2,5‐Dicarboxy­anilinium chloride monohydrate

The title compound, C₈H₈NO₄⁺·Cl⁻·H₂O, is the chloro­hydrated form of 2‐amino­benzene‐1,4‐dicarboxylic acid, the basic crystal structure of which is still not known. Mol­ecules are linked by classical N—H⋯O, O—H⋯O, N—H⋯Cl and O—H⋯Cl hydrogen bonds, mainly along the mol­ecular plane, into sheets built by unusual R₆⁴(26), R₆⁴(22) and R₄³(22) rings. The stacking between layers is stabilized by another N—H⋯Cl hydrogen bond and by π–π inter­actions between aromatic rings facing each other.     

Conformational effects on circular dichroism in the photoelectron angular distribution.

The B-spline density-functional method has been applied to the conformers of the (1R, 2R)-1,2-dibromo-1,2-dichloro-1,2-difluoroethane molecule. The cross section, asymmetry, and dichroic parameters relative to core and valence orbitals, which do not change their nature along the conformational curve, have been systematically studied. While the cross section and the asymmetry parameter are weakly affected, the dichroic parameter appears to be rather sensitive to the particular conformer of the molecule, suggesting that this dynamical property could be a useful tool for conformational analysis. The computational method has also been applied to methyl rotation in methyloxirane. Unexpected and dramatic sensitivity of the dichroic-parameter profile to the methyl rotation, both in the core and valence states, has been found. Boltzmann averaging over the conformers reproduces quite closely the profiles previously obtained for the minimum-energy conformation, which is in good agreement with the experimental results.     

Determination of nitrite, nitrate, and glucose-6-phosphate in muscle tissues and cured meat by IC/MS

The endogenous nitrate concentration in fresh meat and the residual nitrate and nitrite contents after curing are related to food quality and safety. Most ion chromatography (IC) methods suffer from interferences, especially in fresh meat samples, in which the endogenous nitrate content is low, and in cured meat products, in which other nitrogenous compounds can interfere with the separation of inorganic anions. One of the major classes of interfering compounds in fresh meat are sugar phosphates, which originate from glycolysis during the conversion of muscle glycogen to lactic acid. Nitrate can be separated from interfering compounds with a high-capacity anion-exchange column that was manufactured for use with hydroxide eluents (i.e., hydroxide-selective). This column has a different selectivity than traditional IC columns that use carbonate eluents and facilitates the determination of nitrate in both fresh and cured meats. Nitrate was detected by both suppressed conductivity measurement and mass spectrometry (MS). The identifications of nitrate and glucose-6-phosphate were confirmed by MS detection. The described IC/MS method is robust, sensitive to nitrate concentrations as low as 0.10 mg/kg, and can determine sugar phosphates that are useful for monitoring meat freshness. We successfully used this method to determine nitrate in nearly 100 muscle tissues and cured meat samples.     

Excitation energy transfer in ion pairs of polymethine cyanine dyes: efficiency and dynamics

The present work deals with singlet excitation energy transfer (EET) occurring in contact ion pairs (CIPs) of several anionic oxonol analogues (acting as EE donors) and cationic cyanines (acting as acceptors) characterized by off resonance individual transitions. Combining conductometric and spectroscopic measurements with decreasing solvent polarity, we were able to observe a progressive ion pairing leading first to solvent-separated ion pairs (SSIPs) and then to CIPs. Analysis of the absorption spectra of three selected salts (A2,C1, A2,C2, and A1,C4) in chloroform-toluene mixtures showed that the transformation of SSIP into CIP involves the appearance of a certain exciton coupling, the extent of which decreases regularly with increasing gap between the local excitation energies. Fluorescence excitation spectra showed that EET occurs in CIP, and EET efficiencies were evaluated with a procedure expressly devised for weakly emitting donors. These were between 0.2 and 0.65 for the examined ion pairs involving anions A1 and A2. The spectroscopic study was complemented by a theoretical investigation aimed at establishing the dynamic regime of the observed EET. From classical MD simulations and local full geometry optimizations, A2,C1 and A2,C2 were found to form rather stable sandwich-type CIP structures with interchromophore distances (R) of about 0.45-0.50 nm. The donor-acceptor electronic coupling was calculated in terms of Coulombic interactions between atomic transition charges. For CIP, the electronic coupling was decidedly beyond the limit of the weak coupling required for an incoherent F?rster-type mechanism. Thus, we tried to arrange the EET dynamics within the theory developed by Kimura, Kakitani, and Yamato (J. Phys. Chem. B 2000, 104, 9276) for the intermediate coupling case, which provides analytical expressions of time-dependent occupation probability, EET rate, and coherency in terms of two basic quantities: the electronic coupling and a correlation time related to the Franck-Condon factor. The latter was shown to be primarily modulated by F?rster's spectral overlap integral (related in turn to the excitation energy gap). Calculations were carried out for the three sample systems using three values of the electronic coupling roughly corresponding to CIP, 1.0, and 2.0 nm interchromophore distances. At the CIP distance, EET in both A2,C1 and A2,C2 was predicted to occur with a partial exciton mechanism, very short transfer times (about 10 fs), and high degree of coherence. In A1,C4 (having the largest energy gap), EET was found to occur with a hot-transfer mechanism. More or less hot-transfer dynamics appeared to be retained by all three systems at R = 1.0 nm. Fully incoherent EET appeared to become operative only at distances larger than 2.0 nm.     

Solvent-free,microwave-assisted synthesis of thiophene oligomers via Suzuki coupling

The purpose of this study was to obtain a rapid, efficient, and environmentally friendly methodology for the synthesis of highly pure thiophene oligomers. The solvent-free, microwave-assisted coupling of thienyl boronic acids and esters with thienyl bromides, using aluminum oxide as the solid support, allowed us to rapidly check the reaction trends on changing times, temperature, catalyst, and base and easily optimize the experimental conditions to obtain the targeted product in fair amounts. This procedure offers a novel, general, and very rapid route to the preparation of soluble thiophene oligomers. Thus, for example, quaterthiophene was obtained in 6 min by reaction of 2-bromo-2,2'-bithiophene with bis(pinacolato)diboron (isolated yield 65%), whereas quinquethiophene was obtained in 11 min by reaction of dibromoterthiophene with thienylboronic acid (isolated yield 74%). The synthesis of new chiral 2,2'-bithiophenes is reported. The detailed analysis of the byproducts of some reactions allowed us to elucidate a few aspects of reaction mechanisms. While the use of microwaves proved to be very convenient for the coupling between conventional thienyl moieties, the same was not true for the coupling of thienyl rings to thienyl-S,S-dioxide moieties. Indeed, in this case, the targeted product was obtained in low yields because of the competitive, accelerated, Diels-Alder reaction that affords a variety of condensation products.     

Regiocontrolled synthesis of enantiopure 3,3'-thiosubstituted biphenyls

Sulfenylation of 6,6'-dimethoxy-2,2'-dihydroxybiphenyl, used as a racemic mixture and single enantiomers, by phthalimidesulfenyl chloride afforded the corresponding 3,3'-N,N'-dithiophthalimide with complete regioselectivity. Simple manipulations of the latter compound allowed access to the corresponding bis-thiol or o-thioquinone as useful intermediates for the synthesis of new sulfur-containing open-chain and macrocyclic C(2) enantiopure ligands. The application of this methodology to the preparation of a biphenyl bearing two cysteine units as potential HIV-1 protease inhibitor is also described.     

A successful chemical strategy to induce oligothiophene self-assembly into fibers with tunable shape and function

Functional supramolecular architectures for bottom-up organic nano- and microtechnology are a high priority research topic. We discovered a new recognition algorithm, resulting from the combination of thioalkyl substituents and head-to-head regiochemistry of substitution, to induce the spontaneous self-assembly of sulfur overrich octathiophenes into supramolecular crystalline fibers combining high charge mobility and intense fluorescence. The fibers were grown on various types of surfaces either as superhelices or straight rods depending on molecular structure. Helical fibers directly grown on a field effect transistor displayed efficient charge mobility and intrinsic 'memory effect'. Despite the fact that the oligomers did not have chirality centers, one type of hand-helicity was always predominant in helical fibers, due to the interplay of molecular atropisomerism and supramolecular helicity induced by terminal substituents. Finally, we found that the new sulfur overrich oligothiophenes can easily be prepared in high yields through ultrasound and microwave assistance in green conditions.     

Supramolecular chirality transfer to large random aggregates of porphyrins

Chirality is rapidly induced in a fractal aggregate of the porphyrin t-CuPagg by addition of α-helical poly-glutamate. These results demonstrate a facile transfer of chirality via noncovalent interactions to preformed supramolecular assemblies grown in the absence of a chiral template.     

Interaction of cationic liposomes with cell membrane models

The colloidal dispersion stability of nano-sized graphene sheets in supercritical fluid (SCF) media is very important for developing SCF-based exfoliation and dispersion technologies for stabilization and solubilization of graphenes. We carried out molecular dynamics simulations to elucidate the stability mechanism of graphene in supercritical CO(2) (scCO(2)). The potential of mean force (PMF) between two graphene nanosheets in scCO(2) was simulated, and the effect of scCO(2) density and temperature on the PMF behavior has been investigated. The simulation results demonstrate that there exists a free energy barrier between graphenes in the scCO(2) fluid, possibly obstructing the aggregation of graphenes. The single-layer confined CO(2) molecules between the graphene sheets can induce a dominating repulsion interaction between graphene sheets. At higher scCO(2) fluid density, there are more confined CO(2) molecules within the interplate regions, resulting in a stronger repulsive free energy barrier. The effect of temperature on the PMF is relatively minor. The scCO(2) solvent structure shows layered confined arrangement in the interfacial region near the graphene nanosheets, which is correlated well with the PMF profile curve.