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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]

In this work biomimetic monolayers of a MGDG, monogalactosyldiacylglycerol, and DGDG, digalactosyldiacylglycerol mixture (MD), in a ratio close to that of the thylakoid membranes of oxygenic photosynthetic organisms, have been prepared. The lipid mixture incorporates plastoquinone-9 (PQ), that is the electron and proton shuttle of the photosynthetic reaction centres. The MD:PQ mixtures have been firstly studied using surface pressure-area isotherms. Langmuir-Blodgett (LB) films of those mixtures have been transferred onto a substrate forming a monolayer that mimics one of the bilayer sides of the thylakoid membranes. These monolayers have been characterized topographically and electrochemically. The results show the influence of PQ in the MD matrix and its partial expulsion when increasing the surface pressure, obtaining two main PQ positions in the MD matrix. The calculated apparent electron transfer rate constants indicate a different kinetic control for the reduction and the oxidation of the PQ/PQH 2 couple, being k Rapp (I) = 0.7 · 10 − 6 s − 1 , k Rapp (II) = 2.2 · 10 − 9 s − 1 , k Oapp (I) = 7.4 · 10 − 4 s − 1 and k Oapp (II) = 5.2 · 10 − 5 s − 1 , respectively. The comparison of the different galactolipid:PQ systems that our group has studied is also presented, concluding that the PQ position in the galactolipid matrix can be tuned according to several controlled variables. [ABSTRACT FROM AUTHOR]

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]