RESUMO
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.
Assuntos
Membranas/química , Água/química , Fenômenos Biofísicos , Membrana Celular/química , Ligação de Hidrogênio , Bicamadas Lipídicas/química , Lipídeos de Membrana/química , Modelos Biológicos , Relação Estrutura-Atividade , Propriedades de SuperfícieRESUMO
Monolayers spread on Hg drops are shown as a suitable experimental set up to study the influence of external electric fields on the structure of lipid membranes. The electrical response exhibits a sharp transition at 24 degrees C, the transition temperature of DMPC. In addition, voltammetric response of monolayers of mixtures of DMPC/DMPE adsorbed on mercury, shows a similar trend to that found for dipole potential of monolayers of the same composition spread on an air-solution interface. It is concluded that a lipid monolayer adsorbed in a mercury-solution interface, has comparable properties as those found in other experimental models of lipid membranes in similar conditions. In addition, they constitute an ideal set up to study the effect of electrical fields on the dynamic conformation of lipids as a function of packing change produced by the condensation in the gel state or by the interaction of polar head groups.
Assuntos
Campos Eletromagnéticos , Lipídeos/química , Membranas Artificiais , Mercúrio/química , Fenômenos Químicos , Físico-Química , Eletroquímica , TemperaturaRESUMO
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.