Abstract
We report the results of experimental work focusing on host-guest supramolecular complex creation between macrocyclic compound (β-cyclodextrin) and 1-acenaphthenol enantiomers (racemic mixture) in liquid phase composed of 35% acetonitrile in water (v/v) at different temperatures ranging from 0 to 90 °C. Experimental setup involved several analytical protocols based on classical non-forced flow planar chromatography (RP-18 TLC plates), micro-TLC (RP-18 W HPTLC plates), column chromatography (HPLC with C-18 and C-30 stationary phases), as well as UV-Vis spectrophotometry and optical microscopy. It has been found that under various planar chromatographic conditions (stationary plates type, chamber shape and volume, development mode, and saturation) non-typical retention properties (extremely high retention) of 1-acenaphthenol at subambient temperatures can be observed. To our knowledge, reported experimental results are in opposition to currently described retention models based on column chromatographic investigation of host-guest complexes (where in case of strong interaction of given analyte with macrocyclic mobile phases additive, which itself is non strongly retarded by stationary phase—close to the retention of dead volume marker, the retention of target compounds is shortened at low temperatures). To explain this TLC phenomenon that may have in our opinion a number of practical applications, especially for selective high throughput separation involving microchromatographic and/or microfluidic devices as well fractionation and extraction protocols (using, e.g., bar extraction systems), several experiments were conducted focusing on (i) acenaphthenol chromatography under different instrumental conditions, (ii) cyclodextrin retention measured as analyte or mobile phase additive, (iii) plate development time under different mobile phases and temperature settings, (iv) various column chromatographic conditions including C-30 and two C-18 stationary phases, (v) UV-Vis spectrophotometry, and (vi) microscopy inspection of precipitated CD-acenaphthenol crystals. Analysis of collected data has revealed that the most probable reasons for TLC retention behavior of 1-acenaphthenol under β-cyclodextrin additive conditions can be associated with (i) solubility changes of created host-guest complex, (ii) kinetics of solid complex precipitation, and (iii) differences in analysis time between planar and column chromatography. Because precipitation phenomenon may have a massive impact on analytes quantification involving macrocycles as the mobile phase additives, our previously reported data concerning a number of low-molecular compounds (mainly steroids and non steroidal endocrine disrupting chemicals) using HPLC methodology based on binary mobile phases without and with β-cyclodextrin and its hydroxypropyl derivative were re-examined and results discussed. Considering these data and the whole data set reported presently, the enhanced model of chromatographic retention driven by host-guest interaction was proposed.
Highlights
Cyclic oligosaccharides form a wide group of low-molecular mass donut-like-shaped compounds
We demonstrated that for given molecules (1-acenaphthenol enantiomers), there is an enormous difference between column and planar chromatographic retention
We presented a number of experimental data indicating that reversed-phase chromatographic systems, where cyclodextrin additive is weakly retarded by stationary phase, are extremely temperature sensitive, and allow massive retention and selectivity changes for given low-molecular mass compounds [41]
Summary
Cyclic oligosaccharides form a wide group of low-molecular mass donut-like-shaped compounds. (i) A HPLC pump (PU-980; JASCO, Tokyo, Japan) and a UV diode-array detector (MD-915 multiwavelength (i) Classical vertical chamber: 14 × 6 × 6 cm glass container (wall thickness 2 mm), which was placed in temperaturecontrolled water bath Within this chamber, TLC plate was positioned vertically in saturated chamber and chromatography was carried out using TLC Silica gel 60 RP18 F254S glass-based plates obtained from Merck (Darmstadt, Germany). After 15 min of temperature equilibration, mobile phase (0.5 mL, approximately) was injected and chromatographic process was conducted within unsaturated chamber until mobile phase front reached the opposite edge of microplate (developing distance 45 mm) Chromatographic experiments involving this chamber were based on glass HPTLC RP-18 WF254S plates purchased from Merck (Darmstadt, Germany). Light scattering from supramolecular crystals was induced by visible light laser beam (green light 532 ± 10 nm,
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