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Abstract.: Artificial lipid bilayers have been extensively studied as models that mimic natural membranes (biomimetic membranes). Several attempts of biomimetic membranes inserting ubiquinone (UQ) have been performed to enlighten which the position of UQ in the lipid layer is, although obtaining contradictory results. In this work, pure components (DPPC and UQ) and DPPC:UQ mixtures have been studied using surface pressure-area isotherms and Langmuir-Blodgett (LB) films of the same compounds have been transferred onto solid substrates being topographically characterized on mica using atomic force microscopy and electrochemically on indium tin oxide slides. DPPC:UQ mixtures present less solid-like physical state than pure DPPC indicating a higher-order degree for the latter. UQ influences considerably DPPC during the fluid state, but it is mainly expelled after the phase transition at $ \approx$ 26 mN·m^-1 for the 5:1 ratio and at $ \approx$ 21 mN·m^-1 for lower UQ content. The thermodynamic studies confirm the stability of the DPPC:UQ mixtures before that event, although presenting a non-ideal behaviour. The results indicate that UQ position can be tuned by means of the surface pressure applied to obtain LB films and the UQ initial content. The UQ positions in the biomimetic membrane are distinguished by their formal potential: UQ located in 'diving' position with the UQ placed in the DPPC matrix in direct contact with the electrode surface ( -0.04±0.02 V), inserted between lipid chains without contact to the substrate ( 0.00±0.01 V) and parallel to the substrate, above the lipid chains ( 0.09±0.02 V). Graphical abstract: [ABSTRACT FROM AUTHOR]

Abstract: Ubiquinone and plastoquinone are two of the main electron and proton shuttle molecules in biological systems, and monogalactosyldiacylglycerol (MGDG) is the most abundant lipid in the thylakoid membrane of chloroplasts. Saturated MGDG, ubiquinone-10 (UQ) and MGDG:UQ mixed monolayers at the air/water interface have been studied using surface pressure–area isotherms and Brewster Angle Microscopy. Moreover, the transferred Langmuir–Blodgett films have been observed by Atomic Force Microscopy. The results show that MGDG:UQ mixtures present more fluid phase than pure MGDG, indicating a higher order degree for the later. It is also observed an important influence of UQ on the MGDG matrix before UQ collapse pressure and a low influence after this event, due to UQ expulsion from the MGDG matrix. This expulsion leads to a similar remaining UQ content for all the tested mixtures, indicating a limiting content of this molecule in the MGDG matrix at high surface pressures. The thermodynamic studies confirm the stability of the MGDG:UQ mixtures at low surface pressures, although presenting a non-ideal behaviour. Results point to consider UQ as a good candidate for studies of artificial photosynthesis. [Copyright &y& Elsevier]

Abstract: The structure and the electrochemical behaviour of Langmuir and Langmuir–Blodgett (LB) films of the biological ubiquinone-10 (UQ) and a mixture of dipalmytoilphosphatidylcholine (DPPC) and UQ at the molar ratios DPPC:UQ 5:1 and 10:1 have been investigated. The surface pressure-area isotherms of the Langmuir films and the AFM images of the LB films show the formation of a monolayer in the DPPC:UQ mixture till a certain surface pressure is attained, and then at higher surface pressures the UQ is progressively expelled. The cyclic voltammograms of DPPC:UQ LB films formed on indium tin oxide, ITO, at different surface pressures show one reduction and one oxidation peak at low surface pressures, but two or even more reduction and oxidations peaks at medium and high surface pressures. The electrochemical behaviour is correlated with the film structure. [Copyright &y& Elsevier]

Ubiquinone (UQ) is one of the mainelectron and proton shuttlemolecules in biological systems, and dipalmitoylphosphatidylcholine(DPPC) is one of the most used model lipids. Supported planar bilayers(SPBs) are extensively accepted as biological model membranes. Inthis study, SPBs have been deposited on ITO, which is a semiconductorwith good electrical and optical features. Specifically, topographicatomic force microscopy (AFM) images and force curves have been performedon SPBs with several DPPC:UQ ratios to study the location and theinteraction of UQ in the SPB. Additionally, cyclic voltammetry hasbeen used to understand the electrochemical behavior of DPPC:UQ SPBs.Obtained results show that, in our case, UQ is placed in two maindifferent positions in SPBs. First, between the DPPC hydrophobic chains,fact that originates a decrease in the breakthrough force of the bilayer,and the second between the two leaflets that form the SPBs. This secondposition occurs when increasing the UQ content, fact that eventuallyforms UQ aggregates at high concentrations. The formation of aggregatesproduces an expansion of the SPB average height and a bimodal distributionof the breakthrough force. The voltammetric response of UQ dependson its position on the bilayer. [ABSTRACT FROM AUTHOR]