Generalization of ionic partition diagrams to lipophilic compounds and to biphasic systems with variable phase volume ratios

Electrochemical screening of selected β-blockers at a polarized liquid-liquid interface

This paper describes the electrochemical behavior of five β-blockers at the polarized liquid-liquid interface formed between aqueous solution (sodium chloride solution or Britton-Robinson buffers) and bis(triphenylphosphoranylidene)ammonium tetrakis(4-chlorophenyl)borate (BTPPATPBCl) dissolved in 1,2-dichloroethane (the organic phase). All measurements reported in this work were conducted using cyclic voltammetry (CV). The effects of the concentration of analytes, the pH of the aqueous phase, and applied electrochemical parameters on the analytical performance of the studied system are studied and discussed. The linear dynamic ranges (LDRs) of the studied β-blockers were in the range of 5-200 μmol L-1 and the lowest limit of detection (LOD) value was determined for pindolol (LOD = 1.96 μM μmol L-1). The highest LOD value was 4.96 μmol L-1 found for nebivolol. In addition, physicochemical parameters such as the formal Galvani potential difference (Δaqorgϕ), formal Gibbs free energies of the ion transfer reaction (ΔaqorgG') and partition coefficients (log P'aq/org) for all studied molecules were determined. The latter were compared and correlated with the available literature values of log Poctanol. Finally, a standard addition method was used to determine the concentration of nebivolol in pharmaceutical preparations using a platform based on the electrified liquid-liquid interface.

Spectroelectrochemical Analysis of Ion Transfer Mechanisms of Mitoxantrone at Liquid|Liquid Interfaces: Effects of Zwitterionic Dendrimer and Phospholipid Layer

The ionic partition property and transfer mechanism of the anthraquinone antitumor agent mitoxantrone (MTX) were studied in detail at the water|1,2-dichloroethane (DCE) interface by means of surface-sensitive spectroelectrochemical techniques. The interfacial mechanism of the cationic MTX species was composed of potential-driven ion transfer and adsorption processes. The ion association between MTX and zwitterionic polyamidoamine (PAMAM) dendrimers with peripheral carboxy groups was also investigated in terms of the effects of pH and dendritic generation. The monovalent HMTX+ interacted effectively with the negatively charged dendrimers at neutral pH, while the divalent H2MTX2+ exhibited a weak association under acidic conditions. The higher stability of the dendrimer-MTX associates in the interfacial region was found for higher dendritic generations: G3.5 ≥ G2.5 > G1.5. The interfacial behavior of MTX and its dendrimer associates was further analyzed at the phospholipid-modified interface as a model biomembrane surface. The adsorption process of HMTX+ occurred mainly on the hydrophilic side of the phospholipid layer. The spectroelectrochemical results indicated that the dendrimers penetrate into the phospholipid layer and alter the transfer mechanism of HMTX+ across the interface.

Cellular uptake of rose bengal is mediated by OATP1B1/1B3 transporters

Due to its fluorescent properties and high yield of singlet oxygen, rose bengal (RB) is one of the most promising photosensitizers for cancer treatment. However, the negative charge of RB molecule may significantly hamper its intracellular delivery by passive diffusion through the cell membrane. Thus, specific membrane protein transporters may be needed. The organic anion transporting polypeptides (OATPs) are a well-characterized group of membrane protein transporters, responsible for cellular uptake of a number of drugs. To our knowledge, this is the first study that evaluates cellular transport of RB mediated by the OATP transporter family. First, electrified liquid-liquid interface, together with biophysical analysis and molecular dynamics simulations were used to characterize the interaction of RB with several models of a cellular membranes. These experiments proved that RB interacts only with the membrane's surface, without spontaneously crossing the lipid bilayer. Evaluation of intracellular uptake of RB by flow cytometry and confocal microscopy showed significant differences in uptake between liver and intestinal cell line models differing in expression of OATP transporters. The use of specific pharmacological inhibitors of OATPs, together with Western blotting and in silico analysis, indicated that OATPs are crucial for cellular uptake of RB.

Electrocatalysis of molecular oxygen reduction reaction at liquid-liquid interface and DFT computational study of proton transfer from the conjugate acid of 2,2'-dipyridylamineto oxygen

In this study, the catalytic effect of 2,2'-dipyridylamine (DPA) on the reduction of oxygen (O₂) at the polarized water/1,2-dichloroethane (DCE) interface was investigated. Ferrocene (Fc) and tetrathiafulvalene (TTF) were weak electron donors used in this study. Slow reduction of O₂ at the interface containing Fc and TTF was significantly accelerated upon the addition of DPA. Voltammetry and biphasic shake flask experiments revealed that DPA acts as a proton ionophore to transfer protons between the aqueous and organic phases. The PA, GB, and pK_a values of all possible conjugate acids of DPA were calculated. Then, a mechanism was suggested to explain the interaction between protonated DPA and oxygen molecular. The mechanism was computationally analyzed by using density functional theory (DFT). Furthermore, DFT calculations at the B3LYP/6-31G** level of theory showed that the conjugate acid species of DPA transfer proton to O₂ at the interface. The results show that DPA-H²⁺ and DPA-H¹⁺ are the best species to transfer proton to molecular oxygen.

Electrochemically Modulated Liquid-Liquid Extraction for Sample Enrichment

A new sample preparation method is proposed for the extraction of pharmaceutical compounds (Metformin, Phenyl biguanide, and Phenformin) of varied hydrophilicity, dissolved in an aqueous sample. When in contact with an organic phase, an interfacial potential is imposed by the presence of an ion, tetramethylammonium (TMA⁺), common to each phase. The interfacial potential difference drives the transfer of ionic analytes across the interface and allows it to reach up to nearly 100% extraction efficiency and a 60-fold enrichment factor in optimized extraction conditions as determined by HPLC analysis.

Is the Oil | Water Interface the Simplest and Best Suited Model for Understanding Biomembranes?

Many studies have been conducted by using the oil (O) | water (W) interface as a simple model for understanding ion transfer (IT) or electron transfer (ET) across biomembranes. In this review, we revisit the usability of the O | W interface as a biomembrane model. For understanding biomembrane IT, the O | W interface is the simplest and best suited model. For example, the standard Gibbs transfer energy of drug ions at the O | W interface is a useful measure for evaluating their membrane permeability in a conventional in vitro assay, called PAMPA. However, the O | W interface is not necessarily a good model for understanding biomembrane ET. This is because no net current can be observed with the O | W interface, owing to the ET-coupled proton transfer. In such a case, the self-assembled monolayer (SAM) formed on a metal electrode serves as a better model for understanding biomembrane ET.

Evaluation of the artificial membrane permeability of drugs by digital simulation

A digital simulation method has been developed for evaluating the membrane permeability of drugs in the parallel artificial membrane permeation assay (PAMPA). The simulation results have shown that the permeability coefficient (log Ppampa) of drugs is linearly increased with increasing their distribution coefficient (log KD,M) to the lipid membrane, i.e., the hydrophobicity of the drug molecules. However, log Ppampa shows signs of leveling off for highly hydrophobic drugs. Such a dependence of log Ppampa is in harmony with the reported experimental data, and has been well explained in terms of the change in the rate-determining step from the diffusion in the membrane to that in the unstirred water layer (UWL) on both sides of the membrane. Additionally, the effects of several factors, including lag time, diffusion coefficient, pH, and pKa, on the permeability coefficient have been well simulated. It has thus been suggested that the proposed method should be promising for in silico evaluation of the membrane permeability of drugs.

Simple Ion Transfer at Liquid|Liquid Interfaces

The main aspects related to the charge transfer reactions occurring at the interface between two immiscible electrolyte solutions (ITIES) are described. The particular topics to be discussed involve simple ion transfer. Focus is given on theoretical approaches, numerical simulations, and experimental methodologies. Concerning the theoretical procedures, different computational simulations related to simple ion transfer are reviewed. The main conclusions drawn from the most accepted models are described and analyzed in regard to their relevance for explaining different aspects of ion transfer. We describe numerical simulations implementing different approaches for solving the differential equations associated with the mass transport and charge transfer. These numerical simulations are correlated with selected experimental results; their usefulness in designing new experiments is summarized. Finally, many practical applications can be envisaged regarding the determination of physicochemical properties, electroanalysis, drug lipophilicity, and phase-transfer catalysis.

Chemodiversity and molecular plasticity: recognition processes as explored by property spaces

In the last few years, a need to account for molecular flexibility in drug-design methodologies has emerged, even if the dynamic behavior of molecular properties is seldom made explicit. For a flexible molecule, it is indeed possible to compute different values for a given conformation-dependent property and the ensemble of such values defines a property space that can be used to describe its molecular variability; a most representative case is the lipophilicity space. In this review, a number of applications of lipophilicity space and other property spaces are presented, showing that this concept can be fruitfully exploited: to investigate the constraints exerted by media of different levels of structural organization, to examine processes of molecular recognition and binding at an atomic level, to derive informative descriptors to be included in quantitative structure--activity relationships and to analyze protein simulations extracting the relevant information. Much molecular information is neglected in the descriptors used by medicinal chemists, while the concept of property space can fill this gap by accounting for the often-disregarded dynamic behavior of both small ligands and biomacromolecules. Property space also introduces some innovative concepts such as molecular sensitivity and plasticity, which appear best suited to explore the ability of a molecule to adapt itself to the environment variously modulating its property and conformational profiles. Globally, such concepts can enhance our understanding of biological phenomena providing fruitful descriptors in drug-design and pharmaceutical sciences.

Encapsulation of anilinonaphthalenesulfonates in carboxylate-terminated PAMAM dendrimer at the polarized water|1,2-dichloroethane interface

Molecular encapsulation of water-soluble anionic fluorescent dye molecules, 8-anilino-1-naphthalenesulfonic acid (ANS), and its bimolecular derivative (bis-ANS), in the generation 3.5 polyamidoamine (G3.5 PAMAM) dendrimer was investigated in the bulk aqueous phase and at the polarized water|1,2-dichloroethane interface. ANS(-) was electrostatically incorporated in the dendrimer, and the fluorescence enhancement with a blue shift of the emission maximum was observed at pH values <6, where the interior of the dendrimer was positively charged. The fluorescence enhancement of ANS was maximized around pH 3 and then decreased under more acidic conditions. The potential dependences of the molecular encapsulation and the interfacial mechanism were studied in detail by means of potential modulated fluorescence (PMF) spectroscopy. Under acidic conditions, the dendrimer incorporated ANS(-) at the positively polarized interface as well as in the aqueous phase. ANS(-) was released from the dendrimer at the intrinsic transfer potential and independently transferred across the interface. Bis-ANS exhibited relatively strong interaction with the dendrimer over a wide pH range (1 < pH < 8), and a negative shift of the transfer potential was observed under the corresponding pH condition. The PMF analysis clearly demonstrated that the interfacial mechanism of the dendrimer involves transfer and adsorption processes depending on the pH condition and the Galvani potential difference.

Electrochemical study of a dendritic family at the water/1,2-dichloroethane interface

The transfer of six dendritric molecules, DMs, across the water/1,2-dichloroethane interface was investigated using cyclic voltammetry. From the variation of peak potential with pH, two different mechanisms of transfer were postulated depending on the nature of the molecules. Voltammetric parameters were employed to evaluate the hydrophilic/hydrophobic character and calculate the acid dissociation constant of these molecules. The results were explained taking into account the nature and multiplicity of functional surface groups.

Immobilized pH Gradient Gel Cell To Study the pH Dependence of Drug Lipophilicity

An experimental setup to study the pH dependence of standard ion-transfer potentials at the water/NPOE interface is presented. The system is composed of a microhole generated by laser photoablation in a 12-microm polyethylene terephthalate film, the aqueous phase consisting of a commercial immobilized pH gradient gel reswelled in electrolyte solution and a droplet of organic solution. Two electrodes are used, an Ag/AgCl aqueous electrode and an Ag/AgTPBCl organic electrode. This setup is applied to the study of two ionizable compounds (pyridine, 2,4-dinitrophenol). Thermodynamic parameters such as the standard transfer potential, the Gibbs energy of transfer, and the partition coefficients of the ionized forms as well as the neutral forms of these drug compounds are evaluated by differential pulse voltammetry. The data obtained are summarized in ionic partition diagrams, which are a useful tool for predicting and interpreting the transfer behavior of ionized drugs at the liquid/liquid interfaces mimicking the biological membranes.

Electrochemical Study of Ion Transfer of Acetylcholine Across the Interface of Water and a Lipid-Modified 1,2-Dichloroethane

The ion transfer of acetylcholine (AcH(+)) ions across the unmodified and phospholipid-modified water|1,2-dichloroethane (DCE) interface has been studied by means of square-wave and cyclic voltammetry, as well as by electrochemical impedance spectroscopy. After being transferred in the organic phase, the AcH(+) ions undergo chemical reactions with the phospholipids. The overall behavior of the experimental system studied in the presence of phospholipids has been compared with the theoretical results of an ECrev reaction. The kinetic parameters of the chemical interactions between AcH(+) and the phospholipids have been determined from the voltammetric and impedance measurements. Additional characterization of those interactions has been made by using the surface tension measurements.

Numerical simulation of two-phase partition chromatography in microchannels for moderated logP measurements

A finite element simulation has been used in order to study the partition chromatography process of one species between an aqueous mobile phase and an organic stationary phase located at the bottom of a rectangular microchannel. The transient model incorporates convection--diffusion of the species in the water phase coupled to the diffusion in the stationary organic phase by the way of the partition kinetics at the interface. The time evolution of the injected species concentration is analyzed versus the velocity of the mobile phase, the detecting position and the thickness of the stationary phase. The comparison of simulation results with both experimental data and analytical model confirm its validity. These simulations show that thin channels can be used to measure log P of molecules from their retention time. Finally, we have shown how the sample velocity can be optimized for a given geometry of the channel and diffusion coefficient of the species.