Interfacial Complexes between a Protein and Lipophilic Ions at an Oil−Water Interface

Interfacial behaviour and quantitative analysis of hexadecyl phosphocholine drug at a polarized liquid/liquid interface

The interfacial behaviour of the amphiphilic drug hexadecyl phosphocholine (HePC, also called miltefosine) was analysed by cyclic voltammetry applied at the water/1,2-dichloroethane interface. HePC is the only oral drug currently approved for the treatment of visceral, mucosal  and cutaneous leishmaniasis. Because of its amphiphilic character, it can interact with biological membranes, solubilizing their compounds and leading to cell disruption. These interactions are responsible for its side effects and toxicity; therefore, HePC quantification in biological fluids and pharmaceutical preparations is extremely important. However, the lack of a chromophore in its structure prevents its spectroscopic determination. For this reason, the main challenge of this work was to propose an electroanalytical method for the quantification of this drug, which constitutes a simpler alternative than liquid chromatography-tandem mass spectrometry already reported. With this aim, in the first part of this work, the mechanism of the electrochemical process occurring after polarizing the interface was studied. By varying the experimental conditions, it was possible to determine that in a first step, at open circuit or at low potential values, HePC spontaneously adsorbed to the interface. Later, as the potential increased, the transfer of the anions present in the organic phase towards the aqueous side of the interface, where the HePC polar head groups were present, occurred thus forming adsorbed "ion pairs" and producing an increase in positive current. Subsequently, in the negative sweep, the "ion pairs" dissociated and desorbed giving rise to a negative peak. In this way, both negative and positive currents were considered useful for quantitative purposes. In the second part of this work, an appropriate experimental procedure was designed and proposed as a quantitative methodology for the HePC determination, which consisted of cleaning the interface and controlling the time at open circuit, followed by the voltammetric analysis. A linear response of both, positive or negative, peak currents with drug concentration was obtained within an acceptable range, providing a simple solution for the HePC quantification problem. Future studies will be carried out to evaluate the quantification and selectivity in real matrices containing polymer micelles working as HePC nanocarriers with the aim of avoiding the adverse effects of HePC when it is orally or intravenously administered.

Electrogeneration of a Free-Standing Cytochrome c-Silica Matrix at a Soft Electrified Interface

Interactions of a protein with a solid-liquid or a liquid-liquid interface may destabilize its conformation and hence result in a loss of biological activity. We propose here a method for the immobilization of proteins at an electrified liquid-liquid interface. Cytochrome c (Cyt c) is encapsulated in a silica matrix through an electrochemical process at an electrified liquid-liquid interface. Silica condensation is triggered by the interfacial transfer of cationic surfactant, cetyltrimethylammonium, at the lower end of the interfacial potential window. Cyt c is then adsorbed on the previously electrodeposited silica layer, when the interfacial potential, Δowϕ, is at the positive end of the potential window. By cycling of the potential window back and forth, silica electrodeposition and Cyt c adsorption occur sequentially as demonstrated by in situ UV-vis absorbance spectroscopy. After collection from the liquid-liquid interface, the Cyt c-silica matrix is characterized ex situ by UV-vis diffuse reflectance spectroscopy, confocal Raman microscopy, and fluorescence microscopy, showing that the protein maintained its tertiary structure during the encapsulation process. The absence of denaturation is further confirmed in situ by the absence of electrocatalytic activity toward O2 (observed in the case of Cyt c denaturation). This method of protein encapsulation may be used for other proteins (e.g., Fe-S cluster oxidoreductases, copper-containing reductases, pyrroloquinoline quinone-containing enzymes, or flavoproteins) in the development of biphasic bioelectrosynthesis or bioelectrocatalysis applications.

Characterization of Protein-Facilitated Ion-Transfer Mechanism at a Polarized Aqueous/Organic Interface

Protein electrochemistry studies at a polarized interface between two immiscible electrolyte solutions (ITIES) indicate that the detection mechanism of a protein at the interface involves a combination of protein-anion complexation and interfacial adsorption processes. A detailed characterization of the protein-facilitated mechanism of ion transfer at the ITIES will allow the development of new label-free biomolecular detection tools. Molecular dynamics simulations were performed to describe the mechanism of transfer of the hydrophobic anion tetraphenylborate (TPB^-) from a 1,2-dichloroethane (organic) phase to an aqueous phase mediated by lysozyme as a model protein under the action of an external electric field. The anion migrated to the protein at the interface and formed multiple contacts. The side chains of positively charged Lys and Arg residues formed electrostatic interactions with the anion. Nonpolar residues like Trp, Met, and Val formed hydrophobic contacts with the anion as it moved along the protein surface. During this process, lysozyme adopted multiple, partially unfolded conformations at the interface, all involving various anion-protein complexes with small free-energy barriers between them. The general mechanism of protein-facilitated ion transfer at a polarized liquid-liquid interface thus likely involves the movement of a hydrophobic anion along the protein surface through a combination of electrostatic and hydrophobic interactions.

Secondary Structural Changes in Proteins as a Result of Electroadsorption at Aqueous-Organogel Interfaces

The electroadsorption of proteins at aqueous-organic interfaces offers the possibility to examine protein structural rearrangements upon interaction with lipophilic phases, without modifying the bulk protein or relying on a solid support. The aqueous-organic interface has already provided a simple means of electrochemical protein detection, often involving adsorption and ion complexation; however, little is yet known about the protein structure at these electrified interfaces. This work focuses on the interaction between proteins and an electrified aqueous-organic interface via controlled protein electroadsorption. Four proteins known to be electroactive at such interfaces were studied: lysozyme, myoglobin, cytochrome c, and hemoglobin. Following controlled protein electroadsorption onto the interface, ex situ structural characterization of the proteins by FTIR spectroscopy was undertaken, focusing on secondary structural traits within the amide I band. The structural variations observed included unfolding to form aggregated antiparallel β-sheets, where the rearrangement was specifically dependent on the interaction with the organic phase. This was supported by MALDI ToF MS measurements, which showed the formation of protein-anion complexes for three of these proteins, and molecular dynamic simulations, which modeled the structure of lysozyme at an aqueous-organic interface. On the basis of these findings, the modulation of protein secondary structure by interfacial electrochemistry opens up unique prospects to selectively modify proteins.

Electroanalytical Opportunities Derived from Ion Transfer at Interfaces between Immiscible Electrolyte Solutions

This review presents an introduction to electrochemistry at interfaces between immiscible electrolyte solutions and surveys recent studies of this form of electrochemistry in electroanalytical strategies. Simple ion and facilitated ion transfers across interfaces varying from millimetre scale to nanometre scales are considered. Target detection strategies for a range of ions, inorganic, organic, and biological, including macromolecules, are discussed.

Amperometric Ion Sensing Approaches at Liquid/Liquid Interfaces for Inorganic, Organic and Biological Ions

Electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) has become an invaluable tool for the selective and sensitive detection of cationic and anionic species, including charged drug molecules and proteins. In addition, neutral molecules can also be detected at the ITIES via enzymatic reactions. This chapter highlights recent developments towards creating a wide spectrum of sensing platforms involving ion transfer across the ITIES. As well as outlining the basic principles needed for performing these sensing applications, the development of ITIES-based detection strategies for inorganic, organic, and biological ions is discussed.

Recent developments in electrochemistry at the interface between two immiscible electrolyte solutions for ion sensing

The most recent developments on electrochemical sensing of ions at the liquid–liquid interface are reviewed here.

The effect of the functionalization and molecular weight of cationic dextran polyelectrolytes on their electrochemical behavior at the water/1,2-dichloroethane interface

The functionalization and molecular weight of cationic dextran polyelectrolytes have an impact on their adsorption and electrochemical behavior at the water/1,2-dichloroethane interface.

Protein diffusion and long-term adsorption states at charged solid surfaces

The diffusion pathways of lysozyme adsorbed to a model charged ionic surface are studied using fully atomistic steered molecular dynamics simulation. The simulations start from existing protein adsorption trajectories, where it has been found that one particular residue, Arg128 at the N,C-terminal face, plays a crucial role in anchoring the lysozyme to the surface [Langmuir 2010 , 26 , 15954 - 15965]. We first investigate the desorption pathway for the protein by pulling the Arg128 side chain away from the surface in the normal direction, and its subsequent readsorption, before studying diffusion pathways by pulling the Arg128 side chain parallel to the surface. We find that the orientation of this side chain plays a decisive role in the diffusion process. Initially, it is oriented normal to the surface, aligning in the electrostatic field of the surface during the adsorption process, but after resorption it lies parallel to the surface, being unable to return to its original orientation due to geometric constraints arising from structured water layers at the surface. Diffusion from this alternative adsorption state has a lower energy barrier of ∼0.9 eV, associated with breaking hydrogen bonds along the pathway, in reasonable agreement with the barrier inferred from previous experimental observation of lysozyme surface clustering. These results show the importance of studying protein diffusion alongside adsorption to gain full insight into the formation of protein clusters and films, essential steps in the future development of functionalized surfaces.

Formation of dielectric layers and charge regulation in protein adsorption at biomimetic interfaces

Protein charge is an important parameter in the understanding of protein interactions and function. Proteins are subject to dynamic charge regulation, that is, the influence of the local environment (such as charged interfaces and biopolymers) on protein charge. Charge regulation is governed by differences in the dielectric and electrostatic environment between adsorbed protein and the free protein in bulk solution. In this work protein charge regulation is addressed experimentally by employing electrochemistry at interfaces between two immiscible electrolyte solutions (ITIES) as well as theoretically by developing a new protein adsorption model at ITIES. Electrochemistry at ITIES is shown to be particularly well suited to study protein charge regulation as the adsorbed protein experiences a different dielectric environment compared to the bulk phase and the external control of the water/oil potential difference allows systematic studies on how potential induced ion gradients affect protein charge. The theoretical model incorporates all the features of the experimental system and specifically takes into account protein charge regulation at ITIES as well as the impact of the formation of dielectric layers on the experimentally observed impedance. The model parameters include the protein charge-pH profile, bulk pH, and the overall potential difference. It is shown that the formation of a dielectric layer and the associated charge regulation are the main factors dictating the observed experimental behavior. Finally, the theoretical model is used to interpret literature results, and the consistency between the model and the relatively large data set suggests that the model may be used more generally for understanding and predicting protein adsorption.

Melittin adsorption and lipid monolayer disruption at liquid-liquid interfaces

Melittin, a membrane-active peptide with antimicrobial activity, was investigated at the interface formed between two immiscible electrolyte solutions (ITIES) supported on a metallic electrode. Ion-transfer voltammetry showed well-defined semi-reversible transfer peaks along with adsorptive peaks. The reversible adsorption of melittin at the liquid-liquid interface is qualitatively discussed from voltammetric data and experimentally confirmed by real-time image analysis of video snapshots. It is also demonstrated that polarization of the water/1,2-DCE interface results in drastic drop shape variations caused by large variations of the interfacial tension. The experimental data also confirmed that maximum adsorption occurs near the ion transfer potential. Finally, the interaction of melittin with a monolayer of L-α-dipalmitoyl phosphatidylcholine (DPPC) was also investigated showing that melittin destabilizes the lipidic monolayer facilitating its desorption. The non-covalent complex formation between melittin and DPPC was confirmed by mass spectrometry.