Helical- and ahelical-dependent chiral recognition mechanisms in capillary electrophoresis using amylose as the selector

The present study discovered that helical structures of amylose were not always responsible for its chiral recognition abilities in CE. Several enantiomers with different structures were selected as models. Based on ultraviolet-visible spectroscopy and C-13 NMR measurements, it was found that helical structures were gradually destroyed by temperature elevation and almost entirely transformed to extended ahelical structures above 60 degrees C. Then, CE and H-1 NMR chiral recognitions were investigated at different temperatures; chiral selectivity of the enantiomers varied in two different ways. Summarily, helical structures were necessary only for chiral separations of the enantiomers with small (< 0.78 nm) and flexible molecular structures. However, for the gauche enantiomers (> 0.78 nm) with high steric hindrances over their chiral centers, ahelical structures alone can realize chiral recognitions. By using iodine as a helix including competitor, it was further proved that helical structures functioned through the inclusive complexations only in the chiral separations of small enantiomers and had no effect for the others. The underlying mechanisms of the functions of helical and ahelical structures in molecular level were discussed as well.