Electrochemical Impedance Spectroscopy of Tethered Bilayer Membranes

Abstract

The electrochemical impedance spectra (EIS) of tethered bilayer membranes (tBLMs) were analyzed, and the analytical solution for the spectral response of membranes containing natural or artificially introduced defects was derived. The analysis carried out in this work shows that the EIS features of an individual membrane defect cannot be modeled by conventional electrical elements. The primary reason for this is the complex nature of impedance of the submembrane ionic reservoir separating the phospholipid layer and the solid support. We demonstrate that its EIS response, in the case of radially symmetric defects, is described by the Hankel functions of a complex variable. Therefore, neither the impedance of the submembrane reservoir nor the total impedance of tBLMs can be modeled using the conventional elements of the equivalent electrical circuits of interfaces. There are, however, some limiting cases in which the complexity of the EIS response of the submembrane space reduces. In the high frequency limit, the EIS response of a submembrane space that surrounds the defect transforms into a response of a constant phase element (CPE) with the exponent (α) value of 0.5. The onset of this transformation is, beside other parameters, dependent on the defect size. Large-sized defects push the frequency limit lower, therefore, the EIS spectra exhibiting CPE behavior with α ≈ 0.5, can serve as a diagnostic criterion for the presence of such defects. In the low frequency limit, the response is dependent on the density of the defects, and it transforms into the capacitive impedance if the area occupied by a defect is finite. The higher the defect density, the higher the frequency edge at which the onset of the capacitive behavior is observed. Consequently, the presented analysis provides practical tools to evaluate the defect density in tBLMs, which could be utilized in tBLM-based biosensor applications. Alternatively, if the parameters of the defects, e.g., ion channels, such as the diameter and the conductance are known, the EIS data analysis provides a possibility to estimate other physical parameters of the system, such as thickness of the submembrane reservoir and its conductance. Finally, current analysis demonstrates a possibility to discriminate between the situations, in which the membrane defects are evenly distributed or clustered on the surface of tBLMs. Such sensitivity of EIS could be used for elucidation of the mechanisms of interaction between the proteins and the membranes.

Tethered bilayer membranes as a complementary tool for functional and structural studies: The pyolysin case

“…With no rPLO added, the Bode graphs (admittance representation) of EI exhibit typical for tBLMs shapes ( Fig.1 A). The admittance magnitude, Y, plot exhibits sigmoidal shape, with the upper step determined by a solution resistance, and the bottom step-like feature determined by a residual defect density in tBLMs [10]. The phase of admittance, arg Y, exhibits a minimum corresponding to a low frequency step, while, in the high frequency range one may observed a downward trending curve.…”

“…The high frequency part remains unaffected (Fig.1A). The extent of membrane damage can be conveniently assessed by two characteristic points on the Bode plots: 1) frequency f min [10,26], at which low frequency range admittance minimum occurs, and 2) the admittance magnitude value at this frequency point Y fmin = Z fmin -1 [10,12,26,27]. As recently shown [26], both f min and Y min are dependent from the density of defects in tBLMs.…”

“…As recently shown [26], both f min and Y min are dependent from the density of defects in tBLMs. However, within existing theoretical framework [10], for exact numerical evaluation of a number of membrane damaging PLO entities in tBLM one needs to assume homogeneous distribution and monodispersity of the physical properties of the defects. In this work, we will restrict our analysis to a qualitative comparison and use Y min values to compare EIS changes induced by rPLO under different conditions.…”

“…The formation of defects or water-filled pores in A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 3 tBLMs can be easily sensed and followed, in real-time, by electrochemical impedance spectroscopy (EIS) and opens the possibility of tBLMs use in bioanalytical applications [10,11].…”

Polypyrrole-Supported Membrane Proteins for Bio-Inspired Ion Channels

“…[48][49][50][51][52] This methodology provides information not only of the ion transport a complete description of the investigated system, as was recently discussed in a comparative study. 53 Impedance was measured as a function of frequency for steel-supported PNMPy and PNMPy-Omp2a films.…”