Role of galactomannan composition on the binary gel formation with xanthan

The influence of the galactomannan characteristic ratios (M/G) on the temperature of gelation (Tg) and the gel strength of mixtures of galactomannan with xanthan is reported. Two galactomannans were investigated: one highly substituted from the seeds of Mimosa scabrella (M/G = 11), and the other, less substituted, from the endosperm of Schizolobium parahybae, with (M/G = 30) [Ganter JLMS, Zawadzki-Baggio SF, Leitner SC, Sierakowski MR, Reicher F. J Carbohydr Chem 1993;12:753]. The xanthan:galactomannan systems (4:2 g l(-1), in 5 mM NaCl) showed a temperature of gel formation (Tg) of 24 degrees C for that of S. parahybae [Bresolin TMB, Milas M, Rinaudo M and Ganter JLMS. Int J Biol Macromol 1998;23:263] and 20 degrees C for the galactomannan of M. scabrella, determined by viscoelastic measurements and microcalorimetry. A Tg of 40-50 degrees C was found by Shatwell et al. [Shatwell KP, Sutherland IW, Ross-Murphy SB, Dea ICM. Carbohydr Polym 1991;14:29] for locust bean gum-LBG (M/G = 43). Lundin and Hermansson [Lundin L, Hermansson AM. Carbohydr Polym 1995;26:129] reported a difference of 13 degrees C for Tg of two LBG samples with M/G = 3 (40 degrees C) and 5 (53 degrees C), in mixtures with xanthan. It appears that the more substituted galactomannans have lower temperatures of gelation in the presence of xanthan. The mechanism of gelation depends also on the M/G ratio. For the lower values it involves only disordered xanthan chains in contrast to M/G ratios higher than 3. In addition, the presence of the galactomannan from M. scabrella increased slightly the temperature of the conformational change (Tm) of xanthan probably due to the ionic strength contribution of proteins (3.9%) present in the galactomannan. On the other hand, the galactomannans from S. parahybae, with 1.5% of proteins and M. scabrella, with 2.4% of protein, did not show this effect, the Tm of xanthan alone or in a mixture being practically unchanged.

Conformational transition of native and modified gellan

This paper concerns the characterisation of native gellan by differential scanning calorimetry (DSC) and rheology. The stability of the double helix is characterised by Tm and the enthalpy of conformational change. The role of the external salt concentration is investigated; it is shown that Tm is only slightly modified. At ambient temperature, in 10(-2) M NaCl, native gellan behaves as a loose gel (G' > G''). This behaviour disappears when temperature is larger than 60 degrees C. The comparison with deacylated gellan (commercial sample) shows that the position of conformational transition is much more influenced by the salt concentration; the helical structure is less stable and the conformational transition presents a hysteresis between heating and cooling runs when the external salt concentration increases. The rheological behaviour is that corresponding to a solution (G' < G'') at ambient temperature and in 10(-2) M NaCl. When the salt excess increases, then a stronger gel is formed. The differences between the two types of samples are clearly established as well as the relations between the conformation and the rheology of the systems.

Comparison between the physicochemical behaviour of two microbial polysaccharides: RMDP17 and rhamsan

This paper concerns the properties of two ionic polysaccharides with very close chemical structures. pH metric and conductimetric measurements showed that they behave similarly from a polyelectrolytic point of view. From optical rotation and differential scanning calorimetry (DSC) measurements, the two polymers probably adopt a double helical conformation which is destabilised by deacetylation. The main differences concern the stability of this ordered conformation and the ability of these double helices to associate to form gels. The results support a higher thermal stability of the ordered conformation for deacetylated RMDP17 (about 8 degrees C), whereas deacetylated rhamsan has a better ability to form gels.

Physico-chemical characterization of Ca-alginate microparticles produced with different methods

In the present paper the physico-chemical characterization of Ca-alginate microparticles produced with different methods is presented. Ca-alginate microparticles were obtained either by emulsification method or by dripping an aqueous alginate solution into a solution of calcium salt. Inverse Size Exclusion Chromatography (ISEC) was used for the determination of dimensions of the pores and porous volume of microparticles having a mean diameter of 220 microm when obtained by emulsification method. The same technique was used to study the variation of the pore size and porous volume with pH. The results were related with the content of calcium and sodium in the microparticles, before and after their treatment with different HCl solutions. For the particles with a mean diameter of 1.2 mm (obtained by dripping method) we adopted an other approach based on the steric exclusion of solute at equilibrium. For a convenient interpretation of the obtained data, determination of water regain, swelling degree, and scanning electron microscopy (SEM) were performed. Finally, a comparison of the characteristics of microparticles produced by ionic and epichlorohydrin crosslinkings was made. The maximum dimensions of the pores of the microparticles obtained by emulsification were found smaller than those obtained by other technics. The variation of the dimensions of the pores and porous volume with pH is not significant. The structure of the chemically crosslinked beads with epichlorohydrin is more elastic and the swelling is reversible; after drying and reswelling process, the dimensions of the pores and porous volume of these microparticles remain unchanged. On the opposite, for the microparticles obtained by emulsification or dripping method in the presence of calcium ions, these characteristics are changed after a first drying process.

On the interchain associations in aqueous solutions of a succinoglycan polysaccharide

Formation of interchain associations between succinoglycan chains have been studied by comparing weight average molecular weight, intrinsic viscosity of succinoglycan as a function of the conditions to prepare the solutions (polymer concentration, the heating temperature adopted compared with Tm). The different solutions obtained were characterized by their Newtonian viscosity, the storage and loss moduli and their sensitivity to the temperature. It was found that interchain associations, first stabilized mainly during the disorder-order transition convert to more stable associations by aging at temperatures below but not too far from Tm. These associations appear from succinoglycan solutions characterized by an overlap parameter higher than about 8 and modify only very slightly the conformational transition parameters obtained from microcalorimetry measurements.

Xanthan-galactomannan interactions as related to xanthan conformations

The influence of xanthan conformation on the physicochemical behaviour of their mixtures with galactomannan from Schizolobium parahybae mannose:galactose ratio (M/G=3), was studied by viscoelastic measurements, differential scanning calorimetry (DSC) and chiroptical (circular dichroism) methods. The results suggested a more effective interaction of the galactomannan with disordered xanthan segments, which are more abundant in low salt concentrations but are still present in lower proportion at temperatures lower than the temperature of xanthan conformational transition (Tm). The dependence of ellipticity with temperature in a circular dichroism (CD) spectra suggested an ordering of the xanthan chains induced by galactomannan at the temperature of gel formation (Tg approximately 25 degrees C), under conditions where xanthan alone exhibits a disordered conformation. The lower Tg value found (approximately 25 degrees C) compared with that (60 degrees C) usually described in the literature is certainly related to the M/G ratio and the galactosyl unit distribution along the mannan main chain.

Evaluation of molecular strategies to develop a live dengue vaccine

BACKGROUND

Millions of individuals are estimated to become infected with dengue virus each year, particularly in tropical and subtropical regions. Mortality is low but infection can lead to a severe form of dengue, characterised by haemorrhage and shock. A safe and effective vaccine against dengue is still not available.

OBJECTIVE

To use the successful construction of dengue type 4 virus (DEN4) cDNA, which yields infectious RNA transcripts, to provide a new approach to the development of safe and effective dengue vaccines.

STUDY DESIGN

The 3' and 5' noncoding (NC) regions of the genome were targeted to construct DEN4 deletion mutants, because the sequences in these regions are thought to play an important role in the regulation of viral replication. DEN4 cDNA was also employed to construct a viable chimeric virus with dengue type 1, 2 or 3 antigenicity, by substitution of heterotypic structural protein genes.

RESULTS

Most viable mutants, recovered from the cDNA constructs, were partially restricted for growth in simian cells as analysed by plaque morphology assay and viral yield analysis. Several 3' NC deletion mutants which exhibited a range of growth restriction in cell culture were further evaluated for infectivity and immunogenicity in rhesus monkeys. Occurrence and duration of viraemia were reduced for these deletion mutants, compared to the wild type DEN4. Analysis of antibody response to infection in rhesus monkeys also indicated that some of these mutants were attenuated. These DEN4 deletion mutants represent promising live dengue vaccine candidates that merit further clinical evaluation. Chimera DEN1/DEN4 or DEN2/DEN4 which expresses DEN1 or DEN2 antigenicity were also used to infect monkeys. Most monkeys immunised with these chimeric viruses, singly or in combination, developed high titres of neutralising antibodies and were protected against homotypic wild type DEN1 or DEN2 challenge.

CONCLUSIONS

DEN4 and its derived chimeric viruses of other three dengue serotype specificity, that contain appropriate attenuating mutations, have a potential use in a tetravalent live vaccine against dengue.

A phosphatidylcholine phospholipase C inhibitor, D609, blocks interleukin-3 (IL-3)-induced bcl-2 expression but not c-myc expression in human IL-3-dependent cells

Activation of the interleukin-3 (IL-3) receptor is required for the induction of cell proliferation and suppression of apoptosis in primitive hematopoietic progenitor cells. A rapid activation of tyrosine kinases and a phosphatidylcholine-specific phospholipase C has been observed in these cells in response to IL-3. The signal transduction cascades regulating cell proliferation and the suppression of apoptosis are poorly understood. Using human IL-3-dependent TF-1 cells, we have found that the tyrosine kinase inhibitor genistein blocks both the IL-3 suppression of apoptosis and the expression of the cell survival gene bcl-2. In addition, we have found that D609, a specific inhibitor of phosphatidylcholine-specific phospholipase C, also inhibits IL-3-induced expression of the bcl-2 gene without affecting IL-3-induced tyrosine phosphorylation. D609 also drove these cells into apoptosis even in the presence of IL-3. Significantly, genistein inhibited the IL-3 induction of both bcl-2 and c-myc gene. The latter gene is related to the induction of cell proliferation. D609, however, blocked the induction only of the cell survival gene bcl-2. Thus, phosphatidylcholine hydrolysis appears linked to the induction of genes related to cell survival. These data fit with the hypothesis that there is a bifurcation in the signaling pathways downstream of IL-3 receptor-induced tyrosine phosphorylation.

Cassia spectabilis DC seed galactomannan: structural, crystallographical and rheological studies

The seeds of Cassia spectabilis DC (family: Leguminoseae), an Indian fast growing spreading tree, contain about 40% of endosperm and possess the characteristics of becoming a potential source of commercial gum. The purified galactomannan shows Mw 1.1 x 10(6), intrinsic viscosity [eta] 615ml/g with k' = 1.706 x 10(-1), and a mannose to galactose ration of 2.65. The hydrolysis of the fully methylated polysaccharide reveals clearly the expected structure of legume galactomannans. The orthorhombic lattice constants of the hydrated gums are as follows: a = 9.12 A, b = 25.63 A and c = 10.28 A. The results of X-ray fiber studies show that the b dimension of the unit cell is very sensitive to relative humidity (RH), galactose substitution and orientation of the films. The probable space group symmetry of the unit cell is P2(1)2(1)2. Rheological studies of the galactomannan have shown that the transition from semi-dilute to dilute regime occurs at a critical concentration Cc* = 2.75. The slope of the log-log plot of specific viscosity versus C at zero shear rate is 5.87 in the more concentrated regime. The viscoelastic and critical shear rate behavior indicate the characteristics of a coil polymer. The large dependence of the viscosity on the coil overlap parameter is probably due to polymer-polymer interactions and peculiarity of the galactose distribution along the chain. Above 20 g/L concentration, the rheological behavior of the gum is like the one of a weak-gel.

Order-disorder conformational transition in succinoglycan: calorimetric measurements

Differential scanning microcalorimetry was performed on succinoglycan samples from different sources in order to better understand the thermally induced order-disorder conformational transition of the polysaccharide. The shape of thermograms, as well as the melting temperature, were related to the content or distribution of the succinate groups at least in salt-free solution. With increasing polysaccharide and/or salt concentration, the change in the shape of the thermogram was attributed to a progressive screening of the succinate contribution to the order-disorder conformational transition instead of a change in the transition mechanism. In the presence of salt, contrary to the rheological behavior, the calorimetric results were found to be independent of the thermal history of the samples. This suggests a very low enthalpic contribution of the interchain interactions present in succinoglycan solutions. Possible contributions that could explain some discrepancies with results already published in the literature are discussed.

On the specific role of coions and counterions on kappa-carrageenan conformation

The role of the nature and concentration of counterions and coions on the conformation of kappa-carrageenan is discussed; particularly, that of iodide compared with chloride based on the results obtained from optical rotation, viscometry and microcalorimetry. Iodide gives a more stable helical conformation, with a larger degree of order; at the same time, it represses aggregation of the helices and the gelation ability. Concerning the ordered conformation obtained in the presence of iodide coions, the results obtained are not conclusive. From the enthalpy of the conformational change, it seems that, even in the presence of iodide, the ordered conformation remains a double helix. On the other hand, no doubling of molecular weight was obtained by MALLS/GPC. Thus, this aspect remains still under discussion.