Excess white noise to probe transport mechanisms in a membrane channel

Abstract

Current fluctuation analysis has been successfully used over the years to investigate the physical properties of different systems. Here, we perform single-channel time-resolved current experiments in a protein channel to evaluate the different transport mechanisms governing the channel function. Using different salts of monovalent and divalent cations in a wide range of concentrations and applied potentials, we analyze current fluctuations focusing on the voltage dependence of the additional white noise that appears in the low-frequency range of the spectra. We demonstrate that the channel displays two characteristic transport regimes: at low salt concentrations (10 mM to 1 M) ion permeation is controlled by the protein fixed charges that induce accumulation or exclusion of ions to preserve local electroneutrality. At high salt concentrations (>1 M) adsorption processes associated to the binding of cations to the channel charges regulate the transport properties.

Charge, Diffusion, and Current Fluctuations of Single-Stranded DNA Trapped in an MspA Nanopore

“…These contributions can be written as S ¼ S J þ S ion þ S DNA . For solid state pores (30), the OmpF membrane channel (31), and MspA (data not shown), the excess white noise in the open pore, S ion , depends quadratically on the mean ionic current, S ion fI 2 . This relationship is the result of ion number fluctuations in the pore (32), i.e., a resistance fluctuation.…”

Biphasic concentration patterns in ionic transport under nanoconfinement revealed in steady-state and time-dependent properties

“…KCl and LiCl in PI NP). Because time-dependent properties are usually non additive but depend on a critical rate-determining step 73 , and considering that SG/G 2 in OmpF is ruled by surfacegoverned fluctuations 49 , we hypothesize that in this case the particular cation chemistry is the ruling factor and not the pore dimensions or the channel charges. In fact, normalized experimental results in another time-dependent phenomenon like OmpF gating follow the same trend as SG/G 2 because reduced -c curves with KCl and LiCl do not overlap but differ significantly (Figure S6).…”

“…Figure 2B presents the dependence of the current excess noise on concentration using the parabolic coefficient SG/G 2 , which contains relevant kinetic physicochemical information of the intrinsic system fluctuations not directly visible in G measurements [49][50][51] . The power spectral density (PSD) is calculated via the Fourier transform of the recorded current versus time at different applied voltages (Figure S1B), finding a parabolic scaling in the low frequency band <SI> ~ I 2 characteristic of equilibrium fluctuations 51 (Figure S1C).…”

“…Also, recent computational studies show the crucial role of ion−ion correlations in changing the intrinsic channel energetic barriers to ion permeation 84 . Likewise, adsorption of counterions onto the pore wall occurring at high concentration has been reported to change the pore net charge and also hinder the counterion effective mobility 3,49,57,62,85 .…”