Luis García-Río

Gemini surfactant-protein interactions: effect of pH, temperature, and surfactant stereochemistry.

The interactions between bovine serum albumin (BSA) and gemini surfactants derived from cystine have been investigated and were compared with the conventional single-chain surfactant derived from cysteine. The influence of the stereochemistry of the gemini surfactant on its behavior toward BSA was also investigated, as well as the effects of pH and temperature. Electrical conductivity and surface tension measurements were used to obtain important system parameters such as critical aggregation concentration (cac), polymer saturation point (psp), degree of ionization (alpha), and the amount of surfactant binding to protein (M). Stereochemistry was found to influence the surface properties of the surfactants studied and their interaction with BSA but not their micellar properties in solution.

Novel catanionic vesicles from calixarene and single-chain surfactant

The mixed system between p-sulfonatocalix[4]arene and tetradecyltrimethylammonium bromide forms unilamellar vesicles after sonication of the aqueous dispersion. Furthermore these vesicles can be stored, without use of lyoprotectants, by lyophilization and then rehydration without change in size or shape.

Supramolecular Catalysis by Cucurbit[7]uril and Cyclodextrins: Similarity and Differences

To understand the analogies and differences between the cucurbituril and cyclodextrin cavities different solvolytic reactions have been studied in the presence of cucurbit[7]uril, CB7, and beta-CD or its methylated derivative, DM-beta-CD. Solvolysis of 1-bromoadamantane has been used as a test to evaluate the ability of the cavities to solvate the Br(-) leaving group. Obtained results show that in both cases the polarity inside the cavity is similar to that of a 70% ethanol:water mixture. Solvolysis of substituted benzoyl chlorides shows a great difference between the CB7 and DM-beta-CD cavity. Solvolysis of electron withdrawing substituted benzoyl chlorides (associative mechanism) is catalyzed by DM-beta-CD and inhibited by CB7. However, solvolysis of electron donating substituted benzoyl chlorides (dissociative mechanism) is catalyzed by CB7 and inhibited by DM-beta-CD. These experimental behaviors have been explained on the basis of different solvolytic mechanisms. Participation of the hydroxyl groups of the cyclodextrin as a nucleophile can explain the catalytic effect observed for solvolysis of benzoyl chlorides reacting by an associative mechanism. Solvolysis of benzoyl chlorides reacting by a dissociative mechanism is catalyzed by CB7 due to the ability of the CB7 cavity to stabilize the acylium ion developed in the transition state by electrostatic interactions.

Aggregation of p-Sulfonatocalixarene-Based Amphiphiles and Supra-Amphiphiles

p-Sulfonatocalixarenes are a special class of water soluble macrocyclic molecules made of 4-hydroxybenzenesulfonate units linked by methylene bridges. One of the main features of these compounds relies on their ability to form inclusion complexes with cationic and neutral species. This feature, together with their water solubility and apparent biological compatibility, had enabled them to emerge as one the most important host receptors in supramolecular chemistry. Attachment of hydrophobic alkyl chains to these compounds leads to the formation of macrocyclic host molecules with amphiphilic properties. Like other oligomeric surfactants, these compounds present improved performance with respect to their monomeric counterparts. In addition, they hold their recognition abilities and present several structural features that depend on the size of the macrocycle and on the length of the alkyl chain, such as preorganization, flexibility and adopted conformations, which make these molecules very interesting to study structure-aggregation relationships. Moreover, the recognition abilities of p-sulfonatocalixarenes enable them to be applied in the design of amphiphiles constructed from non-covalent, rather than covalent, bonds (supramolecular amphiphiles). In this review, we summarize the developments made on the design and synthesis of p-sulfonatocalixarenes-based surfactants, the characterization of their self-assembly properties and on how their structure affects these properties.

Insights into the Structure of the Supramolecular Amphiphile Formed by a Sulfonated Calix[6]arene and Alkyltrimethylammonium Surfactants

In this work, we have studied the interactions between the water-soluble p-sulfonatocalix[6]arene and cationic surfactants octyltrimethylammonium bromide below the cmc and dodecyltrimethylammonium bromide above the cmc, by saturation transfer difference (STD) NMR spectroscopy. From the STD build-up curves, we have obtained the T1 independent cross relaxation rates, and the results show that the interactions established between the cationic headgroup of the surfactant and the OMe group of the macrocycle play an important role in the stabilization of the complex, both below and above the cmc.

Sulfonated calix[6]arene host-guest complexes induce surfactant self-assembly.

Amphiphilic compounds consist of a hydrophobic alkyl chain and a hydrophilic head group. Owing to their surface active properties these compounds are also designated as surfactants. Driven by hydrophobic effects these systems can self organize into molecular assemblies, such as micelles or vesicles, which have been widely used as capsules for drug delivery and as microreactors for nanoparticle preparation. The control of the assembly and disassembly of these amphiphilic surfactants is of major importance and has been achieved by the incorporation and chemical modification of functional groups that are responsive to external stimuli, such as pH, temperature, redox state, and light. The formation of host–guest complexes between macrocyclic hosts and surfactant monomers represents a different approach to modulate the physicochemical properties of these systems, with the cyclodextrins being the most widely studied hosts, owing to their numerous commercial formulations, their biodegradability and lack of toxicity. There are a number of important features of cyclodextrin-surfactant mixed systems that should be highlighted. Firstly, the formation of a host–guest complex between the surfactant and the cyclodextrin shifts the critical micelle concentration (cmc) to higher values. In practice this means that the addition of cyclodextrin to a medium containing micelles may lead to the dissolution of the self-assembled aggregates. The second point is that there are no interactions between self-assembled surfactants and cyclodextrins. To the best of our knowledge, the information in the literature concerned with lowering the cmc values of simple surfactants by macrocyclic compounds is scarce and is limited to examples of cryptands, which complex the counterions of anionic micelles, thus making the former more hydrophobic and favoring the micellization process. We present here the results obtained in a study of a mixed system based on 5,11,17,23,29,35-hexasulfonato37,38,39,40,41,42-hexamethoxycalix[6]arene (1) and the cationic surfactant dodecyltrimethylammonium bromide (2). Our results show that the host–guest complex formed by 1 and 2 self-aggregates at lower concentrations than the pure surfactant alone. This result can be explained in terms of the surfactant properties of the host–guest complex and its selfaggregation to form micelles. Calixarenes are flexible hosts that can adopt several conformations depending on the substituents present on the lower and upper rims. The unmethylACHTUNGTRENNUNGated precursor of 1 was shown to adopt a partial cone conformation in the solid state with three adjacent sulfonate groups pointing to one side of the cavity and the remaining three sulfonates pointing to the opposite end. It was shown that it is possible to control the conformational flexibility of this host in the presence of crown ether and lanthanideACHTUNGTRENNUNG(III) chlorides and a handful of solid-state structural examples show the calixarene to be in an “up–up” double cone conformation. The only crystal structure reported for 1 shows the molecule to be in a pseudo-“up–up” double cone conformation, whereby one sulfonate group points “downwards”. This more disorganized structure is a consequence of the lack of OH groups that potentially act as conformational directing groups through the formation of intramolecular hydrogen bonds. Molecular modeling calculations also indicated that 1 is highly flexible, as the authors could not locate a clearly defined energy minimum. Information about the preferred conformation in solution can be achieved by observing the signals of the ArCH2Ar methylene protons in the H NMR spectra. In the absence of 2 the H NMR spectrum of 1 contains one singlet for the ArCH2Ar protons, indicating that the calixarene is exchanging rapidly in the NMR time scale between several possible conformations. In the presence of 2 we also observed one singlet for the methylene protons for all the concentrations studied. This observation suggests that complexation of 2 with 1 and aggregation of the host–guest complex does not make the host rigid to an extent that the H NMR signals can be differentiated. [a] N. Basilio, Prof. L. Garc a-R o Departamento de Qu mica F sica, Facultade de Qu mica Universidade de Santiago, 15782 Santiago (Spain) Fax: (+34) 981-595012 E-mail : luis.garcia@usc.es

Dimeric and monomeric surfactants derived from sulfur-containing amino acids

Anionic urea-based dimeric (gemini) surfactants derived from the amino acids L-cystine, D-cystine and DL-cystine, as well as monomeric surfactants derived from L-cysteine, L-methionine and L-cysteic acid were synthesized and their solution properties characterized by electrical conductivity, equilibrium surface tension, and steady-state fluorescence spectroscopy techniques. The geminis studied showed the lowest critical micelle concentration (cmc) values, however the monomeric cysteine counterpart exhibited superior efficiency in lowering surface tension, an unusual finding that can be attributed to the free sulfhydryl group. Chirality seems to play a role in the surface active properties of the gemini surfactants, but not on micelle formation. All the surfactants studied showed a higher preference for adsorption at the air/water interface rather than to form micelles, a fact that may be related to the urea moiety. The polarity of the interfacial region, measured with the solvatochromic probe E(T)(30) (Reichardts betaine dye), was similar to sodium dodecyl sulphate (SDS) micelles.

Mixed micelle formation between amino acid-based surfactants and phospholipids

The mixed micelle formation in aqueous solutions between an anionic gemini surfactant derived from the amino acid cystine (C(8)Cys)(2), and the phospholipids 1,2-diheptanoyl-sn-glycero-3-phosphocholine (DHPC, a micelle-forming phospholipid) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC, a vesicle-forming phospholipid) has been studied by conductivity and the results compared with the ones obtained for the mixed systems with the single-chain surfactant derived from cysteine, C(8)Cys. Phospholipid-surfactant interactions were found to be synergistic in nature and dependent on the type of phospholipid and on surfactant hydrophobicity. Regular solution theory was used to analyse the gemini surfactant-DHPC binary mixtures and the interaction parameter, β(12), has been evaluated, as well as mixed micelle composition. The results have been interpreted in terms of the interplay between reduction of the electrostatic repulsions among the ionic head groups of the surfactants and steric hindrances arising from incorporation of the zwitterionic phospholipids in the mixed micelles.

Counterion Binding in Solutions of p-Sulfonatocalix[4]arene

(23)Na relaxation NMR measurements and self-diffusion coefficients for sodium cations and p-sulfonatocalix[n]arenes (SCn) were obtained to confirm that monovalent inorganic cations are complexed by SC4. In the absence of added salts and at neutral pH, the cavity of p-sulfonatocalix[4]arene (SC4) fully binds an Na(+) counterion. Our results provide evidence that when investigating the complexation of inorganic cations by SC4 a competitive binding scheme must be considered if more than one cation is present in solution. Moreover, it has been shown for the first time that SC6 and SC8 can also complex monovalent inorganic (sodium) cations.

Microemulsions as microreactors in physical organic chemistry

Microemulsions are very versatile reaction media which nowadays find many applications, ranging from nanoparticle templating to preparative organic chemistry. The thermodynamically stable and microheterogeneous nature of microemulsions, used as reaction media, induces drastic changes in the reagent concentrations, and this can be specifically used for tuning the reaction rates. In particular, amphiphilic organic molecules can accumulate and orient at the oil-water interface, inducing regiospecificity in organic reactions. In this review, we will show the recent tendencies of the use of microemulsions as organic reaction media.