Interaction of a new anticancer prodrug, gemcitabine-squalene, with a model membrane: coupled DSC and XRD study.

Gemcitabine is an anticancer nucleoside analogue active against a wide variety of solid tumors. However it is rapidly deaminated to an inactive metabolite, leading to short biological half-life and induction of resistance. A new prodrug of gemcitabine, coupling squalene to gemcitabine (GemSq), has been designed to overcome the above drawbacks. It has been previously shown that this prodrug displays significantly higher anticancer activity than gemcitabine against leukemia. In the present study the structural modifications of dipalmitoylphosphatidylcholine (DPPC) model membranes induced by increasing concentrations of GemSQ have been investigated using small and wide angle X-ray scattering (SWAXS) and differential scanning calorimetry (DSC). At room temperature an unusual inverse bicontinuous cubic phase formed over a broad composition range. The basic bilayer structure displayed an intermediate order between those of the gel and fluid phases of DPPC. A reversible transition to a fluid lamellar phase occurred upon heating. The transitions between these two phases were governed by different mechanisms depending on the GemSq concentration in the membrane. Finally, the biological relevance of these observations for the cytotoxic activity of GemSq has been discussed.

Compaction of crystallographic forms of pharmaceutical granular lactoses. II. Compacts mechanical properties.

It is well known that the choice of the crystal form affects the physicochemical properties such as compaction behaviour. In this work, the mechanical properties of compacts obtained from compaction of lactoses by using a micropress prototype are calculated. Tensile strength, Young's modulus, toughness and Brinell hardness were measured and used to compare the various crystalline forms: alpha-lactose monohydrate (LalphaM), anhydrous beta-lactose (LbetaA), anhydrous alpha-lactose (LalphaA) and partly amorphous lactose (FF). With all the mechanical properties measured, the lactoses could be differentiated. Then, the specific energy of failure G*(IC) was obtained from the toughness and the Young's modulus for each lactose. LalphaM showed small specific energy of failure due to its low toughness which is not balanced by its Young's modulus. The highest values were obtained with the two anhydrous forms, LalphaA and LbetaA. Finally, these mechanical properties were linked with general compaction behaviour and cohesive energy density which is a characterization at a molecular level.

Thermal and structural behavior of milk fat. 3. Influence of cooling rate and droplet size on cream crystallization.

Crystallization of triacylglycerols (TG) within milk fat globules of creams is studied with an instrument coupling time-resolved synchrotron X-ray diffraction (XRDT) at both small and wide angles and high-sensitivity differential scanning calorimetry (DSC) at cooling rates of -3 and -1 degrees C/min from 60 to -10 degrees C and compared to that of the anhydrous milk fat (AMF). Simultaneous thermal analysis permits correlation of the formation of the different crystalline species monitored by XRDT to the DSC events. Under the above cooling conditions, milk fat TG sequentially crystallize, within the globules, from about 19 degrees C, in three different lamellar structures with double-chain length (2L) stackings of 47 and 42 A and a triple-chain length (3L) stacking of 71 A, all of alpha type, which are correlated to two or three overlapped exothermic peaks recorded by DSC. Compared to what is observed for AMF, TG crystallization in emulsion (i) favors sub-alpha formation at low temperature and (ii) induces layer stacking defects in 3L crystals. Subsequent heating at 2 degrees C/min shows numerous structural rearrangements before final melting, confirming that (i) cooling at -1 degrees C/min leads to the formation of unstable crystalline varieties in the dispersed state and (ii) a monotropic transition alpha-->beta' takes place. Similar behavior is observed for cooling at -3 degrees C/min and subsequent heating. An overall comparison of the thermal and structural properties of the crystalline species formed as a function of the cooling rate, between >1000 and 0.15 degrees C/min, and stabilization time at 4 degrees C is given. Depending on the cooling rate, at least five crystalline subcell species are observed at wide angles, alpha and sub-alpha, two beta' and one beta. At small angles, at least six lamellar stackings are identified, three 3L and three 2L. However, a single subcell packing (e.g., alpha) might correspond to several longitudinal chain stackings, demonstrating the usefulness of the small-angle XRD technique. Reconstituted emulsions homogenized under different pressures are used to determine the influence of droplet size on crystallization. The decrease of droplet size induces (i) a higher supercooling/supersaturation and (ii) a higher disorder and/or a smaller size of TG crystals within the emulsion droplets. At the supramolecular scale, polarized light microscopy shows that various cooling rates applied in situ using a temperature-controlled stage directly influence crystal sizes and their type of organization within milk fat globules. The faster the cooling rate, the smaller the size of the crystals within the globules.

Thermal and Structural Behavior of Milk Fat.

Milk fat crystallization was studied using X-ray diffraction as a function of temperature (XRDT) and differential scanning calorimetry (DSC) analysis considering crystals formed during slow cooling of natural milk fat globules of cream. During cooling at |dT/dt|=0.15 degrees C/min from 55 to -8 degrees C, the crystalline varieties formed in fat globules by triacyglycerols (TGs) correspond to two double-chain-length organizations (2L) of 46.5 and 40 Å and to two triple-chain-length stackings (3L) of 71.3 and 65 Å. Nucleation occurs in the alpha form; then the alpha+beta' polymorphic forms coexist until the end of the cooling. The four crystalline varieties start to form within a 10 degrees C range, from about 21 degrees C, preventing separation of overlapped peaks by DSC recording. In a second step, the sample of cream was heated at 2 degrees C/min in the range -8 to +60 degrees C to follow the melting behavior of the crystals. XRDT measurements show the progressive transformations of the crystalline varieties correlated with endotherms and exotherms recorded by DSC. The 40-Å structure takes advantage of the melting of the other species to grow until its melting. The comparison made with anhydrous milk fat behavior under the same conditions shows that crystallization is different in emulsion and in bulk. Copyright 2001 Academic Press.

Interaction between artificial membranes and enflurane, a general volatile anesthetic: DPPC-enflurane interaction.

The structural modifications of the dipalmitoylphosphatidylcholine (DPPC) organization induced by increasing concentration of the volatile anesthetic enflurane have been studied by differential scanning calorimetry, small-angle, and wide-angle x-ray scattering. The interaction of enflurane with DPPC depends on at least two factors: the enflurane-to-lipid concentration ratio and the initial organization of the lipids. At 25 degrees C (gel state), the penetration of enflurane within the lipids induces the apparition of two different mixed lipid phases. At low anesthetic-to-lipid molar ratio, the smectic distance increases whereas the direction of the chain tilt changes from a tilt toward next-neighbors to a tilt between next-neighbors creating a new gel phase called L(beta')(2NNN). At high ratio, the smectic distance is much smaller than for the pure L(beta') DPPC phase, i.e., 50 A compared to 65 A, the aliphatic chains are perpendicular to the membrane and the fusion temperature of the phase is 33 degrees C. The electron profile of this phase that has been called L(beta)(i), indicates that the lipids are fully interdigitated. At 45 degrees C (fluid state), a new melted phase, called L(alpha)(2), was found, in which the smectic distance decreased compared to the initial pure L(alpha)(1) DPPC phase. The thermotropic behavior of the mixed phases has also been characterized by simultaneous x-ray scattering and differential scanning calorimetry measurements using the Microcalix calorimeter of our own. Finally, titration curves of enflurane effect in the mixed lipidic phase has been obtained by using the fluorescent lipid probe Laurdan. Measurements as a function of temperature or at constant temperature, i.e., 25 degrees C and 45 degrees C give, for the maximal effect, an enflurane-to-lipid ratio (M/M), within the membrane, of 1 and 2 for the L(alpha)(2) and the L(beta)(i) lamellar phase respectively. All the results taken together allowed to draw a pseudo-binary phase diagram of enflurane-dipalmitoylphosphatidylcholine in excess water.

Biomimetic organization: Octapeptide self-assembly into nanotubes of viral capsid-like dimension.

The controlled self-assembly of complex molecules into well defined hierarchical structures is a promising route for fabricating nanostructures. These nanoscale structures can be realized by naturally occurring proteins such as tobacco mosaic virus, capsid proteins, tubulin, actin, etc. Here, we report a simple alternative method based on self-assembling nanotubes formed by a synthetic therapeutic octapeptide, Lanreotide in water. We used a multidisciplinary approach involving optical and electron microscopies, vibrational spectroscopies, and small and wide angle x-ray scattering to elucidate the hierarchy of structures exhibited by this system. The results revealed the hexagonal packing of nanotubes, and high degree of monodispersity in the tube diameter (244 A) and wall thickness (approximately equal to 18 A). Moreover, the diameter is tunable by suitable modifications in the molecular structure. The self-assembly of the nanotubes occurs through the association of beta-sheets driven by amphiphilicity and a systematic aromatic/aliphatic side chain segregation. This original and simple system is a unique example for the study of complex self-assembling processes generated by de novo molecules or amyloid peptides.