Chiral Interactions and Sensing at
Liquid|Liquid Interface

Abstract

Chiral interactions of compounds with therapeutic interest and its study predicting and interpreting transport process across biological barriers represents one of the most important topics in research. This thesis is devoted to the study of chiral ion transfer at the interface between two immiscible electrolytes solutions (ITIES), as a promising
method of simplifying chiral detection and separation. As a proof of concept, for the study of chiral compounds at liquid|liquid interface, three different approaches were used: i) chiral stationary phases based on modified
cyclodextrins, AcαCD and AcβCD, ii) chiral acute phase protein, α1-acid-glycoprotein (AGP) and iii) thick film electrode modified with an ethylated cyclodextrin ferrocene
(EtCDFc). The chiral selectors used, AcαCD, AcβCD, AGP and EtCDFc display complex three-dimensional structures that are capable of recognising specifically the enantiomers
of a drug molecule, with different affinity. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to investigate the enantioselective interaction between the chiral molecules. In the study of chiral ion transfer using AcαCD and AcβCD, as a chiral organic phase, it was observed that the two lipophilic CDs facilitated the transfer of ephedrine ions by the formation of inclusion complexes. The enantioselectivity was achieved as the complexes between the protonated ephedrine ions and the CDs lead two different signal responses as a result of different affinities in the complex formation. Furthermore, the positive enantiomer (+)EPH+ showed to be consistently the cation being transferred at less positive potentials suggesting that it binds preferentially with the chiral selectors, in comparison with (-)EPH+, indicating that its transfer is more facile. The difference in stability constant between the (+)EPH+ and (-)EPH+ complexes was found to be 1.41±0.1 for AcβCD and 1.20±0.1 for AcαCD. When investigating the chiral interactions between the AGP and the three basic drugs (propranolol, lidocaine and procaine hydrochloride), it was found, that the plasma protein binds to the protonated drugs with clear different affinities. The formation of a complex between the drugs and AGP was shown as a decrease in the CV and DPV responses, corresponding to the reduction in the transfer of the cationic drugs, as only the unbound (free) drug was able to be transferred across the liquid|liquid interface. The bound and unbound drug concentration was estimated in a different range of concentrations based on the responses obtained in the presence and absence of the protein. The differences in current responses, observable in the measurements, lead to chiral discrimination between R- and S-propranolol. Scatchard analysis was employed to calculate the association constant and the number of binding sites of the drugs with AGP. The calculated association constants were 2.7x105 M-1 for S- and 1.3x105 M-1 for R-propranolol,
which were significantly higher than those for lidocaine, 1.2x104 M-1, and for procaine,8.4x103 M-1. This showed that AGP has more affinity for R- and S-propranolol than
lidocaine or procaine hydrochloride. A thick film modified electrode with a chiral redox probe, ethylated ferrocene cyclodextrin (EtCDFc) was used to study chiral ion transfer across the liquid|liquid interface coupled to a redox reaction. EtCDFc has a dual role, a redox active moiety and a cyclodextrin moiety which is able to form chiral complexes. Thus, the redox reaction of EtCdF was accompanied by the complexation of mandelic acid enantiomers with the cyclodextrin part of EtCDFc, with the two reactions mutually influencing each other. In addition the thick film ensured that the generated product of the molecular probe was within the diffusion layer and away from the aqueous|organic solvent interface, so that the charge neutrality of the organic film was only maintained by the presence of ions from the aqueous phase.