Ramon Pons

Cationic surfactants from lysine: Synthesis, micellization and biological evaluation

Biocompatible cationic surfactants from the amino acid lysine (hydrochloride salts of N(epsilon)-lauroyl lysine methyl ester, N(epsilon)-myristoyl lysine methyl ester and N(epsilon)-palmitoyl lysine methyl ester) have been prepared in high yields by lysine acylation in epsilon position with three natural saturated fatty acids. The micellization process of these surfactants has been studied using the PGSE-NMR technique. The compounds were tested as antimicrobial agents against Gram-positive and Gram-negative bacteria. The surfactants show moderate antimicrobial activity against the Gram-positive bacteria but Gram-negative bacteria are resistant to these surfactants in the concentration range tested. The haemolytic activity is considerably lower than those reported for other cationic N(alpha)-acyl amino acid analogues. The acute toxicity against Daphnia magna and biodegradability was studied. The toxicity is clearly lower than that reported for conventional cationic surfactants from quaternary ammonium and the three surfactants from lysine can be classified as ready biodegradable surfactants.

Formation and properties of miniemulsions formed by microemulsions dilution

Mini-emulsions have been formed in quaternary systems water/hexanol/sodium dodecyl sulfate/decane by dilution of a microemulsion with an excess of water. We have investigated systematically the effect of composition variables in the droplet size and Ostwald Ripening rate. This droplet size has been investigated by using dynamic light scattering of samples submitted to further dilution in water. According to the dynamic light scattering results, the initial droplet size depends on the initial microemulsion water content, the larger the initial water concentration, the smaller the initial droplet size. This is probably related to the structure of the initial phase. The rate of Ostwald Ripening depends on the final surfactant concentration as expected for differences in interfacial tension. At high surfactant and dispersed phase contents, instability due to flocculation has been observed. Other experiments in which a fifth component (a more hydrophobic oil) was added to slow down Ostwald Ripening showed an initial droplet size increasing comparable to the systems without additive but after a relatively long time of approximately 2 h, a decrease was observed.

Gemini surfactants from natural amino acids

In this review, we report the most important contributions in the structure, synthesis, physicochemical (surface adsorption, aggregation and phase behaviour) and biological properties (toxicity, antimicrobial activity and biodegradation) of Gemini natural amino acid-based surfactants, and some potential applications, with an emphasis on the use of these surfactants as non-viral delivery system agents. Gemini surfactants derived from basic (Arg, Lys), neutral (Ser, Ala, Sar), acid (Asp) and sulphur containing amino acids (Cys) as polar head groups, and Geminis with amino acids/peptides in the spacer chain are reviewed.

Supramolecular Properties of the Proline-Rich γ-Zein N-Terminal Domain

Zeins are maize storages proteins that accumulate inside large vesicles called protein bodies. gamma-Zein lines the inner face of the protein body membrane, and its N-terminal proline-rich repetitive domain with the sequence (VHLPPP)(8) appears to be necessary for the accumulation of the protein within the organelle. Synthetic (VHLPPP)(8) adopts an amphipathic polyproline II conformation. In a preliminary recent work we used atomic force microscopy to study the surface organization of the octamer and transmission electron microscopy to visualize aggregates of the peptide from aqueous solution. We previously envisioned two self-assembly models (i.e., the geometric and the micellar) that take into account the observed features. In the present work we studied in detail the self-assembly of the peptide in solution and found that the peptide is able to form cylindrical micelles. Fibrils formed on graphite are generated by assembly of solution micelles. Based on the results of these studies, we focused exclusively on the micellar model. To our knowledge we have characterized for the first time supramolecular aggregates of polyproline structures other than collagen. The spontaneous arrangement of (VHLPPP)(8) suggests a role for the N-terminal domain of gamma-zein in the process of the whole protein deposition in protein bodies.

Novel preparation methods for highly concentrated water-in-oil emulsions

Abstract Kinetically stable water-in-oil (W/O) high-internal-phase-volume-ratio emulsions with gel-like appearance have been formed in water/hydrogenated non-ionic surfactant/oil systems. Their visual aspect varies from transparent to translucent or white depending on composition variables and temperature. Systematic studies undertaken to characterize these emulsions, referred to as gel emulsions, have revealed that they form above the hydrophilic-lipophilic balance (HLB) temperature of the corresponding ternary system. They consist of two isotropic liquid phases; the dispersed phase is composed of aqueous droplets and the continuous phase is a W/O microemulsion. These emulsions can be prepared by gradual addition of the internal phase to the external phase while stirring, the most common method for preparing highly concentrated emulsions. In the ternary systems water/non-ionic surfactant/hydrocarbon we found two new procedures for preparing gel emulsions. (a) Mixing of the three components, at their final composition, with vigorous stirring, can lead, at the appropriate temperature, to gel emulsion formation. (b) Increasing the temperature of an isotropic phase, the composition of which is that of the final emulsion. These methods of preparation have been rationalized in terms of the evolution of system properties during the process.

Conformational Changes in Stratum Corneum Lipids by Effect of Bicellar Systems

Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy was applied to study the effects of the bicelles formed by dimyristoyl-glycero-phosphocholine (DMPC) and dihexanoyl-glycero-phosphocholine (DHPC) in porcine stratum corneum (SC) in vitro. A comparison of skin samples treated and untreated with bicelles at different temperatures was carried out. The analysis of variations after treatment in the position of the symmetric CH2 stretching, CH2 scissoring, and CH2 rocking vibrations reported important information about the effect of bicelles on the skin. Bicellar systems caused a phase transition from the gel or solid state to the liquid crystalline state in the lipid conformation of SC, reflecting the major order-disorder transition from hexagonally packed to disordered chains. Grazing incidence small and wide X-ray scattering (GISAXS and GIWAXS) techniques confirmed this effect of bicelles on the SC. These results are probably related to with the permeabilizing effect previously described for the DMPC/DHPC bicelles.

Unconventional vesicle-to-ribbon transition behaviour of diacyl glycerol amino acid based surfactants in extremely diluted systems induced by pH-concentration effects

A vesicle-to-ribbon transition has been observed in extremely dilute 1-O-(L-arginyl)-2, 3-O-di-decanoyl-sn-glycerol dichlorohydrate (1010R) surfactant systems by means of static light scattering. At concentration as low as 0.005 mM the solutions scattered significantly. From the angular dependence and molecular weight of the aggregates a vesicular structure is suggested. Increasing surfactant concentration in water induces the vesicle-to-ribbon transition at concentrations as low as 0.5 mM. This transition is accompanied by a strong decrease of scattered intensity and change in angular dependence. Lowering the pH at a fixed concentration can induce the same transition. Both parameters change the protonation of the surfactant, inducing an increase in preferred surfactant head-group area. Those findings are congruent with the observed surface tension behaviour as a function of surfactant concentration and suggests an explanation for the widely different critical micellar concentration (c.m.c.) as determined by different techniques.

Cross-linked chitosan/liposome hybrid system for the intestinal delivery of quercetin

Quercetin is a flavonoid with antioxidant/anti-inflammatory properties, poorly absorbed when administered orally. To increase its bioavailability and optimize its release in the intestine, a hybrid system made of liposomes coated with cross-linked chitosan, named TPP-chitosomes, was developed and characterized by light scattering, transmission electron microscopy, differential scanning calorimetry, X-ray powder diffraction and Turbiscan® technology. The TPP-chitosomes were nanosized (∼180 nm), fairly spherical in shape and unilamellar. The actual coating of the surface of liposomes with the cross-linked chitosan was demonstrated by Small-Angle X-ray Scattering. The release of quercetin in simulated gastric and intestinal pH was investigated, the results showing that the system provided resistance to acidic conditions, and promoted the release in alkaline pH, mimicking the intestinal environment. The proposed hybrid system represents a promising combination of nanovesicles and chitosan for the delivery of quercetin to the intestine in the therapy of oxidative stress/inflammation related disorders.

Isolation and partial characterization of a biosurfactant mixture produced by Sphingobacterium sp. isolated from soil

Strain 6.2S, isolated from soil and identified as a Sphingobacterium sp., is the first strain in this genus to be reported as a biosurfactant producer, being able to reduce the surface tension of its culture supernatant to 32 mN/m. In this work, biosurfactants from the culture supernatant were purified and partially characterized. The crude extract (10 g/L) was very effective in reducing surface tension (22 mN/m). Thin layer chromatography (TLC) indicated that a mixture of various biosurfactants was present in the 6.2S crude extract. After purification, Fraction A, a phospholipid mixture, reduced surface tension to 33 mN/m. Fraction B was a mixture of lipopetides and at least one glycolipid. The surface tension-concentration curve showed two plateaux, the first of which can be attributed to a critical aggregation concentration of the biosurfactant with a protein (2.7 g/L) and the second to the true cmc in water (6.3g/L).

Application of Bicellar Systems on Skin: Diffusion and Molecular Organization Effects

The effect of bicelles formed by dipalmitoylphosphatidylcholine (DPPC)/dihexanoylphosphatidylcholine (DHPC) on stratum corneum (SC) lipids was studied by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy at different temperatures. Analysis of the lipid organization in terms of chain conformational order and lateral packing shows that the use of bicelles hampers the fluidification of SC lipids with temperature and leads to a lateral packing corresponding to a stable hexagonal phase. Grazing incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS) techniques confirm these results and give evidence of higher lamellar order after treatment with these bicelles. Additionally, the effects of DPPC/DHPC and dimyristoylphosphatidylcholine (DMPC)/DHPC bicelles at different SC depths were compared. The combination of ATR-FTIR spectroscopy and the tape-stripping method was very useful for this purpose.