Dan Rizkov

β-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‐β‐amino alcohols, is employed for the chiral separation of D,L‐dansyl‐amino acids, unmodified amino acid racemates, phenylalanine and tryptophan, and β‐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 β‐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.

Selected Contemporary Topics in Sol-Gel Electro-Chemistry

This review provides an overview of some exciting, new as well as somewhat older, directions in sol-gel electrochemical applications of silicates and composite silicate electrodes. Rather than provide an exhaustive account of all the many papers that have been published on sol-gel electrochemistry and composite sol-gel electrodes, we prefer to illustrate the versatility of sol-gel chemistry by a few select examples which on the one hand illustrate the power entailed in sol-gel technology for electrochemical applications, and on the other hand point to hot electrochemical fields in which more research is due and exciting developments are to be expected. We start this review with a brief historical perspective. The inorganic sol-gel and silicone electrochemistry fields are both rather old though never extensively dealt with particularly when it comes to sensing applications. In contrast, the sol-gel electrochemistry of inorganic-organic hybrids is a relatively young field whose importance was recognized only in the last 30 years, and despite, or maybe even owing to the late recognition it is being very extensively studied nowadays. The use of composite electrodes for sensing and other applications is emphasized in this review, and the fast evolving electrodriven deposition techniques are reviewed.