Rheological behaviour of a succinoglycan polysaccharide in dilute and semi-dilute solutions

We report the rheological behaviour of a succinoglycan polysaccharide in dilute and semi-dilute solutions as a function of temperature, ionic strength and the nature of counterion. We have examined the viscosity dependence as a function of molecular weight using samples obtained by ultrasonication. We have also prepared samples lacking succinate substitutes and compared their behaviour with that of the native polymer. In both cases, we observed that, after heating a polymer solution for the first time above the conformational transition temperature, a different ordered state was obtained on cooling. This state had a lower molecular weight and intrinsic viscosity but identical chemical structure and local properties. A role for the side chain in the stabilization of breaks in the backbone is suggested. Nevertheless, a unique curve is obtained for the specific viscosity as a function of the overlap parameter c[eta] for different polymer concentrations of both the native and heated forms. However, different curves are obtained for normal and succinate-free polymers, and the succinate-free polymer is characterized by a lower Huggins constant.     

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 T_m). 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 T_m. 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.     

Conformational transitions in polyelectrolyte adsorption

The mole fraction X of nucleic acid chromophores, adsorbed on cationic and anionic exchange chromatography paper disks, was calculated [O.L. H?rer and A. Grigorescu, Rev. Roum. Med. Virol., 43 (1992) 33] in terms of the standard Debye screening length D (in the 0.01 to 1.2 M ionic strength range), using data from MLSEA (multiple light scattering enhanced absorption) measurements. In contrast to the cases of mononucleotides or double-stranded and relatively rigid DNA structures, highly polymerized and single-stranded RNA shows sigmoidal-shaped X curves, suggesting that polymer adsorption may be regarded as a conformational transition from a three-dimensional statistical coil to a two-dimensional one.     

Application of conductivity studies and polyelectrolyte theory to the conformation and order-disorder transition of xanthan polysaccharide

The conductivity of xanthan (extracellular polysaccharide from Xanthomonas campestris) in the potassium salt form has been studied over the temperature range 5–80°C spanning the order-disorder conformational transition. In salt-free solution data analysis using Manning's polyelectrolyte-conductivity theory gives a charge spacing, b, of 0.58±0.04 nm for the low temperature ordered form, consistent with a single rather than a double helix (b=0.58 and 0.29 nm respectively). In solutions with 0.01 M added KBr the increase in counterion condensation on conformational ordering is found from conductivity studies to be — ^–1= 0.20 ± 0.02, in good agreement with the value 0.20±0.02 using polyelectrolyte-equilibrium theory for the variation of transition-midpoint temperature with added salt determined from opticalrotation data.     

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.     

Solution properties of a new polyelectrolyte derived from the polysaccharide scleroglucan

Selective, quantitative oxidation of pendant 1,6-β-linked glucopyranose residues along scleroglucan chains has yielded a new carboxylated polyelectrolyte. Study of the aqueous solution properties of the latter (named sclerox-I) has disclosed rather anomalous behaviour. Viscosity, calorimetric and chiro-optical measurements show that the interaction of sclerox-I with Ca²⁺ ions leads to a change in conformation of the polyanions, probably of the type random coils → ordered, elongated forms.     

Conformational studies of polysaccharide multiple helices

The possibility of the existence of multiple helices in various homopolysaccharides has been explored by the calculation of conformational-energy contour maps. The structures include homopolymers ofd-xylose,d-glucose,d-mannose, andd-galactose, linked α- and β-(1→2), -(1→3), and -(1→4). A number of double parallel-stranded, double antiparallel-stranded, and triple parallel-stranded helices are predicted, all of which are stabilized by interstrand hydrogen-bonds. At least three of the predicted, multiple-helical structures are known to exist. A classification scheme to predict the probabilities of multiple-helix formation is suggested. Possible structure-function relationships of homopolysaccharides are discussed.     

Conformational transition in DNA on a cold surface

The contour length of DNA fragments, deposited and imaged on mica under buffer, was measured as a function of deposition temperature. Extended DNA molecules (on Ni- and silane-treated surfaces) contract rapidly with falling temperature, approaching the contour length of A-DNA at 2°C. The contraction is not unique to a specific sequence and does not occur in solution at 2°C or on a surface at 25°C, indicating that it arises from a combination of low temperature and surface contact. It is probably a consequence of reduced water activity at a cold surface.     

Conformational investigation on the bacterial polysaccharide xanthan

The conformation of xanthan has been investigated as a function of temperature, ionic strength, and polymer concentration. A reversible transition induced by temperature is demonstrated; the melting temperature (T_M) is directly correlated to the total ionic-strength and is independent of the polymer concentration. Measurements of circular dichroism show that the polysaccharide exists in a combination of only two characteristic conformations (random and ordered), regardless of the temperature and the concentrations of salt and polymer. Hydrodynamic measurements show that the hydrodynamic volume of both conformations is almost constant over the range of temperature investigated. The mechanism proposed by Morris for melting is confirmed, and a multichain process is excluded. The birefringence stability of the concentrated solutions is discussed.     

Conformational behavior of the polysaccharide backbone of murein

The energetically possible conformations for the alternating heteropolysaccharide backbone of murein, consisting of N-acetylglucosamine and N-acetylmuramic acid, were calculated using an empirical approach. The calculations were carried out for regular as well as for random-chain polymers, resulting in a model for the saccharide strands featuring extended chains with a length increment of 0.98–1.02 nm per disaccharide unit and peptide attachment sites at every second saccharide residuum pointing into all directions with propagation angles of 80–100° between consecutive sites.     

Conformational behaviour of some tachykinin C-terminal heptapeptides

The conformational behaviour of substance P, physalaemin and eledoisin C-terminal heptapeptides was investigated using empirical energy calculations. Although the conformational distributions of the three heptapeptides are somewhat different, they have a few common low energy conformations. Some of them, which satisfy the structure-activity relationships, may fit the well known SP-P receptors.     

Accumulation and utilization of polysaccharide by hot-spring phototrophs during a light-dark transition

Abstract One hundred and sixty-four strains of myxobacteria were isolated from soil collected in the British Isles and tested for their antimicrobial activity. The organisms were indentified as belonging to the genera Corallococcus, Cystobacter and Myxococcus with smaller numbers of Archangium, Sorangium and Stigmatella . The majority of the isolates showed antimicrobial activity when overlaid with agar inoculated with test organisms and 77% inhibited Micrococcus luteus and 23% Botrytis cinerea . The activity was shown to be due to antibiotics excreted into the medium and soluble in chloroform. The results are discussed in relation to the importance of myxobacteria in soil ecology.     

Melting behaviour of a triple helical polysaccharide schizophyllan in aqueous solution

Two sonicated samples of schizophyllan in aqueous solution at temperatures from 20 to 160°C were investigated by viscometry. The temperature dependence of the viscosity coefficient η showed that schizophyllan in water undergoes an irreversible thermal transition at about 135°C. The values of $\text{(ln η}{}_{\mathrm{r}}\text{)}\text{c}$ (ηr is the relative viscosity and c is the polymer concentration (w/v)) at 25°C determined after preheating aqueous schizophyllan indicated that the major conformations of schizophyllan in water at 120 and 150°C are triple helix and single random coil, respectively. Thus, it was concluded that the change in η at about 135°C with an increase in temperature is due to the melting of triple helices to single chains. Schizophyllan denatured to single chains at about 150°C did not restore the intact triple helix, but formed aggregates, when the solution was cooled to 25°C. It was also found that the aggregates form a gel when c is higher than a certain value.     

Conformational transition between four and five-stranded phenylalanine zippers determined by a local packing interaction

Alpha-helical coiled coils play a crucial role in mediating specific protein-protein interactions. However, the rules and mechanisms that govern helix-helix association in coiled coils remain incompletely understood. Here we have engineered a seven heptad "Phe-zipper" protein (Phe-14) with phenylalanine residues at all 14 hydrophobic a and d positions, and generated a further variant (Phe-14(M)) in which a single core Phe residue is substituted with Met. Phe-14 forms a discrete alpha-helical pentamer in aqueous solution, while Phe-14(M) folds into a tetrameric helical structure. X-ray crystal structures reveal that in both the tetramer and the pentamer the a and d side-chains interlock in a classical knobs-into-holes packing to produce parallel coiled-coil structures enclosing large tubular cavities. However, the presence of the Met residue in the apolar interface of the tetramer markedly alters its local coiled-coil conformation and superhelical geometry. Thus, short-range interactions involving the Met side-chain serve to preferentially select for tetramer formation, either by inhibiting a nucleation step essential for pentamer folding or by abrogating an intermediate required to form the pentamer. Although specific trigger sequences have not been clearly identified in dimeric coiled coils, higher-order coiled coils, as well as other oligomeric multi-protein complexes, may require such sequences to nucleate and direct their assembly.     

Conformational states governing the rates of spontaneous transition mutations

We report the results of a theoretical study, combining the results of sequence analysis and integral equation structural methods for nucleic acids in aqueous solutions, on the effects of nearest neighbors on the (T.G) mispair in solution, for 12 nearest neighbor contexts. Attempts have been made to classify the structural and energetic effects of the 5′ and 3′ neighbors with respect to the observed spontaneous mutation rates in vertebrates. It is found that 5′ nearest neighbor is probably the most critical structural factor in facilitating or discouraging mutations. Local conformational states correlate with discrimination of bases to be excised in mispairs. Our study confirms the role of the flexibility of the DNA molecule in governing the rates of spontaneous mutations. © 1995 John Wiley & Sons, Inc.     

Conformational transition dynamics and the mechanism of long-range effects in DNA

A two-component model is proposed to describe the conformational dynamics of DNA macromolecule. A hypothesis on the mechanism of long-distance action transfer in DNA by local conformational transitions is presented. The conditions under which local transitions of B-A type are transmitted along a DNA chain are established.     

Conformational epitope of the type III group B Streptococcus capsular polysaccharide.

The protective epitope of the type III group B streptococcal polysaccharide (GBSPIII) is length dependent and conformational. To obtain a more accurate characterization of the conformational epitope, ELISA inhibition and surface plasmon resonance studies were conducted on two GBSPIII-specific mAbs using a large panel of oligosaccharide probes. The results of the studies confirmed that 2 repeating units (RU) is the minimum binding unit and that, while increases in chain length from 2 RU to 7 RU caused further optimization of the epitope, it remained monovalent. A 3-fold increase in affinity was observed between 7 RU and 20 RU, which, by surface plasmon resonance studies on a Fab, was shown to be due to both further optimization of the individual epitope and the occurrence of multivalency of epitope. The data support our hypothesis that the conformational epitope is an extended helical segment of the GBSPIII. GBSPIII exists mainly in the random coil form, which structurally mimics short oligosaccharide self Ags, but it can infrequently and spontaneously form extended helices. Although not prevalent in GBSPIII, the immune system preferentially selects these helical epitopes because they are unique to the polysaccharide. Contrary to a previously proposed model of GBSPIII binding in which the binding of the first Ab propagates a continuum of helical epitopes, our binding kinetics are consistent only with the helical epitope's being discontinuous and infrequent.