Cytochrome c provokes the weakening of zwitterionic membranes as measured by force spectroscopy

Influence of membrane galactolipids and surface pressure on plastoquinone behaviour

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/PQH2 couple, being kRapp(I)=0.7·10(-6)s(-1), kRapp(II)=2.2·10(-9)s(-1), kOapp(I)=7.4·10(-4)s(-1) and kOapp(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.

Monogalactosyldiacylglycerol and digalactosyldiacylglycerol role, physical states, applications and biomimetic monolayer films

The relevance of biomimetic membranes using galactolipids has not been expressed in any extensive experimental study of these lipids. Thus, on the one hand, we present an in-depth article about the presence and role of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) in thylakoid membranes, their physical states and their applications. On the other hand, we use the Langmuir and Langmuir-Blodgett (LB) techniques to prepare biomimetic monolayers of saturated galactolipids MGDG, DGDG and MGDG:DGDG 2:1 mixture (MD)--biological ratio--. These monolayers are studied using surface pressure-area isotherms and their data are processed to enlighten their physical states and mixing behaviour. These monolayers, once transferred to a solid substrate at several surface pressures are topographically studied on mica using atomic force microscopy (AFM) and using cyclic voltammetry for studying the electrochemical behaviour of the monolayers once transferred to indium-tin oxide (ITO), which has good optical and electrical properties. Moreover, MD presents other differences in comparison with its pure components that are explained by the presence of different kinds of galactosyl headgroups that restrict the optimal orientation of the MGDG headgroups.

Biomimetic monolayer films of digalactosyldiacylglycerol incorporating plastoquinone

The photosynthesis is the process used by plants and bacteria cells to convert inorganic matter in organic thanks to the light energy. This process consist on several steps, being one of them the electronic transport from the photosystem II to the cytochrome thanks to plastoquinone-9 (PQ). Here we prepare membranes that mimic the characteristics and composition of natural photosynthetic cell membranes and we characterize them in order to obtain the PQ molecules position in the membrane and their electrochemical behaviour. The selected galactolipid is digalactosyldiacylglycerol (DGDG) that represents the 30% of the thylakoid membrane lipid content. The results obtained are worthful for several science fields due to the relevance of galactolipids as anti-algal, anti-viral, anti-tumor and anti-inflammatory agents and the antioxidant and free radical scavenger properties of prenylquinones. Both pure components (DGDG and PQ) and the DGDG:PQ mixtures have been studied using surface pressure-area isotherms. These isotherms give information about the film stability and indicate the thermodynamic behaviour of the mixture and their physical state. The Langmuir-Blodgett (LB) film has been transferred forming a monolayer that mimics the bottom layer of the biological membranes. This monolayer on mica has been topographically characterized using AFM and both the height and the physical state that they present have been obtained. Moreover, these monolayers have been transferred onto ITO that is a hydrophilic substrate with good optical and electrical features, so that, it is suitable for studying the electrochemical behaviour of these systems and it is a good candidate for energy producing devices.

Incorporation of ubiquinone in supported lipid bilayers on ITO

Ubiquinone (UQ) is one of the main electron and proton shuttle molecules 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. In this study, SPBs have been deposited on ITO, which is a semiconductor with good electrical and optical features. Specifically, topographic atomic force microscopy (AFM) images and force curves have been performed on SPBs with several DPPC:UQ ratios to study the location and the interaction of UQ in the SPB. Additionally, cyclic voltammetry has been used to understand the electrochemical behavior of DPPC:UQ SPBs. Obtained results show that, in our case, UQ is placed in two main different 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 second position occurs when increasing the UQ content, fact that eventually forms UQ aggregates at high concentrations. The formation of aggregates produces an expansion of the SPB average height and a bimodal distribution of the breakthrough force. The voltammetric response of UQ depends on its position on the bilayer.