ß-Amino alcohol selectors for enantioselective separation of amino acids by ligand-exchange capillary zone electrophoresis in a low molecular weight organogel.

A new family of copper ligand-exchange selectors, L- or D-ß-amino alcohols, is employed for the chiral separation of D,L-dansyl-amino acids, unmodified amino acid racemates, phenylalanine and tryptophan, and ß-blocker L,D-propranolol by SDS-micellar electrokinetic chromatography and by electrophoretic chromatography in a low molecular weight organogel (LMOG)-filled capillary. The LMOG comprised a self-assembled fibrillar gel of trans-(1S,2S)-1,2-bis-(dodecylamido) cyclohexane in methanol. The di-L-valinol-copper complex exhibited the best performance on LMOG-CE compared with all other ß-amino alcohol-copper selectors. The dependence of chiral resolution on the pH*, the ratio between the copper and the L-valinol ligand and the concentration of added selector complex in the run buffer were investigated revealing a marked difference between the activity of the copper-valinol and the previously studied copper-valine selector. The optimal separation conditions were achieved using a 2:1 valinol/copper ratio, in accordance with the 2:1 structure of the complex, which was proven by single crystal and powder X-ray diffractions and by elemental analysis. Unlike the copper-valine selectors that could be used only under acidic conditions (pH* 3.5), the copper-valinol selectors could be used under near-neutral conditions and even at pH* 9.1. A comparison between SDS-micellar electrokinetic chromatography and LMOG-CE under otherwise identical conditions revealed a significant superior separation on the LMOG-filled capillaries.

Chiral separation of dansyl amino acids by ligand exchange capillary electrochromatography in a low molecular weight organogel.

Chiral electroseparation is demonstrated, for the first time, by a low molecular weight organogel filled capillary. Five pairs of dansylated amino acids were separated by copper ligand exchange on a trans-(1S,2S)-1,2-bis-(dodecylamido) cyclohexane (1) gel in methanol. Low molecular weight organogels are emerging materials that form stable, fibrillar, thermoreversible and thixotropic gels without covalent bonding of their monomeric building blocks. The dependence of chiral resolution and complex formation stability on the pH*, the ratio between copper and the D-valine selector, as well as other parameters were investigated revealing trends that were unparalleled in previously reports on copper ligand exchange of dansylated amino acids. These observations were explained in view of a simple stacking model of (1) and the difference in axial ligation of the amide carbonyl backbone of the gel to the dansyl D- or L-amino acid:D-valine:copper ternary complexes.

Nonstoichiometric gelation of cyclodextrins and included planar guests.

A generic family of low molecular weight binary gels comprising beta-cyclodextrin (beta-CD) and one of a large variety of polyaromatic hydrocarbons (PAHs) in dimethylformamide (DMF), pyridine, and other polar solvents is described. The system is rather general and robust. It tolerates large changes in each of the major ingredients without losing gelation ability. alpha- and gamma-CD, and negatively or positively modified beta-CD (e.g., sulfate-, phosphate-, or amine-tethered beta-CD) as well as methylated beta-CD are all effective gelators. The cogelators encompass a similarly large variety of compounds characterized by the ability to form an ovular inclusion complex with the CD molecules and a capability to stack outside the CD cap to give long-range order far from the CD cap. Despite the low ratio between the CD and the cogelators, we show that most of the CD molecules are retained in the liquid phase and do not participate directly in the actual construction of the gel network. In fact, most of the sulfated and phosphated beta-CDs can be cleaned off the gel structure by electrophoresis, leaving an intact gel porous structure. The nonstoichiometric nature of the gel is underscored by the fact that one molecule of beta-CD can combine with as few as three molecules of chrysene or as many as 450 molecules of chrysene to gelate an additional 35,000-40,000 molecules of the solvent.

A low molecular weight hydrogel which exhibits electroosmotic flow and its use as a bioreactor and for electrochromatography of neutral species.

A low molecular weight hydrogel which exhibits electroosmotic flow is described, and its use for separation and biocatalytic applications that require passage of a solvent stream through the gel is demonstrated.

Equilibrium distribution of polysulfide ions in aqueous solutions at different temperatures by rapid single phase derivatization.

Polysulfides are abundant form of reduced sulfur compounds whose distribution in aquatic systems continues to pose environmental challenge. The Gibbs free-energy of formation, enthalpy of formation, and standard entropy of inorganic polysulfides were derived based on measurements of the temperature-dependent distribution of inorganic polysulfides in supersaturated aqueous polysulfide solutions. The data complements the relevant Gibbs free-energy data that were derived in our recent publication. The thermodynamic data show that the average polysulfide length is increased and polysulfides dissolve better at elevated temperatures, though the extent of this increase is pH dependent. At high pH (pH > 10) increasing the temperature from 25 to 80 degrees C results in a 5.6% increase in the concentration of polysulfide bound sulfur (i.e., dissolved zerovalent sulfur) and increases the average chain length (n) by 0.2 sulfur atoms, whereas at pH 8.2 the n increases by 0.25, and the dissolved polysulfide sulfur increases threefold.

Donor-acceptor-promoted gelation of polyaromatic compounds.

Low molecular mass organogels are nonconventional polymeric structures in which a minute amount of low molecular weight compound can reversibly gelify the whole solution without forming covalent bonds between the monomers. In this article, we demonstrate that certain electron acceptors (taking dinitrobenzoates as model compounds) that are incapable of gelifying the solvent on their own can assemble as much as a 15-16-fold larger amount of polyaromatic hydrocarbons (PAHs) and form two-component donor-acceptor organogels in different solvents. At the core of the long-range order stand donor-acceptor pairs. We assess our claims by detailed 1H NMR, spectrophotometry, fluorescence, and time-resolved fluorescence methods. The thermodynamics of the gelation process is described on the basis of temperature dependent 1H NMR studies. We believe that, in this case, 1H NMR provides direct quantification of the dissolved concentrations of the different species and therefore provides a direct way to measure the enthalpy, entropy, and free energy associated with gel formation.

Equilibrium distribution of polysulfide ions in aqueous solutions at 25 degrees C: a new approach for the study of polysulfides' equilibria.

A new approach based on rapid, chemical derivatization in a single phase was used to determine the disproportionation constants and the underlying thermodynamics of inorganic polysulfides in aqueous solutions. This method resolves the dispute over the existence of hexasulfide in aqueous solutions and establishes the presence of even higher polysulfide chains in water. The Gibbs free energies of formation (G(Sn)(o)2-) for the polysulfide species are 77.4, 71.6, 67.4, 66.1, 67.2, 70.5, and 73.6 kJ/mol for n = 2-8, respectively. Our approach is based on single phase, fast methylation of polysulfides with methyl trifluoromethanesulfonate (methyl triflate) and subsequent determination of the dimethylpolysulfides by HPLC. Two independent methods were used in order to confirm quantitative equivalence between the observed distribution of dimethylpolysulfides and the polysulfide distribution in the water: (i) Kinetic studies of each competing reaction step showed that the kinetics of the derivatization are faster than each of the competing reactions that may lead to disproportionation and deviation of the observed distribution of dimethylpolysulfides from that of the aqueous polysulfides. (ii) Determination of isotope mixing during the derivatization of a mixture of two solutions, one containing polysulfide of natural isotopic distribution and the second containing 34S-rich polysulfide revealed that polysulfide mixing during derivatization is rather low. The systematic error due to redistribution of pentasulfide during derivatization is 3% based on isotope dilution tests and less than 5% of total zero-valent sulfur based on kinetic considerations.