(Keynote) Ion Transport across Bilayer Lipid Membranes
Authors: Shirai Osamu 
Published: December 2024

Ion transports across cell membranes are related to various important biofunctions such as respiration, metabolism, and signal propagation [1]. Bilayer lipid membranes (BLMs) have been used as base materials to analyze the characteristics of ion transports across biomembranes containing various transporters. Although a BLM, which forms the structural frame of biomembrane, serves as a permeation barrier for hydrophilic ions such as K+, Na+ and Cl‒, it has been reported that electrolyte ions (M+ and X–) slightly transport across a BLM between two aqueous phases (W1 and W2) in the absence of any transporter [2]. In this case, a pair of M+ and X– distributes from aqueous phases to the BLM, and M+ and X– transport oppositely each other by applying the potential difference (membrane potential, E
W1-W2). The higher the hydrophobicity of the ion is, the higher the ion transport current flows when the same membrane potential is applied. Based on the Goldman-Hodgkin-Katz equation, the ion transport current density (j
W1-W2) is described as Eq. (1) when only MX exists in W1 and W2 [2].

j
W1-W2 = - (c
0
F
2
E
W1-W2
β)(D
M+ + D
X-)/(RTd) (1)
Here, c
0, F, R, T, d, b and D are the current density for the ion transport, the concentration of the electrolyte (MX), the Faraday constant, the absolute temperature, the thickness of the BLM, the distribution coefficient, and the diffusion coefficient, respectively. Assuming that the effect of ion-pair formation is negligible small, b is described as Eq. (2).
ln β = (ΔG
°
tr, M+ + ΔG
°
tr, X-)/(2RT) (2)
ΔG
°
tr, M+ and ΔG
°
tr, X- are the standard Gibbs energies for the ion transfer of M+ and X–, respectively, from the aqueous phase to the BLM. It is impossible to evaluate and values. Because the standard Gibbs energies for the ion transfer of M+ and X– from the aqueous phase to several organic solution are proportional to hydration energies of M+ and X– ( and ), respectively, and can be expressed as Eq. (3) .
ΔG
°
tr, M+ = a ΔG
°
hyd, M+ + b (ΔG
°
tr, X- = a' ΔG
°
tr, X- + b') (3)
Here, a and b (a' and b') are constant values and depend on the media such as organic solvents. Based on Eqs. (1)-(3), it is clear that the ion permeability depends on the species and concentration of the electrolyte. Accordingly, or can be used as a measure of hydrophobicity of the objective ion.
It is well-known that the ion transport is facilitated in the presence of hydrophobic ions, carrier compounds, and/or ion channels [1]. When only 10–7-10–5 mol dm–3 (M) of hydrophobic ions such as tetraphenylborate (TPhB–), dipicrylaminate (DPA–), etc. were added into W1 and/or W2 containing 0.1 M of hydrophilic electrolytes such as KCl, NaCl, etc., a pair of anodic and cathodic peaks symmetrical to 0 V appeared in cyclic voltammograms [3]. The peak currents were found to be related to the adsorption and desorption of hydrophobic ions on the surface of the BLM. Under the steady-state condition, the relationship between E
W1-W2 and j
W1-W2 indicated a sigmoidal curve. This steady-state current was mainly caused by the transport of the counter ion and hydrophobic ions served as carriers of the counter ion. Similarly, the ion transport was facilitated by an ionophore such as valinomycin (Val) and nonactin (Non). It is thought that these carriers facilitate the transport of alkali ions across the BLM as carriers, because these cations are strongly complexed with these ionophores. The author’s group revealed that the j
W1-W2 values depended on not only cation species but also anion species [4]. In the case of the carrier-type transport, it was proved that both the cation and the anion distributed to the BLM and that both two ions simultaneously transported in the opposite direction. We also found that the ion transport through ion channels such as gramicidin A occurs based on the same mechanism [5].
References
[1] R. B. Gennis, “Biomembranes: Molecular Structure and Function”, 1989, Springer-Verlag, New York, Berlin, Heidelberg, London, Paris, Tokyo.
[2] K. Kimura, O. Shirai, Y. Kitazumi, and K. Kano, J. Electroanal. Chem., 779 (2016) 131-136.
[3] T. Naruse, Y. Yamada, K. Sowa, Y. Kitazumi, and O. Shirai, Anal. Sci., 38 (2022) 683-688.
[4] J. Onishi, O. Shirai, and K. Kano, Electroanalysis, 22 (2010) 1229-1238.
[5] S. Kubota, O. Shirai, Y. Kitazumi, and K. Kano, Anal. Sci., 32 (2016) 189-192.