Gabriel J. Gordillo

Structural and functional properties of hydration and confined water in membrane interfaces

The scope of the present review focuses on the interfacial properties of cell membranes that may establish a link between the membrane and the cytosolic components. We present evidences that the current view of the membrane as a barrier of permeability that contains an aqueous solution of macromolecules may be replaced by one in which the membrane plays a structural and functional role. Although this idea has been previously suggested, the present is the first systematic work that puts into relevance the relation water-membrane in terms of thermodynamic and structural properties of the interphases that cannot be ignored in the understanding of cell function. To pursue this aim, we introduce a new definition of interphase, in which the water is organized in different levels on the surface with different binding energies. Altogether determines the surface free energy necessary for the structural response to changes in the surrounding media. The physical chemical properties of this region are interpreted in terms of hydration water and confined water, which explain the interaction with proteins and could affect the modulation of enzyme activity. Information provided by several methodologies indicates that the organization of the hydration states is not restricted to the membrane plane albeit to a region extending into the cytoplasm, in which polar head groups play a relevant role. In addition, dynamic properties studied by cyclic voltammetry allow one to deduce the energetics of the conformational changes of the lipid head group in relation to the head-head interactions due to the presence of carbonyls and phosphates at the interphase. These groups are, apparently, surrounded by more than one layer of water molecules: a tightly bound shell, that mostly contributes to the dipole potential, and a second one that may be displaced by proteins and osmotic stress. Hydration water around carbonyl and phosphate groups may change by the presence of polyhydroxylated compounds or by changing the chemical groups esterified to the phosphates, mainly choline, ethanolamine or glycerol. Thus, surface membrane properties, such as the dipole potential and the surface pressure, are modulated by the water at the interphase region by changing the structure of the membrane components. An understanding of the properties of the structural water located at the hydration sites and the functional water confined around the polar head groups modulated by the hydrocarbon chains is helpful to interpret and analyze the consequences of water loss at the membranes of dehydrated cells. In this regard, a correlation between the effects of water activity on cell growth and the lipid composition is discussed in terms of the recovery of the cell volume and their viability. Critical analyses of the properties of water at the interface of lipid membranes merging from these results and others from the literature suggest that the interface links the membrane with the aqueous soluble proteins in a functional unit in which the cell may be considered as a complex structure stabilized by water rather than a water solution of macromolecules surrounded by a semi permeable barrier.

Adsorption kinetics of charged thiols on gold nanoparticles

Colloidal Au nanoparticles have been functionalized with mercaptopropane sulfonate (MPS) and mercaptoethylamine (MEA) short alkanethiols in aqueous solutions. Adsorption kinetics of these charged thiols onto gold nanoparticles has been followed by the decrease in the surface plasmon band at 530 nm due to the red shift caused by particle-to-particle aggregation. Titration curves resulted from plotting the 530 and 800 nm-absorbance versus the added amount of thiol in solution, and yielded the mercaptane coverage on Au. While the adsorption of positively charged MEA on negatively charged Au colloid is very rapid, slower and more complex adsorption kinetics have been found for the negatively charged MPS.

Redox properties of ubiquinon (UQ10) adsorbed on a mercury electrode

The electrochemical behaviour of ubiquinone 10 (UQ10) adsorbed on mercury in contact with aqueous electrolytes has been investigated at coverages smaller than a monolayer. Stability regions, acid–base ionisation constants and standard potentials for redox equilibria between conjugate stable species were determined and reaction mechanisms proposed. The standard potential for the ubiquinone/ubihydroquinone couple obtained was 0.276 V vs. SCE (–0.138 at pH 7).The values of pKa obtained for the two acid–base dissociation equilibria for the ubihydroquinone, 12 and 13.6, are higher than those predicted from the Hammett equation and lower than the value obtained in a low-relative-permittivity medium (80 % w/w ethanol). The ubisemiquinone ion redical has been found to be stable at pH > 13.6 with a disproportionation constant of 0.4. The corresponding constant for the protonated radical was estimated to be 1014, showing the high instability of this form. This fact, together with kinetic considerations, suggests that for pH values lower than 12 the redox chemistry proceeds via dismutation. Two-phase transitions for the reduced and oxidised forms were observed.

Electrocatalytic Hydrogen Redox Chemistry on Gold Nanoparticles

Electrocatalytic proton reduction leading to the formation of adsorbed molecular hydrogen on gold nanoparticles of 1-3 and 14-16 nm diameter stabilized by 1-mercapto-undecane-11-tetra(ethyleneglycol) has been demonstrated by cyclic voltammetry using a hanging mercury drop electrode. The nanoparticles were adsorbed to the electrode from aqueous dispersion and formed robust surface layers transferrable to fresh base electrolyte solutions. Unique electrocatalytic proton redox chemistry was observed that has no comparable counterpart in the electrochemistry of bulk gold electrodes. Depending on size, the nanoparticles have a discrete number of electrocatalytically active sites for the two-electron/two-proton reduction process. The adsorbed hydrogen formed is oxidized with the reverse potential sweep. These findings represent a new example of qualitative different behavior of nanoparticles in comparison with the corresponding bulk material.

The electrochemistry of ubiquinone-10 in a phospholipid model membrane

The electrochemistry of ubiquinone-10, UQ, incorporated over a phospholipid layer adsorbed on a mercury drop electrode has been investigated over a wide pH range. It is shown that the position of the quinone headgroup in relation to the lipid determines the reversibility of the redox chemistry. For pH <7, the reaction follows a disproportionation route involving the ubiquione radical. There is evidence for the presence of a parallel reaction sequence. The bifurcation point appears to occur for the UQ molecule, which disproportionates after protonation and reduction, in parallel with direct electron transfer to yield the UQ--radical anion. The incorporation of UQ in a lipid monolayer makes its reduction very irreversible for pH > 7.

Structural and dynamical surface properties of phosphatidylethanolamine containing membranes.

The hydration of solid dimyristoylphosphatidylethanolamine (DMPE) produces a negligible shift in the asymmetric stretching frequency of the phosphate groups in contrast to dimyristoylphosphatidylcholine (DMPC). This suggests that the hydration of DMPE is not a consequence of the disruption of the solid lattice of the phosphate groups as occurs in DMPC. The strong lateral interactions between NH(3) and PO(2)(-) groups present in the solid PEs remain when the lipids are fully hydrated and seem to be a limiting factor for the hydration of the phosphate group hindering the reorientation of the polar heads. The lower mobility is reflected in a higher energy to translocate the phosphoethanolamine (P-N) dipoles in an electrical field. This energy is decreased in the presence of increasing ratios of PCs of saturated chains in phosphoethanolamine monolayer. The association of PC and PE in the membrane affecting the reorientation of the P-N groups is dependent of the chain-chain interaction. The dipole potentials of PCs and PEs mixtures show different behaviors according to the saturation of the acyl chain. This was correlated with the area in monolayers and the hydration of the P-N groups. In spite of the low hydration, DMPE is still able to adsorb fully hydrated proteins, although in a lower rate than DMPC at the same surface pressure. This indicates that PE interfaces possess an excess of surface free energy to drive protein interaction. The relation of this free energy with the low water content is discussed.

Adsorption and electrochemical reduction of Co(II)-dimethylglyoxime on mercury

Abstract Cobalt and nickel in aqueous solutions can be determined by adsorptive striping voltammetry, using dimethylglyoxime as a complexing agent in order to accumulate the metals efficiently on a mercury surface. In borate buffer solution (pH 9.3) capacity results suggest that Co(II)-dimethylglyoxime (Co(HDMG) 2 ) adsorbs in two different orientations each undergoing different reaction pathways: for low surface concentrations Co(0) is produced and two electrons are transferred; for high surface concentration both Co(II) and the ligand are reduced, this process involving the transfer of more than two electrons.

The adsorption of nickel dimethylglyoxime complex on mercury

Abstract The adsorption of the Ni(II) dimethylglyoxime complex, Ni(DMGH) 2 , on mercury was studied as a function of time by voltammetric and differential capacitance methods in ammonia and borate buffers. The influence of the adsorption potential and the complex concentration was also studied. The adsorption equilibrium shows a reversible behaviour. The experimental data conform to the Frumkin adsorption isotherm. The standard electrochemical Gibbs energy of adsorption and the interaction parameter obtained from the fit indicate the presence of attractive forces between adsorbed molecules and a standard electrochemical Gibbs energy of adsorption comparable with those of pentanol and hexanol. The experimental rate of adsorption, at short times, is independent of time. It is also linearly dependent on the complex concentration and exponentially dependent on the adsorption potential. The experimental rate law can be explained with a model considering the adsorption kinetics as being controlled by the adsorption of the complex.

Water defects induced by expansion and electrical fields in DMPC and DMPE monolayers: Contribution of hydration and confined water

The values of capacitance of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE) monolayers on Hg, derived from cyclic voltammetry studies indicate that when the lipids are near the phase transition temperature fractures are formed at a critical area beyond that corresponding to the hydration shell of the lipids in the liquid expanded state. Similar fractures are inferred to be formed when an electric field is applied at constant area, at a breaking potential which is a function of the lipid species. These voltage values denote that energy involved in the transition induced by the electrical field is much higher for DMPE than for DMPC at low areas. This can be explained by the higher intermolecular lateral interactions by H-bonds between the ethanolamine and phosphate groups. However, at larger areas, the energy values for DMPC are as high as for DMPE which is understood to be due to the higher hydration of phosphocholine head groups. This finding gives a new insight in relation to the dynamics of the lipid head groups at the membrane interphase region in terms of the states of water between the lipids. This is congruent with previous results evaluated with the well known ΔΠ vs. surface pressure plots in monolayers of the same lipids at air-water interfaces.

The reduction of nickel dimethylglyoxime complex preadsorbed on mercury

Abstract The voltammetric reduction of Ni II (DMGH) 2 complex preadsorbed on mercury both in 1 M NH 4 Cl + NH 3 buffer and in 0.1 M borate buffer solutions was studied. In this work experimental evidence points to Ni II (DMGH) 2 reduction with simultaneous decomposition of the ligand DMGH − and reduction of Ni(II) to Ni(O). The experimental sweep rate dependence of peak current and potential is explained in terms of a E r C i surface reaction scheme and a rate expression that takes into account attractive forces between the adsorbed reactant complex.