This is an accepted version of a paper published in Bioelectrochemistry. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination.Citation for the published paper: Mårtensson, C., Agmo Hernańdez, V. (2012 AbstractIt is reported that the reduction of ubiquinone incorporated into supported lipid bilayers and into immobilized liposome layers on gold electrodes is kinetically and thermodynamically enhanced by the presence of acetylcholine and tetrabutylammonium (TBA + ) in solution. The reduction peak and the mid-peak potentials of the redox reactions, determined by cyclic voltammetry, are displaced towards more positive potentials by approximately 500 and 250 mV, respectively, in the case of TBA + ; and by approximately 750 and 530 mV, respectively, in the case of acetylcholine. The intensity of the signal varies with the cation concentration, allowing for quantitative determinations in the millimolar range. It is proposed that the enhanced reduction of ubiquinone arises from the formation of tetraalkylammonium cationubiquinone radical anion ion-pairs. Electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) measurements confirmed that the potential shift and the intensity of the redox signal are coupled with the adsorption of the tetraalkylammonium cations on the lipid membrane. The Langmuir adsorption equilibrium constant (K) of TBA + on lipid membranes at physiological pH is determined. In supported lipid bilayers K = 440.7 ± 160 M -1 , while in an immobilized liposome layer K = 35.53 ± 3.53 M -1 . 1

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