Mechanistic Aspects of Chiral Discrimination on Modified Cellulose

Abstract

Cellulose and cellulose derivatives are biopolymers which are often used as stationary phases for the separation of enantiomers. Describing the mechanism of such separations is a difficult task due to the complexity of these phases. In the present study, we attempt to elucidate the types of interactions occurring between a diol intermediate for a LTD(4) antagonist and a tris(4-methylbenzoate)-derivatized cellulose stationary phase. Thermodynamic studies indicate that, at low temperatures, the enantioselectivity is entropy driven. At higher temperatures, the separation is enthalpy driven. DSC and IR experiments reveal that the transitions between the enthalpic and the entropic regions of the van't Hoff plots are a result of a change in conformation of the stationary phase. Investigation of chromatographic kinetic parameters reveals that, at low temperature, the second eluted enantiomer undergoes sluggish inclusion interactions. Subtle changes in the structure of the analyte indicates that π-π interactions do not contribute to enantioselectivity. Finally, molecular modeling of (R)- and (S)-diol and the stationary phase suggests that hydrogen bonding is a primary factor in the separation, and the calculated energy values obtained from the molecular modeling correlate well with the chromatographic elution order.

Enantioselective Extraction of Racemic Ofloxacin by Di(2-Ethylhexyl)phosphoric Acid and Tartaric Acid Derivatives as Mixed Complex Chiral Selectors

“…But in fact, the effects of extraction resolution was impaired due to the increase of confusion degree of the whole extraction system caused by increasing temperature. The plots in Figure 6 are well in conformity with the Van't Hoff model, from another point of view, indicating that the complexes do not change in conformation 17,18 and that enantioselective interactions are unchanged in the temperature ranges studied 19 .…”

Thermodynamic and Extrathermodynamic Studies of Enantioseparation of Imidazolinone Herbicides on Chiralcel OJ Column

“…It showed thereby that the origin of the nonlinear van't Hoff behaviors may result from conformational changes of stationary phases, analyte, or their combinations. In the case of this study, more likely, the conformation change of the OJ phase caused the nonlinear van't Hoff plots under the low temperature [19].…”

“…The Δ(Δ ∘ ) and Δ(Δ ∘ ) of imazapic (2) were negative at 5 to 10 ∘ C. At 10 ∘ C, imazapyr (1) and imazaquin (5) had positive Δ(Δ ∘ ). Nonlinear van't Hoff behaviors have been observed in many studies, for example, enantioseparation of dihydropyrimidinone on Chiralcel AD column [28], and a diol compound on Chiralcel OJ column [19]. Since small analytes were used, the AD and OJ phases based on polysaccharide were concluded to undergo thermally induced conformational changes.…”

“…O'Brien et al [19] reported that Δ(Δ ∘ ), and Δ(Δ ∘ ) of a diol compound over a temperate range of 20 to 50 ∘ C increased with increasing 2-propanol content from 30% to 45%, which is consistent with our observations for imazapyr (1), imazapic (2), imazethapyr (3), and imazamox (4) over the temperature range of 15 to 50 ∘ C and the 2-propanol content range of 30 to 40%. Moreover, the dependences of Δ ∘ , Δ ∘ , Δ(Δ ∘ ) and Δ(Δ ∘ ) on 2-propanol content from 15 to 40% showed that there was a deviation at 30% 2-propanol content.…”

“…The five imidazolinone compounds are a homologous series that has different additive groups on imazapyr (1). Although there have been many studies on enantioseparation of OJ column, few studies have been addressed with the effect of temperature and thermodynamic characteristics [19][20][21]. The main objective of this study was to examine thermodynamic and extrathermodynamic characteristics of chromatographic behaviors for imidazolinone herbicides on the OJ column.…”