Yurii A. Shchipunov

Sol–gel-derived biomaterials of silica and carrageenans

A new precursor, tetrakis(2-hydroxyethyl) orthosilicate (THEOS), introduced by Hoffmann et al. (J. Phys. Chem. B 106 (2002) 1528-1533), was used to synthesize monolithic hybrid biomaterials on the basis of silica and three main types of carrageenans, kappa-, iota-, and lambda-carrageenans. The advantage of THEOS over the currently applied TEOS and TMOS is in its complete solubility in water. This negated the need to add organic solvents, thus excluding a denaturating effect on biopolymers. In their turn, carrageenans introduced into the precursor solution made use of common catalysts unneeded to trigger the sol-gel transition. It was found that they promoted the mineralization, acting as a template for the inorganic component. The kinetics of sol-gel processes, mechanical properties, phase behavior, and structure of novel hybrid biomaterials were studied by dynamic rheology, differential scanning calorimetry, and scanning electron microscopy. The material properties were regulated by both the precursor and carrageenan. The increase of silicate concentration led to a rise in the stiffness and brittleness of the material, whereas the polysaccharide addition made it softer and more elastic. It was shown that the formation and properties of mixed gels were determined by the nature of carrageenan. kappa-Carrageenans brought about shrinkage of hybrid materials that led to water separation, while iota- and lambda-carrageenans did not induce the syneresis. This is in line with the difference in polysaccharide properties when they are in aqueous solutions without silicate. Furthermore, kappa- and iota-carrageenans experienced a thermoreversible phase transition in the hybrid materials owing to the helix-coil transition. This resulted in a step like change in the mechanical properties of mixed systems in the corresponding temperature range. lambda-Carrageenan is a nongelling polysaccharide, which is why the rheological parameters of its hybrid gel were unchanged with the temperature. It was found that the polysaccharides modified the structure of silica-based materials. They transformed a three-dimensional network of connected silica particles into that consisting of crossed fibers.

A new precursor for the immobilization of enzymes inside sol-gel-derived hybrid silica nanocomposites containing polysaccharides

Tetrakis(2-hydroxyethyl) orthosilicate (THEOS) introduced by Hoffmann et al. (J. Phys. Chem. B., 106 (2002) 1528) was first used to prepare hybrid nanocomposites containing various polysaccharides and immobilize enzymes in these materials. Two different types of O-glycoside hydrolyses (EC3.2.1), 1-->3-beta-D-glucanase LIV from marine mollusk Spisula sacchalinensis and alpha-D-galactosidase from marine bacterium Pseudoalteromonas sp. KMM 701, were taken for the immobilization. To reveal whether the polysaccharide inside the hybrid material influences the enzyme entrapment and functioning, negatively charged xanthan, cationic derivative of hydroxyethylcellulose and uncharged locust bean gum were examined. The mechanical properties of these nanocomposites were characterized by a dynamic rheology and their structure by a scanning electron microscopy. It was found that 1-->3-beta-D-glucanase was usually immobilized without the loss of its activity, while the alpha-D-galactosidase activity in the immobilized state depended on the polysaccharide type of material. An important point is that the amount of immobilized enzymes was small, comparable to their content in the living cells. It was shown by the scanning electron microscopy that the hybrid nanocomposites are sufficiently porous that allows the enzymatic substrates and products to diffuse from an external aqueous solution to the enzymes, whereas protein molecules were immobilized firmly and not easily washed out of the silica matrix. A sharp increase of the enzyme lifetime (more than a hundred times) was observed after the immobilization. As established, the efficient entrapment of enzymes is caused by few advantages of new precursor over the currently used TEOS and TMOS: (i) organic solvents and catalysts are not needed owing to the complete solubility of THEOS in water and the catalytic effect of polysaccharides on the sol-gel processes; (ii) the entrapment of enzymes can be performed at any pH which is suitable for their structural integrity and functionality; (iii) a gel can be prepared at reduced concentrations of THEOS (1-2%) in the initial solution that excludes a notable heat release in the course of its hydrolysis.

Lecithin bridging by hydrogen bonds in the organogel

A series of polar solvents have been studied in order to reveal those capable of producing the thickening effect on hydrocarbon solutions of lecithin. It has been established that glycerol, formamide and ethylene glycol, in addition to water (previously known), have the ability to induce organogel formation. The IR results show that molecules of both the gel-forming solvents and non-gel-forming ones are attached to the lecithin phosphate group via hydrogen bonds. The latter influence the mobility of the choline residue. The ability to promote thickening of lecithin solutions has correlated with the polarity of molecules. The correlation is particularly pronounced in the series of such structurally related compounds as glycerol, ethylene glycol and 1,3-propanediol. It has been inferred from the results that the difference between gel-forming and non-gel-forming polar solvents is caused by their orientation and localization in the polar moiety of a lecithin molecule. In a proposed model of the organogel, the solvent molecules bridge phosphate groups of neighbouring lipid molecules, thus allowing their association into tubular aggregates through an extensive ribbon-like hydrogen bonding network.

Hybrid organic–inorganic nanocomposites fabricated with a novel biocompatible precursor using sol-gel processing

The synthesis of hybrid organic–inorganic nanocomposite materials containing biopolymers by the sol-gel processing is often faced with severe difficulties owing to their poor compatibility with the common precursors. The problem was solved when tetrakis(2-hydroxyethyl) orthosilicate (THEOS) was recently suggested for the preparation of sol-gel derived nanocomposites. This article summarizes recent data on novel types of hybrid polysaccharide-silica nanocomposite materials fabricated with the help of THEOS. Their formation, properties and structure as well as features and possible mechanism for the sol-gel processes are considered. It is demonstrated that the novel approach is particularly suitable in situations where others are unsatisfactory. This includes systems of which supramolecular organization or phase state is sensitive to the pH of solutions, temperature and addition of organic solvents. One example of a successful application is the immobilization of labile enzymes. The biocompatibility of THEOS allows their activity to be retained, thereby enhancing long-term and thermal stabilities.

Phospholipids at the oil/water interface: adsorption and interfacial phenomena in an electric field

Interfacial effects produced in an immiscible liquid system by the action of an external electric field have been considered. The addition of small amounts of neutral phospholipids to the nonaqueous phase has been shown to result in a marked increase in the sensitivity of the interfacial boundary to the voltage applied, which is manifested by: (i) an accelerated decrease of the interfacial tension after the two immiscible liquid phases have been brought into contact; (ii) reduced interfacial tension, by 20-30 mN/m, at the oil/water interface at field strengths of 1-10 kV/m (the interfacial tension drop in the absence of phospholipids does not exceed 5 mN/m); (iii) development of electrohydrodynamic instability at the planar dividing surface between phases; and (iv) dispersion of water into the nonaqueous phase at smaller field strengths by a factor of about 100 as compared to those normally required in the absence of phospholipids. In order to gain a deeper insight into the mechanisms of interfacial phenomena, mainly exemplified by the n-heptane/water system containing phosphatidylcholine, three major issues have been considered: (1) Kinetics of the adsorption of phospholipid at the oil/water interface from the nonaqueous phase, and effects produced by exposure to an external electric field; also, the adsorption under equilibrium conditions, and the structure of the adsorption layer formed. (2) Interactions between neutral phospholipid and inorganic or organic ions at the interfacial boundary under the voltage applied. (3) Conditions for the occurrence of electrohydrodynamic instability at the dividing surface between oil and water and the formation of a water-in-oil emulsion; also aggregation and gelation processes induced in the nonaqueous phospholipid solution bulk by the action of a weak external electric field. Throughout the present paper, an attempt has been made to relate the microscopic behaviour of phospholipids under an external electric field to macroscopically observable properties at the movable interfacial boundaries. The adsorption studies have shown that phosphatidylcholine is prone to self-organization into a liquid-crystalline state at an immiscible liquid interface. The disintegration of the interfacial lipid film thus formed by the action of a weak electric field has been explained as due to an enhanced electrohydrodynamic instability of liquid crystals. This results in the formation of either an emulsion, or a microemulsion in the nonaqueous solution bulk. The formation of a microemulsion is manifested by the appearance of an optically anisotropic gel, stable only under an external applied electric field, in the nonaqueous solution bulk.(ABSTRACT TRUNCATED AT 400 WORDS)

Adsorption at the interface of two immiscible liquids and association of surface-active substances in bulk phase. 1. Application of the modified gibbs equation

On the basis of the literature and our own experimental data, the adsorption and association of sodium dodecyl sulfate and cetyltrimethylammonium bromide were examined. The number (n) of molecules in the aggregate was found by means of the Freundlich adsorption isotherm, and the adsorption of associated surface-active substances is described by the modified Gibbs equation: dπ = 2RTΓ/nd In C, presented in (Shchipunov, Yu. A., Zh. Fiz. Khim. 56, 2783 (1982)). Discussion of experimental data was devoted to demonstration of the reality of estimations obtained from measurement of interfacial tension of the beginning of aggregate formation in the aqueous bulk phase and of the calculating areas occupied by molecules of surface-active substances in the adsorption layer.

The formation of anisotropic mesophase of phosphatidylcholine in alkane/water system by the action of electric field

Abstract The formation of a phosphatidylcholine anisotropic mesophase in n-heptane induced by the action of weak electric fields has been observed. As evidenced by preliminary results, the mesophase is a microemulsion formed by swollen lipid micelles.

The formation of bilayer membranes and liquid-crystalline properties of amphiphiles☆

Abstract Dialkylammonium salts and alkylethylenediamine derivatives have been shown to be capable of forming planar bilayer membranes and myelin figures in aqueous solutions when the amphiphiles contain charged functional groups. This is a major distinction between the species studied and neutral phospholipids, whose liquid crystals swell in water due mainly to repulsive hydration forces. As has been shown, the charge on the molecule is not conducive to an effective stabilization of bilayer membranes. It has been suggested that stable bimolecular films can be generated from amphiphiles carrying functional groups capable of adding a significant number of water molecules to the hydration shell.

Interaction of ions with phosphatidylcholine and phosphatidylethanolamine at the n-heptane/water interface: II. Hydrophobic ions

Abstract Electrocapillary phenomena at the n-heptane/water interface in the presence of phosphatidylcholine or phosphatidylethanolamine in alkane were studied by the drop-volume technique. The regulating role of the electric field was established in phospholipid interaction with inorganic ions: changes in interfacial pressure at the n-heptane/water interface with addition of electrolytes were observed only when an electric field was applied to the system of two immiscible liquids. It is suggested that ions can penetrate and bond by phospholipid functional groups at the phase boundary only after reorientation of the latter in the electric field. As in the case of ionic surfactants, change in interfacial pressure was directly proportional to the cube root of the concentration of electrolyte. It is assumed that as a result of preferential binding by phospholipid functional groups of ions of one sign, lipid molecules are charged and at the phase boundary behave themselves like ionic surfactants. Experiments with salts of multicharged ions, La3+, [Fe(CN)6]3−, and [Fe(CN)6]4−, have shown that mainly phospholipid functional groups bind cations.

Molecular shape of amphiphiles self-organised into bimolecular films

The dependence of the appearance of the black spot in thick nonaqueous films and the formation of bimolecular lipid membranes on the shape of amphiphilic molecules has been studied using a wide-ranging homologous series of alkyl derivatives of diamines. The optimum molecular shape and limits to amphiphile self-organisation into the bimolecular state have been ascertained.