the scope of empirical methods.     

Concentration as the switch for chiral recognition in biomembrane models

Chiral recognition was observed in a biomembrane model. Micellar aggregates formed by enantiopure N-alkyl-N,N-dimethyl-N-(1-phenyl)ethylammonium bromide were in fact able to convert the racemic mixture of bilirubin-IXalpha into an enantiomerically enriched mixture. The stereochemical preference and the extent of enantiomeric enrichment depend on the length of the hydrophobic portion of the surfactant and on the concentration conditions, and changes in the stereochemical bias are reversible.     

Role of the hydrophobic effect in the transfer of chirality from molecules to complex systems: from chiral surfactants to porphyrin/surfactant aggregates

The interaction between the achiral sulfonated porphyrin 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin, H 2TPPS 4 (4-), and two chiral cationic surfactants has been studied by optical absorption, fluorescence, and circular dichroism (CD) spectroscopies. At surfactant concentrations above the critical micellar concentration (cmc) the porphyrin is included in the micellar aggregates, but it is CD silent. Below the cmc at a definite porphyrin/surfactant stoichiometry the formation of heteroaggregates with transfer of chirality to the porphyrin chromophore occurs. The preferred surfactant/porphyrin stoichiometry is 3:1, which suggests a structure driven by electrostatic and hydrophobic interactions between porphyrin and surfactant and dipolar and ionic interactions with the water solution. At surfactant concentrations above the cmc, depending on the protocol of preparation of the samples, the formation of the two kinds of aggregates can be observed, reversible for the simple surfactant micelles incorporating the porphyrin, but irreversible for the heteroaggregates.     

Decomposition of formamide assisted by microwaves, a tool for synthesis of nitrogen-containing heterocycles

Generation of ammonia via thermal decomposition of formamide was studied under microwave conditions to provide an efficient tool for the synthesis of nitrogen-containing heterocycles. Quinazolin-4-ones, which are known as building blocks for molecules with pharmaceutical interest, were chosen as examples. Our work confirms that reactants may have different behaviours under microwaves, depending on power input, reached temperature and pressure in the vials. Full control and fine tuning of these parameters are achievable using modern microwave technology.     

Chiral recognition of dipeptides in Langmuir monolayers

The recognition of the enantiomeric couples of ditryptophan in Langmuir films of N-hexadecanoyl-l-proline was investigated by surface pressure–area (π–A) isotherm measurements and Brewster angle microscopy experiments. The π–A isotherms relative to the films including the enantiomeric dipeptides show small differences whereas an evident enantiodiscrimination is observed by Brewster angle microscopy images.     

Compression elastic modulus of neutral,ionic, and amphoteric hydrogels based on N‐vinylimidazole

Several hydrogels of N‐vinylimidazole and sodium styrenesulfonate have been prepared by radical cross‐linking copolymerization in aqueous solution, using N,N′‐methylene‐bisacrylamide as crosslinker. Depending on composition, these hydrogels were neutral, amphoteric, cationic or anionic. Compression‐strain measurements were performed on samples as‐synthesized and swollen in deionized water or in acid aqueous solutions, with and without salt. It was thus found that the cross‐linking densities determined by compression measurements on as‐synthesized samples are in good accordance with those calculated by means of the model of polymer networks with pendant vinyl groups. A non‐Gaussian parameter (β) was introduced to explain that the elastic moduli (G) of samples swollen at equilibrium are larger than predicted by the Gaussian model. The β values of the neutral or ionized systems increase with swelling and fall into a single curve, which denotes a common behavior. Swelling has two opposite effects on G; on the one hand G decreases because the polymer volume fraction diminish and the system shifts from the affine limit to the phantom one; on the other, β increases and contributes to increasing G. The balance of those two opposite effects determines the variation of G with swelling. The possible contribution of ionic crosslinks to ν_e for the polyampholyte and for the polycation wearing divalent counteranions was discussed. A peculiar system is poly(sodium styrenesulfonate), whose cross‐linking density is much lower than expected. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1078–1087, 2009     

On the M‐essential spectra of two‐group transport equations

In this paper, we deal with the M‐essential spectra of unbounded linear operators in Banach spaces where some generalizations of earlier work are given. Furthermore, we give an application from transport theory. Copyright © 2013 John Wiley & Sons, Ltd.