Maria Frias

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.

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.

Configuration of Carbonyl Groups at the Lipid Interphases of Different Topological Arrangements of Lipid Dispersions

The purpose of this work is to analyze the conformation of the carbonyl groups of acyl phospholipids at the hydrocarbon-water interphase in different topological ensembles and phase states, such as micelles and bilayers. The separation of the band components in lipids dispersed in D(2)O is compared with that of PCs in a low hydrated state. When hydrated, the differences in the frequencies of the band components corresponding to the carbonyl groups identified as low hydrated and hydrated populations increase when dimyristoylphosphatidylcholine (DMPC) bilayers go from the lamellar gel to the ripple corrugated phase at the pretransition temperature. Below the pretransition, at which the membrane in the gel state is planar, the two components overlap making the deconvolution unreliable. A further analysis shows that the frequency of the highly hydrated population increases more noticeable than that corresponding to the low hydrated one following the sequence: micelles, fluid phase, ripple gel phase, and lamellar gel phase. This is confirmed by the increase in the separation of the band components when the liposomes are subjected to an osmotic dehydration suggesting that the hydrated population loses water and the dehydrated one partially hydrates. It is concluded that this behavior is a feature conferred by hydration of the different topological arrangements. The relevance of these results on the interphase properties of lipid membranes is discussed.

Lysophosphatidylcholine-arbutin complexes form bilayer-like structures.

Arbutin is known to suppress melanin production in murine B16 melanoma cells and inhibit phospholipase action. This encourages the possibility to stabilize it in lipid aggregates for its administration in medical applications. Thus, it was of interest to demonstrate that monomyristoylphosphatidylcholine (14:0 lysoPC) and arbutin may form association complexes. This was studied by Electron Microscopy (EM), 31P Nuclear Magnetic Resonance (31P NMR), Electronic Paramagnetic Resonance (EPR) and Fourier Transform Infrared Spectroscopy (FTIR). EM images show the formation of particles of c.a. 6 nm in diameter. For a 1:1 lysoPC-arbutin molar ratio 31P NMR shows a spectrum with a shoulder that resembles the axially symmetric spectrum characteristic of vesicles. The addition of La3+ ions to the arbutin-lysoPC complex allows one to distinguish two phosphorous populations. These results suggest that arbutin-lysoPC forms vesicles with bilayers stabilized in an interdigitated array. FTIR spectroscopy shows that arbutin interacts with the hydrated population of the carbonyl groups and with the phosphates through the formation of hydrogen bonds. It is interpreted that hydrophobic interactions among the phenol group of arbutin and the acyl chain of lysoPC are responsible for the decrease in acyl chain mobility observed at the 5th C level by EPR. A model proposing the formation of interdigitated bilayers of arbutin-lysoPC could explain the experimental results.

Coordination forces between lipid bilayers produced by ferricyanide and Ca2

Attractive forces usually invoked to take place in membrane-membrane contact in aggregation are hydrogen bonding cross-linkings and hydrophobic interactions between opposing surfaces. However, little is known in relation to the presence of coordination forces in the membrane-membrane interaction. These are understood as those that may be favoured by the formation or the participation of coordination complexes between surface specific groups. In this work, we have analyzed the formation of this type of aggregates between phosphatidylcholine vesicles mediated by a coadsorption of ferricyanide and Ca(2+) ions to the interface. The results obtained by surface potential measures, optical and electronic microscopy, FTIR and (1)H NMR spectroscopies indicate that ferricyanide [Fe(CN)(6)](3-) but not of ferrocyanide [Fe(CN)(6)](4-) can form the complex when Ca(2+) has been adsorbed previously to the membrane surface. In this condition, the anion is likely to act as a bridge between two opposing membranes causing a tight aggregation in which geometry and the polarizability of the ligands to Fe(3+) play a role.