Electron transport in supported and tethered lipid bilayers modified with bioelectroactive molecules

Tethered bilayer lipid membranes (tBLMs) are commonly used as model membranes. However, in biophysical studies, free-standing membranes (“black” lipid membranes) are more realistic models of cellular processes. In this article, we discuss the rates of electron transfer in both types of bilayer lipid membranes. These BLMs were then modified using two very important mitochondrial membrane-associated molecules: ubiquinone-10 (UQ10) and α-tocopherol (VitE). The electron transfer rates in the unmodified films were studied with three redox couples, Fe(CN)6(3-/4-), Ru(NH3)6(3+/2+), and NAD+/NADH, using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The rate of electron transfer in the modified films was studied using the biologically relevant NAD+/NADH electroactive couple using the same methods. It is shown that when the BLMs are modified with only UQ10, it is possible to observe electron transfer. However, when the antioxidant VitE is added to the modification, the electron transfer provided by UQ10 is inhibited. Following initial studies using CV, a comparison of electron transfer theory and data was used to investigate this phenomenon in more detail, using EIS data. The standard rate constant caused by electron tunneling across the film, k(th)(0), depends on the value of β used. Two different values of the potential independent electron tunneling coefficient, β, were fitted, and it is shown that a β value half of those usually reported in literature (refereed here as β(app)) gives better agreement between the theory and the experimental results. The unmodified films present k(th)(0) values on the order of 10(-15) cm s(-1) when β = 0.72 Å(-1) and k(th)(0) values on the order of 10(-9) cm s(-1) when β(app) = 0.38 Å(-1). For the modified films, the values of k(th)(0) are on the order of 10(-15) cm s(-1) when β = 0.72 Å(-1) and 10(-9) cm s(-1) for β(app) = 0.38 Å(-1). The experimental electron transfer rate constant, k(app)(0), is on the order of 10(-8) cm s(-1) for unmodified and modified (with (i) UQ10, (ii) VitE, and (iii) UQ10 + VitE) films.