Lipid-surfactant interactions

By far the most detailed thennodynamic analysis of binding to or incorporation of molecules into membranes has up to now been formulated for the binding or incorporation of surfactants into lipid bilayers, including the solubilization phenomenon. There is a simple reason for this surfactants and synthetic lipids of high purity are available in large quantities and the experimental titration curves are highly reproducible because the reactants remain soluble in the sense that no macroscopic settling occurs. In addition, the reactions are fast on the time 

The heat effects observed wlien surfactants are incorporated into membranes arise mainly from changes in hydrophobic hydration , i.e. a change of the water exposure of hydrophobic groups when a surfactant monomer in water is transferred into a hydrophobic surrounding, i.e. the bilayer. This heat of reaction is strongly temperature dependent and usually changes sign at a temperature of 20-30°C when no other heat effects are superimposed. 

An example where these characteristic changes are easily observable, are titration experiments with micellar solutions of surfactants which are titrated into the vessel filled with pure water. Initially, dilution of the micellar solution leads to complete demicellization. When the concentration of monomers in the cell approaches the critical micellar concentration cmc the heat effect connected with the transfer of surfactants from the micelle to the aqueous solution disappears and the heat of reaction approaches zero [118-123]. 

From the slope of the A// vs. 7 curve the change in heat capacity Ac/7 can be determined. In a first approximation, Acy is constant, at least over a temperature 

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In Part Four (Chapter eight) we focus on the interactions of mixed systems of surface-active biopolymers (proteins and polysaccharides) and surface-active lipids (surfactants/emulsifiers) at oil-water and air-water interfaces. We describe how these interactions affect mechanisms controlling the behaviour of colloidal systems containing mixed ingredients. We show how the properties of biopolymer-based adsorption layers are affected by an interplay of phenomena which include selfassociation, complexation, phase separation, and competitive displacement. 

Ewert KK, Samuel CE, Safinya CR (2008) Lipid-DNA interactions structure-function studies of nanomaterials for gene delivery. In Dias R, Lindman B (eds) Interaction of DNA with surfactant and polymers. Blackwell, Boston, MA... 

SC. Surfactant interaction with lipids and proteins leads to a fundamental breakdown of biological processes that underpin skin health. Mild surfactants have lead to cleansers with significantly reduced drying and damaging potential but only within the last decade have truly moisturizing cleansers begun to emerge. 

Although a few general guidelines exist, the design of such mentioned extraction schemes is still a trial-and-error proposition. Consequently, more basic information on the nature of the lipid -protein - surfactant interactions is still required. It should also be noted that in most instances, the micellar "extraction" step is merely the prelude to further fractionation (usually by electrophoretic, column or hydrophobic chromatographic techniques) and purification of the desired biological components (402-404). 

Tn nature proteins interact with ions, lipids, and other proteins as part of the broad spectrum of necessary biological processes including membrane functionality and antigen-antibody effects. Protein functionality can be altered greatly by the interaction of proteins with surface-active agents, and the subject of protein-surfactant interaction is important in relation to food, cosmetic, and biomedical areas. 

When an irritating surfactant interacts with the skin surface, it partially or totally removes the hydrolipidic film, disorganizes the lipidic barrier of the stratum comeum, and... 

Membrane-bound enzymes, as several authors have emphasized, are generally bound to lipid and resistant to solubilization unless the lipid is removed or lipid-protein interactions reduced. The forces between the lipid and protein elements are unlikely to be the same for all enzymes nor the same for the same enzymes from different species. Lipid removal by surfactants results in the exchange of bound lipid for bound detergent molecules [44] whether or not this occurs without change in the conformation of the protein is open to question. Siekewitz [91] has pointed out other problems in interpreting results of solubilized proteins ... 

The interfacial thickness of emulsion droplets is an important parameter affecting lipid oxidation reaction rates. Increasing interfacial membrane thickness can conceivably hinder the physical interaction between aqueous phase prooxidants (e.g., transition metals) and emulsified lipids(Chaiyasit et al., 2000 Silvestre et al., 2000). For example, Silvestre and co-workers (2000) showed that iron-catalyzed cumenehydroperoxide reduction, as well as salmon oil-in-water emulsion oxidation, was slower when Brij 700 was used in place of Brij 76. Brij 700 and 76 are small molecule surfactants with identical hydrophobic tail group lengths (CHjlCH lj -), but vary only with respect to the size of their polar head groups Brij 700 and Brij 76 consist of 100 and 10 oxyethylene head groups, respectively. Lower hydroperoxide decomposition and lipid oxidation rates in Brij 700-stabilized emulsions suggest that a thicker interfacial layer was able to act as a physical barrier to decrease lipid-prooxidant interactions (Silvestre et al., 2000). 

In the context of DNA-surfactant interactions we will briefly comment on the amphiphilic nature of DNA and its consequences for the solution behavior In discussing the self-assembly behavior of DNA, we begin by broadly discussing other amphiphilic compounds and their self-assembly. Amphiphilic compounds - those that have distinct hydrophilic and lipophilic parts - range from low molecular weight molecules, like surfactants and lipids, to macromolecules, consisting of synthetic graft and block copolymers, and biomacromolecules, like proteins, lipopolysacchar-ides and nucleic acids. 

Bruni, R., Taeusch, H.W., and Waring, A.J. Surfactant protein B Lipid interactions of synthetic peptides representing the amino-terminal amphipathic domain. Proc. Natl. Acad. Sci. USA 1991, 88, 7451-7455. 

It is important for the theoretical understanding of the formation of various topologies that these aggregates have entropic contributions on the scale of the objects, i.e. on a much larger scale than set by the molecules. These cooperative entropic effects should be included in the overall Helmholtz energy, and they are essential to describe the full phase behaviour. It is believed that the mechanical parameters discussed above kc,k and J0, control the phase behaviour, where it is understood that these quantities may, in principle, depend on the overall surfactant (lipid) concentration, i.e. when the membranes are packed to such a density that they strongly interact. 

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