Liquid chromatographic chiral recognition of phytoalexins on immobilized polysaccharides chiral stationary phases. Unusual temperature behavior

Three immobilized polysaccharide chiral stationary phases, Chiralpak IA, Chiralpak IB and Chiralpak IC, were used for the study of enantioseparation of 36 derivatives of natural indole phytoalexins, in most cases bioactive, including racemic spirobrassinin, 1-methoxyspirobrassinin and 1-methoxyspirobrassinol methyl ether. Almost all analytes were baseline resolved at least on two different polysaccharide columns in normal phase mode. The effects of mobile phase composition, the analyte structure and the column temperature on the retention and enantioseparation were investigated. Evaluation of the corresponding thermodynamic parameters using vane Hoff plots (In k versus 1/T) in the temperature range -15 to 50 degrees C indicated that separations were enthalpy controlled in most cases, but some entropy controlled separations were also observed. Moreover, unusual phenomenon, an increase retention with increasing temperature accompanied with increased resolution was observed on the Chiralpak IC column. The elution order of enantiomers was determined in some cases and reversed elution order was also observed. (C) 2019 Elsevier B.V. All rights reserved.