Studies on human insulin adsorption kinetics at an organic–aqueous interface determined using a label-free electroanalytical approach

Structural Changes in Insulin at a Soft Electrochemical Interface

Understanding the interaction of proteins at interfaces, which occurs at or within cell membranes and lipoprotein vesicles, is central to our understanding of protein function. Therefore, new experimental approaches to understand how protein structure is influenced by protein-interface interactions are important. Herein we build on our previous work exploring electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) to investigate changes in protein secondary structure that are modulated by protein-interface interactions. The ITIES provides an experimental framework to drive protein adsorption at an interface, allowing subsequent spectroscopic analysis (e.g., Fourier transform infrared spectroscopy) to monitor changes in protein structure. Here, we reveal that the interaction between insulin and the interface destabilizes native insulin secondary structure, promoting formation of α helix secondary structures. These structural alterations result from protein-interface rather than protein-protein interactions at the interface. Although this is an emerging approach, our results provide a foundation highlighting the value of the ITIES as a tool to study protein structure and interactions at interfaces. Such knowledge may be useful to elucidate protein function within biological systems or to aid sensor development.

Quantitative Analysis of Redox-Inactive Ions by AC Voltammetry at a Polarized Interface between Two Immiscible Electrolyte Solutions

The interface between two immiscible electrolyte solutions (ITIES) is ideally suited to detect redox-inactive ions by their ion transfer. Such electroanalysis, based on the Nernst-Donnan equation, has been predominantly performed using amperometry, cyclic voltammetry, or differential pulse voltammetry. Here, we introduce a new electroanalytical method based on alternating-current (AC) voltammetry with inherent advantages over traditional approaches such as avoidance of positive feedback iR compensation, a major issue for liquid|liquid electrochemical cells containing resistive organic media and interfacial areas in the cm² and mm² range. A theoretical background outlining the generation of the analytical signal is provided and based on extracting the component that depends on the Warburg impedance from the total impedance. The quantitative detection of a series of model redox-inactive tetraalkylammonium cations is demonstrated, with evidence provided of the transient adsorption of these cations at the interface during the course of ion transfer. Since ion transfer is diffusion-limited, by changing the voltage excitation frequency during AC voltammetry, the intensity of the Faradaic response can be enhanced at low frequencies (1 Hz) or made to disappear completely at higher frequencies (99 Hz). The latter produces an AC voltammogram equivalent to a "blank" measurement in the absence of analyte and is ideal for background subtraction. Therefore, major opportunities exist for the sensitive detection of ionic analyte when a "blank" measurement in the absence of analyte is impossible. This approach is particularly useful to deconvolute signals related to reversible electrochemical reactions from those due to irreversible processes, which do not give AC signals.

Electrostatic Shielding Guides Lateral Deposition for Stable Interphase toward Reversible Magnesium Metal Anodes

Compared with lithium, magnesium shows a low propensity toward dendritic deposition due to its low surface self-diffusion barriers. However, due to the intrinsic surface roughness of the metal and the nonuniformity of the formed solid-electrolyte interphase, uneven deposition of Mg still happens, which brings about high local current density and continuous proliferation of the interphase, greatly exacerbating the passivation. Unfortunately, little attention has been paid to the deposition uniformity and the interfacial stability of Mg metal anodes, which result in a potential penalty. Herein, we modify the electrolyte with cathodically stable cations to guide smooth deposition via an electrostatic shielding strategy. The cations adsorbed on the initial protuberances effectively homogenize the charge flux by repulsing the incoming Mg²⁺ away from the tips. Importantly, we prove the lateral growth can benefit the interphase stability and electrochemical reversibility.

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.

Monosize microbeads for pseudo-affinity adsorption of human insulin

Affinity adsorption technique is increasingly used for protein purification, separation and other biochemical applications. Therapeutic molecules such as antibodies, cytokines, therapeutic DNA and plasma proteins must be purified before characterization and utilization. The aim of this study was to prepare micronsized spherical polymeric beads and to investigate the extent of their human insulin adsorption capability. Monosize poly(ethylene glycol dimethacrylate-N-methacryloyl-(L)-histidine) [poly(EDMA-MAH)] beads were prepared by modified suspension copolymerization. Functional monomer (MAH) was synthesized using methacryloyl chloride and L-histidine. The beads were characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, swelling test and elemental analysis. MAH incorporation into monosize polymeric beads, having an average size around 2-3 μm, was estimated as 55.3 μmol MAH/g bead. Equilibrium swelling ratios of poly(EDMA-MAH) and poly(EDMA) beads were 65% and 55%, respectively. Adsorption experiments were performed under different conditions (i.e., pH, temperature, protein concentration and ionic strength). It was found that adsorption characteristics are strongly depend on these conditions. Maximum insulin adsorption capacity was achieved as 24.7 mg insulin/g poly(EDMA-MAH) beads. Results were well fitted to the Langmuir isotherm model. Compared with poly(EDMA-MAH), nonspecific insulin adsorption onto poly(EDMA) beads was very low (0.61 mg insulin/g bead) and can be negligible. It was observed that insulin could be repeatedly adsorbed and desorbed (at least 10 times) without significant loss in adsorption capacity.

Dual behavior of sodium dodecyl sulfate as enhancer or suppressor of insulin aggregation and chaperone-like activity of camel alphaS(1)-casein

Sodium dodecyl sulfate (SDS) at low concentrations considerably enhanced insulin aggregation and reduced the chaperone-like activity of purified camel alphaS(1)-casein (alphaS(1)-CN). These observed changes were the result of repulsive electrostatic interactions between both negative charged head groups of SDS and alphaS(1)-CN, and the net negative charge of insulin molecules, resulting in the greater exposure of hydrophobic patches of insulin and its enhanced aggregation. In contrast, enhanced hydrophobic interactions were primarily responsible for the conformational changes observed in insulin and alphaS(1)-CN at high SDS concentrations, resulting in increased binding of SDS and alphaS(1)-CN to insulin and its reduced aggregation.