Perfluoroalkylphosphocholines are poor protein-solubilizing surfactants, as tested with neutrophil plasma membranes

Nitrone-Trolox conjugate as an inhibitor of lipid oxidation: Towards synergistic antioxidant effects

Free radical scavengers like α-phenyl-N-tert-butylnitrone (PBN) and 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) have been widely used as protective agents in various biomimetic and biological models. A series of three amphiphilic Trolox and PBN derivatives have been designed by adding to those molecules a perfluorinated chain as well as a sugar group in order to render them amphiphilic. In this work, we have studied the interactions between these derivatives and lipid membranes to understand how they influence their ability to prevent membrane lipid oxidation. We showed the derivatives better inhibited the AAPH-induced oxidation of 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLiPC) small unilamellar vesicles (SUVs) than the parent compounds. One of the derivatives, bearing both PBN and Trolox moieties on the same fluorinated carrier, exhibited a synergistic antioxidant effect by delaying the oxidation process. We next investigated the ability of the derivatives to interact with DLiPC membranes in order to better understand the differences observed regarding the antioxidant properties. Surface tension and fluorescence spectroscopy experiments revealed the derivatives exhibited the ability to form monolayers at the air/water interface and spontaneously penetrated lipid membranes, underlying pronounced hydrophobic properties in comparison to the parent compounds. We observed a correlation between the hydrophobic properties, the depth of penetration and the antioxidant properties and showed that the location of these derivatives in the membrane is a key parameter to rationalize their antioxidant efficiency. Molecular dynamics (MD) simulations supported the understanding of the mechanism of action, highlighting various key physical-chemical descriptors.

(Cryo)Transmission Electron Microscopy of Phospholipid Model Membranes Interacting with Amphiphilic and Polyphilic Molecules

Lipid membranes can incorporate amphiphilic or polyphilic molecules leading to specific functionalities and to adaptable properties of the lipid bilayer host. The insertion of guest molecules into membranes frequently induces changes in the shape of the lipid matrix that can be visualized by transmission electron microscopy (TEM) techniques. Here, we review the use of stained and vitrified specimens in (cryo)TEM to characterize the morphology of amphiphilic and polyphilic molecules upon insertion into phospholipid model membranes. Special emphasis is placed on the impact of novel synthetic amphiphilic and polyphilic bolalipids and polymers on membrane integrity and shape stability.

Hybrid Fluorinated and Hydrogenated Double-Chain Surfactants for Handling Membrane Proteins

Two hybrid fluorinated double-chain surfactants with a diglucosylated polar head were synthesized. The apolar domain consists of a perfluorohexyl main chain and a butyl hydrogenated branch as a side chain. They were found to self-assemble into small micelles at low critical micellar concentrations, demonstrating that the short branch increases the overall hydrophobicity while keeping the length of the apolar domain short. They were both able to keep the membrane protein bacteriorhodopsin stable, one of them for at least 3 months.

2-DE using hemi-fluorinated surfactants

The synthesis of hemi-fluorinated zwitterionic surfactants was realized and assessed for 2-DE, a powerful separation method for proteomic analysis. These new fluorinated amidosulfobetaine (FASB-p,m) were compared to their hydrocarbon counterparts amidosulfobetaine (ASB-n) characterized by a hydrophilic polar head, a hydrophobic and lipophilic tail, and an amido group as connector. The tail of these FASB surfactants was in part fluorinated resulting in the modulation of its lipophilicity (or oleophobicity). Their effect on the red blood cell (RBC) membrane showed a specific solubilization depending on the length of the hydrophobic part. A large number of polypeptide spots appeared in the 2-DE patterns by using FASB-p,m. The oleophobic character of these surfactants was confirmed by the fact that Band 3, a highly hydrophobic transmembrane protein, was not solubilized by these fluorinated structures. The corresponding pellet was very rich in Band 3 and could then be solubilized by using a strong detergent such as amidosulfobetaine with an alkyl tail containing 14 carbon atoms (ASB-14). Thus, these hemi-fluorinated surfactants appeared as powerful tools when used at the first step of a two-step solubilization strategy using a hydrocarbon homologous surfactant in the second step.

Hemifluorinated surfactants: a non-dissociating environment for handling membrane proteins in aqueous solutions?

The instability of membrane proteins in detergent solution can generally be traced to the dissociating character of detergents and often correlates with delipidation. We examine here the possibility of substituting detergents, after membrane proteins have been solubilized, with non-detergent surfactants whose hydrophobic moiety contains a perfluorinated region that makes it lipophobic. In order to improve its affinity for the protein surface, the fluorinated chain is terminated by an ethyl group. Test proteins included bacteriorhodopsin, the cytochrome b(6)f complex, and the transmembrane region of the bacterial outer membrane protein OmpA. All three proteins were purified using classical detergents and transferred into solutions of C(2)H(5)C(6)F(12)C(2)H(4)-S-poly-Tris-(hydroxymethyl)aminomethane (HF-TAC). Transfer to HF-TAC maintained the native state of the proteins and prevented their precipitation. Provided the concentration of HF-TAC was high enough, HF-TAC/membrane protein complexes ran as single bands upon centrifugation in sucrose gradients. Bacteriorhodopsin and the cytochrome b(6)f complex, both of which are detergent-sensitive, exhibited increased biochemical stability upon extended storage in the presence of a high concentration of HF-TAC as compared to detergent micelles. The stabilization of cytochrome b(6)f is at least partly due to a better retention of protein-bound lipids.

Interaction of membrane proteins and lipids with solubilizing detergents

Detergents are indispensable in the isolation of integral membrane proteins from biological membranes to study their intrinsic structural and functional properties. Solubilization involves a number of intermediary states that can be studied by a variety of physicochemical and kinetic methods; it usually starts by destabilization of the lipid component of the membranes, a process that is accompanied by a transition of detergent binding by the membrane from a noncooperative to a cooperative interaction already below the critical micellar concentration (CMC). This leads to the formation of membrane fragments of proteins and lipids with detergent-shielded edges. In the final stage of solubilization membrane proteins are present as protomers, with the membrane inserted sectors covered by detergent. We consider in detail the nature of this interaction and conclude that in general binding as a monolayer ring, rather than as a micelle, is the most probable mechanism. This mode of interaction is supported by neutron diffraction investigations on the disposition of detergent in 3-D crystals of membrane proteins. Finally, we briefly discuss the use of techniques such as analytical ultracentrifugation, size exclusion chromatography, and mass spectrometry relevant for the structural investigation of detergent solubilized membrane proteins.

Carbohydrate- and related polyol-derived fluorosurfactants: an update

A short review of recent literature is presented on the synthesis, biological properties, colloid and surface chemistry, and applications of carbohydrate- and related polyol-derived amphiphiles with perfluoroalkyl hydrophobes.

Stabilization of integral membrane proteins in aqueous solution using fluorinated surfactants

Surfactants carrying either a hydrocarbon or a fluorocarbon alkyl chain have been synthesized. The polar head was either tris(hydroxymethyl)acrylamidomethane (THAM), telomerized THAM, or a glycosylated THAM moiety. The aqueous solubility of some of these molecules was increased by oxidizing to a sulfoxide the thioether function that associates their hydrophobic and hydrophilic moieties. In all cases, the critical micellar concentration was principally determined by the length and chemical nature of the alkyl chain. The usefulness of these surfactants in handling integral membrane proteins in solution has been examined using as test materials chloroplast thylakoid membranes and the photosynthetic complex cytochrome b6f. In keeping with earlier observations in other systems, none of the fluorinated surfactants was able to solubilize thylakoid membranes. Transfer to a solution of fluorinated surfactant of b6f complexes that had been solubilized and purified in the presence of a classical detergent usually resulted in aggregation and precipitation of the protein, while most homologous molecules with hydrocarbon chains did keep the b6f complex soluble. Two of the fluorinated surfactants, however, proved able to maintain the b6f complex water-soluble, intact, and enzymatically active. Because of their limited affinity for lipid alkyl chains and other hydrocarbon surfaces, fluorinated surfactants appear as potentially interesting tools for the study of membrane proteins that do not stand well exposure to classical detergents.

Highly fluorinated amphiphiles and colloidal systems, and their applications in the biomedical field. A contribution

Fluorocarbons and fluorocarbon moieties are uniquely characterized by very strong intramolecular bonds and very weak intermolecular interactions. This results in a combination of exceptional thermal, chemical and biological inertness, low surface tension, high fluidity, excellent spreading characteristics, low solubility in water, and high gas dissolving capacities, which are the basis for innovative applications in the biomedical field. Perfluoroalkyl chains are larger and more rigid than their hydrogenated counterparts. They are considerably more hydrophobic, and are lipophobic as well. A large variety of well-defined, modular fluorinated surfactants whose polar head groups consist of polyols, sugars, sugar phosphates, amino acids, amine oxides, phosphocholine, phosphatidylcholine, etc, has recently been synthesized. Fluorinated surfactants are significantly more surface active than their hydrocarbon counterparts, both in terms of effectiveness and of efficiency. Despite this, they are less hemolytic and less detergent. Fluorosurfactants appear unable to extract membrane proteins. Fluorinated chains confer to surfactants a powerful driving force for collecting and organizing at interfaces. As compared to non-fluorinated analogs, fluorosurfactants have also a much stronger capacity to self-aggregate into discrete molecular assemblies when dispersed in water and other solvents. Even very short, single-chain fluorinated amphiphiles can form highly stable, heat-sterilizable vesicles, without the need for supplementary associative interactions. Sturdy microtubules were obtained from non-chiral, non-hydrogen bonding single-chain fluorosurfactants. Fluorinated amphiphiles can be used to engineer a variety of colloidal systems and manipulate their morphology, structure and properties. Stable fluorinated films, membranes and vesicles can also be prepared from combinations of standard surfactants with fluorocarbon/hydrocarbon diblock molecules. In bilayer membranes made from fluorinated amphiphiles the fluorinated tails segregate to form an internal teflon-like hydrophobic and lipophobic film that increases the stability of the membrane and reduces its permeability. This fluorinated film can also influence the behavior of fluorinated vesicles in a biological milieu. For example, it can affect the in vivo recognition and fate of particles, or the enzymatic hydrolysis of phospholipid components. Major applications of fluorocarbons currently in advanced clinical trials include injectable emulsions for delivering oxygen to tissues at risk of hypoxia; a neat fluorocarbon for treatment of acute respiratory failure by liquid ventilation; and gaseous fluorocarbon-stabilized microbubbles for use as contrast agents for ultrasound imaging. Fluorosurfactants also allow the preparation of a range of stable direct and reverse emulsions, microemulsions, multiple emulsions, and gels, some of which may include fluorocarbon and hydrocarbon and aqueous phases simultaneously. Highly fluorinated systems have potential for the delivery of drugs, prodrugs, vaccines, genes, markers, contrast agents and other materials.