Electrochemistry of LB films of mixed MGDG:UQ on ITO

Abstract

. C/ Colom 1, 08222 Terrassa (Barcelona), Spain. Tl: 34 937398043. AbstractThe electrochemical behaviour of biomimetic monolayers of monogalactosyldiacylglycerol (MGDG) incorporating ubiquinone-10 (UQ) has been investigated. MGDG is the principal component in the thylakoid membrane and UQ seems a good substitute for plastoquinone-9, involved in photosynthesis chain. The monolayers have been performed using the Langmuir and Langmuir-Blodgett (LB) techniques and the redox behaviour of the LB films, transferred at several surface pressures on a glass covered with indium-tin oxide (ITO), has been characterized by cyclic voltammetry. The cyclic voltammograms show that UQ molecules present two redox processes (I and II) at high UQ content and high surface pressures, and only one redox process (I) at low UQ content and low surface pressures. The apparent rate constants calculated for processes I and II indicate a different kinetic control for the reduction and the oxidation of UQ/UQH2 redox couple, being kRapp(I)= 2.2·10 -5 s , respectively. The correlation of the redox response with the physical states of the LB films allows determining the positions of the UQ molecules in the biomimetic monolayer, which change with the surface pressure and the UQ content. These positons are known as diving and swimming.

Influence of membrane galactolipids and surface pressure on plastoquinone behaviour

“…At more anodic potentials than -0.20 V a continuous increase of the intensity was obtained, indicating hydrogen evolution. This behaviour was also reported for the ITO-MGDG electrode in which the hydrogen evolution started at potentials more cathodic than -0.40 V [12]. The cyclic voltammogram of the ITO-MD/electrolyte system has been used to evaluate its Cd (Inset of Fig.…”

“…3) from the values of the voltammetric charging current [14]. The ITO-MD electrode presents higher Cd values than bare ITO, increasing from 2 to 4.5 μF•cm -2 when scanning from 0.90 to -0.30 V. Cd values ≈ 1.8 μF•cm -2 have been reported for high quality lipid monolayers [12] so indicating that our monolayer is not completely homogeneous and presents few defects. The fact that the voltamogram presents reproducible behaviour indicates that the lipid layer is permeable to water molecules, and after the third scan, stable water content is achieved in the monolayer.…”

“…The head groups of negatively charged PG and SQDG are exclusively located on the cytoplasmic (stroma) side, those of the uncharged DGDG on the luminal side and those of MGDG on both sides. In previous work, our group has prepared biomimetic membranes composed of one kind of lipid (MGDG, DGDG or DPPC), mixed with PQ or ubiquinone-10 (UQ) (Schematic 1B) that is similar in size and shape to PQ [9][10][11][12][13][14][15]. Several lipid:quinone ratios have been studied and we have determined the position in the lipid matrix of the PQ or UQ.…”

“…With this aim, in this work, we have prepared biomimetic monolayers of MGDG-DGDG at a ratio of 2:1, that we designate as MD in the work, and MD and PQ (MD:PQ) systems at a ratio of 5:1. The selection of the 5:1 ratio is based on our experience [9][10][11][12], and is the ratio closest to the biological one and shows the best-defined characteristics, especially when using cyclic voltammetry. The MGDG:DGDG ratio in the MD mixture is close to the natural one of thylakoid membranes and, in addition, it represents ≈ 80% of the thylakoid membrane lipid composition.…”

Tuning ubiquinone position in biomimetic monolayer membranes

“…Therefore, this movement disturbs the DPPC monolayer causing a molecular area increase [51,55]. Considering the results (section 3.1) and the relative heights observed at each surface pressure permit obtaining the physical state corresponding to each shade [56], being dark zones corresponding to LE and fair zones to LC. DPPC and DPPC:UQ mixtures present grooves of depth < 4 Å at the LC phase, which suggest that the continuous phase has been formed by fusion of rigid domains.…”