Oftentimes, a relatively large amount of S-(–)-α-lipoic acid is detected in dietary supplements claiming to contain R-(+)-α-lipoic acid on the bottle label. We found that chiral conversion was promoted by heating in the presence of glucose. As candidates for coexisting components, neutral nonionic and highly polar substances were suggested, such as sugars. The chiral conversion rate changed depending on sample purity, particularly the presence or absence of coexisting components in the dietary supplements. The α-lipoic acid enantiomers were reciprocally converted by heating at 180☌, and finally became a racemate. Samples were cleaned up by the solid-phase dispersive extraction method using Oasis MAX and MCX as the solid-phase gel. We elucidated factors for the chiral conversion of α-lipoic acid enantiomers in α-lipoic acid-containing dietary supplements. These results indicate that π interactions contribute to practical separation science, such as removal of environmental pollutants and quantitative determination of medicinal compounds. Briefly, we applied various π interactions to specific separation analyses, and we succeeded in separating halogenated compounds, H/D isotopologue pairs, and saccharides by effective π interactions. Furthermore, π interactions can contribute to the separation of various samples, which are difficult to achieve by the available retention mechanisms. In this focusing review, we introduce a few specific π interactions by columns modified with fullerenes and polycyclic aromatic hydrocarbons (PAHs), which showed strong π-π interactions due to spherical recognition and multiple CH-π interactions. We investigated the properties of π interactions by developing new silica-monolithic capillary columns modified with carbon materials providing strong π interactions. On the other hand, liquid chromatography (LC) is a powerful separation technique, which is able to distinguish the partition coefficients of solutes between the mobile and stationary phases, and can sensitively reflect the strength of molecular interactions. Despite the importance of π interactions, they are still challenging to study because π interactions are much weaker than most other molecular interactions, such as hydrophobic interaction, hydrogen bonding, and electrostatic bonding. Therefore, further deep understanding and control of π interactions will greatly facilitate the development of new functional materials. Π interactions have recently received considerable attentions due to principal factor governing molecular recognitions and self-assemble abilities, as accumulated in the database on proximate arrangements in structures of biological systems and organic functional materials. In this review, analysis and characterization methods utilizing reaction dynamics in a separation capillary are summarized. Michaelis-Menten constants have successfully been determined through the plateau height by CE/DFA. In CE/DFA, kinetically generated product is continuously resolved from the equilibrium species, and a plateau signal would be detected when the reaction rate is constant. ![]() A novel analysis technique of capillary electrophoresis/dynamic frontal analysis (CE/DFA) has also been proposed for the analysis of such reactions as involving equilibria and kinetic reactions. Since CE is operated in an open-tubular capillary, it is also suitable for the characterization of carbon nanoclusters such as graphene and carbon nanotube, and measurement of effective electrophoretic mobility helps characterization of nanoclusters. Characteristics of the CE analysis have been applied to analyses of acid-base equilibria of degradable substances and ion-association equilibria in an aqueous solution. Analysis of equilibria under CE separation possesses several advantages against traditional analyses in homogeneous solution coexisting substances including impurities and kinetically generated substances are resolved by CE from the equilibrium species of interest. Precise measurement of effective electrophoretic mobility allows analyzing the equilibrium. In affinity CE, an analyte of interest interacts with a modifier added in the separation buffer in fast equilibrium, and effective electrophoretic mobility of the analyte is contributed from its equilibrium species. Electrophoretic migration of an analyte in capillary electrophoresis (CE) reflects reaction dynamics of the analyte in solution.
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