Exploring the Binding of Barbital to a Synthetic Macrocyclic Receptor. A Charge Density Study.

Experimental charge density distribution studies, complemented by quantum mechanical theoretical calculations, of a host-guest system composed of a macrocycle (1) and barbital (2) in a 1:1 ratio (3) have been carried out via high-resolution single-crystal X-ray diffraction. The data were modeled using the conventional multipole model of electron density according to the Hansen-Coppens formalism. The asymmetric unit of macrocycle 1 contained an intraannular ethanol molecule and an extraannular acetonitrile molecule, and the asymmetric unit of 3 also contained an intraannular ethanol molecule. Visual comparison of the conformations of the macrocyclic ring shows the rotation by 180° of an amide bond attributed to competitive hydrogen bonding. It was found that the intraannular and extraannular molecules inside were orientated to maximize the number of hydrogen bonds present, with the presence of barbital in 3 resulting in the greatest stabilization. Hydrogen bonds ranging in strength from 4 to 70 kJ mol-1 were the main stabilizing force. Further analysis of the electrostatic potential among 1, 2, and 3 showed significant charge redistribution when cocrystallization occurred, which was further confirmed by a comparison of atomic charges. The findings presented herein introduce the possibility of high-resolution X-ray crystallography playing a more prominent role in the drug design process.

A comparison of the experimental and theoretical charge density distributions in two polymorphic modifications of piroxicam.

Experimental charge density distribution studies of two polymorphic forms of piroxicam, ß-piroxicam (1) and piroxicam monohydrate (2), were carried out via high-resolution single crystal X-ray diffraction experiments and multipole refinement. The asymmetric unit of (2) consists of two discrete piroxicam molecules, (2a) and (2b), and two water molecules. Geometry differs between (1) and (2) due to the zwitterionic nature of (2) which results in the rotation of the pyridine ring around the C(10)-N(2) bond by approximately 180°. Consequently, the pyridine and amide are no longer co-planar and (2) forms two exclusive, strong hydrogen bonds, H(3)O(4) and H(2)O(3), with bond energies of 66.14 kJ mol-1 and 112.82 kJ mol-1 for (2a), and 58.35 kJ mol-1 and 159.51 kJ mol-1 for (2b), respectively. Proton transfer between O(3) and N(3) in (2) results in significant differences in surface electrostatic potentials. This is clarified by the calculation of atomic charges in the zwitterion that shows the formally positive charge of the pyridyl nitrogen which is redistributed over the whole of the pyridine ring instead of concentrating at N-H. Similarly, the negative charge of the oxygen is distributed across the benzothiazine carboxamide moiety. The multipole derived lattice energy for (1) is -304 kJ mol-1 and that for (2) is -571 kJ mol-1, which is in agreement with the experimentally determined observations of higher solubility and dissolution rates of (1) compared to (2).

MhbT is a specific transporter for 3-hydroxybenzoate uptake by Gram-negative bacteria.

Klebsiella pneumoniae M5a1 is capable of utilizing 3-hydroxybenzoate via gentisate, and the 6.3-kb gene cluster mhbRTDHIM conferred the ability to grow on 3-hydroxybenzoate to Escherichia coli and Pseudomonas putida PaW340. Four of the six genes (mhbDHIM) encode enzymes converting 3-hydroxybenzoate to pyruvate and fumarate via gentisate. MhbR is a gene activator, and MhbT is a hypothetical protein belonging to the transporter of the aromatic acid/H(+) symporter family. Since a transporter for 3-hydrxybenzoate uptake has not been characterized to date, we investigated whether MhbT is responsible for the uptake of 3-hydroxybenzoate, its metabolic intermediate gentisate, or both. The MhbT-green fluorescent protein (GFP) fusion protein was located on the cytoplasmic membrane. P. putida PaW340 containing mhbRΔTDHIM could not grow on 3-hydroxybenzoate; however, supplying mhbT in trans allowed the bacterium to grow on the substrate. K. pneumoniae M5a1 and P. putida PaW340 containing recombinant MhbT transported (14)C-labeled 3-hydroxybenzoate but not (14)C-labeled gentisate and benzoate into the cells. Site-directed mutagenesis of two conserved amino acid residues (Asp-82 and Asp-314) and a less-conserved residue (Val-311) among the members of the symporter family in the hydrophilic cytoplasmic loops resulted in the loss of 3-hydroxybenzoate uptake by P. putida PaW340 carrying the mutant proteins. Hence, we demonstrated that MhbT is a specific 3-hydroxybenzoate transporter.

Status assessment of New Zealand's naturally uncommon ecosystems.

Globally, ecosystems are under increasing anthropogenic pressure; thus, many are at risk of elimination. This situation has led the International Union for Conservation of Nature (IUCN) to propose a quantitative approach to ecosystem-risk assessment. However, there is a need for their proposed criteria to be evaluated through practical examples spanning a diverse range of ecosystems and scales. We applied the IUCN's ecosystem red-list criteria, which are based on changes in extent of ecosystems and reductions in ecosystem processes, to New Zealand's 72 naturally uncommon ecosystems. We aimed to test the applicability of the proposed criteria to ecosystems that are naturally uncommon (i.e., those that would naturally occur over a small area in the absence of human activity) and to provide information on the probability of ecosystem elimination so that conservation priorities might be set. We also tested the hypothesis that naturally uncommon ecosystems classified as threatened on the basis of IUCN Red List criteria contain more threatened plant species than those classified as nonthreatened. We identified 18 critically endangered, 17 endangered, and 10 vulnerable ecosystems. We estimated that naturally uncommon ecosystems contained 145 (85%) of mainland New Zealand's taxonomically distinct nationally critical, nationally endangered, and nationally vulnerable plant species, 66 (46%) of which were endemic to naturally uncommon ecosystems. We estimated there was a greater number of threatened plant species (per unit area) in critically endangered ecosystems than in ecosystems classified as nonthreatened. With their high levels of endemism and rapid and relatively well-documented history of anthropogenic change, New Zealand's naturally uncommon ecosystems provide an excellent case-study for the ongoing development of international criteria for threatened ecosystems. We suggest that interactions and synergies among decline in area, decline in function, and the scale of application of the criteria be used to improve the IUCN criteria for threatened ecosystems.

Octenyl-succinylated inulins for the delivery of hydrophobic drug.

The efficacy of hydrophobic anticancer drugs is limited by their poor solubility in water, inefficient target delivery, and toxic side effects. In this work, doxorubicin (DOX) was solubilized using OSA-inulins which created micellar aggregates in aqueous solution above a critical concentration. In vitro delivery of OSA-inulin-DOX micelles resulted in strong inhibition of the growth of MCF-7 breast cancer cells as compared to free DOX. They also displayed a faster cellular uptake rate, indicating that the micelles were promptly internalized into the cells through CD44 receptor-mediated endocytosis. During in vivo tumor suppression experiments in tumor-bearing mice, the OSA-inulin-DOX micelles strongly hindered tumor growth and showed substantially lower systemic toxicity compared with free DOX. Our achievements demonstrate that OSA-inulin has great potential for the encapsulating, dissolving, and targeted delivery of hydrophobic drugs, especially antitumor drugs, for nutraceutical, medical, and pharmaceutical applications.

An experimental and theoretical charge density study of theophylline and malonic acid cocrystallization.

The pharmaceutical agent theophylline (THEO) is primarily used as a bronchodilator and is commercially available in both tablet and liquid dosage forms. THEO is highly hygroscopic, reducing its stability, overall shelf-life, and therefore usage as a drug. THEO and dicarboxylic acid cocrystals were designed by Trask et al. in an attempt to decrease the hygroscopic behaviour of THEO; cocrystallisation of THEO with malonic acid (MA) did not improve the hygroscopic stability of THEO in simulated atmospheric humidity testing. The current study employed high-resolution X-ray crystallography, and Density Functional Theory (DFT) calculations to examine the electron density distribution (EDD) changes between the cocrystal and its individual components. The EED changes identified the reasons why the THEO:MA cocrystal did not alter the hygroscopic profile of THEO. The cocrystal was equally porous, with atomic packing factors (APF) similar to those of THEO 0.73 vs. 0.71, respectively. The THEO:MA (1) cocrystal structure is held together by an array of interactions; a heterogeneous synthon between the imidazole and a carboxylic fragment stabilising the asymmetric unit, a pyrimidine-imidazole homosynthon, and an aromatic cycle stack between two THEO moieties have been identified, providing 9.7-12.9 kJ mol-1 of stability. These factors did not change the overall relative stability of the cocrystal relative to its individual THEO and MA components, as shown by cocrystal (1) and THEO being equally stable, with calculated lattice energies within 2.5 kJ mol-1 of one other. The hydrogen bond analysis and fragmented atomic charge analysis highlighted that the formation of (1) combined both the EDD of THEO and MA with no net chemical change, suggesting that the reverse reaction - (1) back to THEO and MA - is of equal potential, ultimately producing THEO hydrate formation, in agreement with the work of Trask et al. These results highlight that a review of the EDD change associated with a chemical reaction can aid in understanding cocrystal design. In addition, they indicate that cocrystal design requires further investigation before becoming a reliable process, with particular emphasis on identifying the appropriate balance of synthon engineering, weak interactions, and packing dynamics.

Strong human association with plant invasion success for Trifolium introductions to New Zealand.

It has proven difficult, when focused only on biological determinants, to explain why some plant species become naturalized in or invade new locations, whereas others fail. We analyzed the invasion of Trifolium (true clover) species into New Zealand, assessing a range of human, biogeographic, and biological influences at three key invasion stages: introduction, naturalization, and spread. We used sparse principal component analysis (SPCA) to define suites of related attributes and aggregated boosted trees to model relationships with invasion outcomes. Human and biogeographic attributes were strongly associated with success at all stages. Whereas biogeographic attributes, notably large native range, were consistently associated with success, different human factors appeared to favor success at different stages, such as presence in early trade/immigration hotspots (introduction), intentional large-scale planting (naturalization), and frequent presence as a seed contaminant (relative spread rate). Biological traits were less strongly associated with success for introduction and spread and little if at all for naturalization; we found that tall perennials with long flowering periods were more frequently selected for introduction, whereas species with extended flowering in New Zealand spread more rapidly. In addition to causal relationships, the importance of human factors may reflect indirect associations, including ecological traits associated with both human use and invasion. Nevertheless, our results highlight key roles that humans can play in facilitating plant invasion via two pathways: (i) commercial introduction leading to widespread planting and concomitant naturalization and spread and (ii) unintentional introduction and spread of species associated with human activities, such as seed contaminants.

A kinetic analysis of three modified novel nitroreductases.

A kinetic comparison between three nitroreductase enzymes isolated from the genome of Bacillus licheniformis ATCC 14580 for prospective use as immobilised enzymes for explosives detection has been conducted. The genes encoding the three enzymes (yfkO [BLNfnB] encoding an NfsB-like enzyme; nfrA [BLNfrA1] and ycnD [BLNfrA2] encoding PnrA-like enzymes) have been PCR amplified from the native genome and cloned into pET-28a(+) and a modified cysteine((6))-tagged pET-28a(+) and subsequently over-expressed, purified, and biochemically characterised. The previously uncharacterised nitroreductases exhibited activity against a wide range of explosives, including cyclic nitramines. Amino acid alignments and overall structural comparisons with other nitroreductase family members suggest that the B. licheniformis enzymes are members of the NfsA-Frp/NfsB-FRase I family group. Despite the overall low amino acid identity, regions for flavin mononucleotide binding and active site residues were highly conserved.

Interaction of gum arabic with fatty acid studied using electron paramagnetic resonance.

Electron paramagnetic resonance (EPR) is here used to study the interaction between gum arabic and a fatty acid. The EPR spectra of 5-doxyl stearic acid (5-DSA), a spin-labeled fatty acid analog, displayed increasingly anisotropic line features upon addition of gum arabic, indicating a strong immobilization of the nitroxyl moiety when the fatty acid is bound to gum arabic. To understand the nature of the interaction, EPR measurements were carried out at different pHs and using two fractions of gum arabic separated by hydrophobic interaction chromatography (HIC). 5-DSA bound favorably to the hydrophobic fraction, which contains mainly glycoprotein, and a small amount of high molecular weight arabinogalactan protein (AGP). Binding occurred to a less extent to the hydrophilic fraction, which contains essentially arabinogalactan (AG). Such a hydrophobic binding mechanism is further supported by a sharp drop in the binding when pH is raised above the pK(a) value of 5-DSA (approximately pH 5). This is because the ionization of carboxylic groups would lead to increased polarity and hydrophilicity of the fatty acid. A secondary effect involving the formation of ionic hydrogen bonds between carboxylic groups in fatty acid and lysine residues in gum arabic might also contribute. This is consistent with the reduction in binding ability when the pH was elevated above the pK(a) value of lysine residue (approximately pH 10). The biological significance of these findings is considered.

Antimony biomethylation in culture media revisited in the light of solubility and chemical speciation considerations.

Laboratory culture experiments have shown that antimony biomethylation can result from bacterial and fungal activity under both aerobic and anaerobic conditions. However, in the light of our current knowledge of antimony solubility and equilibria, critical analysis of the conditions used in published laboratory studies reveals that solution chemistry was generally overlooked and oversaturated solutions were used. As a result, it is very difficult, if not impossible, to establish reliable observed effect-concentration relationships in the experiments published.

Characterization of the polysaccharide from Cola millenii seeds.

We have isolated and characterized a water soluble polysaccharide from Cola millenii seeds. It was found to be composed of a total of 59% neutral sugars (mainly rhamnose, galactose and arabinose ~24, 13 and 8% respectively) and 41% uronic acids [mainly galacturonic acid]. The weight and number average molar mass values were found to be 4.7 × 106 g/mol and 3.5 × 106 g/mol, respectively. The polysaccharide exhibited polyelectrolyte properties with the intrinsic viscosity varying with salt concentration. The polysaccharide formed a highly viscous solution in water with apparent zero shear viscosities of 0.59-772 Pa·s at concentrations 0.3-2.5 wt%. The solutions were shear thinning even at very low concentrations. The mechanical spectra showed gel-like characteristics at concentrations >2 wt%. The rheological behavior indicates the polysaccharide has potential for application as a thickener and suspending agent in food, pharmaceutical and cosmetic formulations.

Octenyl-succinylated inulin for the encapsulation and release of hydrophobic compounds.

Octenyl-succinylated inulins (OSA-inulin) were synthesized in aqueous solutions using inulin with varying degrees of polymerization (DP). They were characterized using 1H NMR and FTIR and their degrees of substitution were determined. All the samples formed micellar aggregates in aqueous solution above a critical aggregation concentration (CAC) and solubilized beta-carotene. The amount of beta carotene solubilized within the micelles ranged from 12 to 25 mg/g of OSA-inulin and depended on the inulin molar mass. Dynamic light scattering showed that the aggregates, with and without dissolved beta-carotene, were ∼10-15 nm in size and this was confirmed by Transmission Electron Microscopy which also indicated that the micelles had a globular shape. OSA-inulin particles containing encapsulated beta-carotene were produced by freeze-drying. The encapsulated beta-carotene was not released from the freeze-dried particles when introduced into simulated gastric fluid at pH 2.5 but was readily released in simulated small intestinal fluid at pH 7. The results demonstrate the potential application of OSA-inulin in the encapsulation, dissolution and targeted delivery of hydrophobic drug molecules for nutraceutical, pharmaceutical and medical applications.

Learning from failures: testing broad taxonomic hypotheses about plant naturalization.

Our understanding of broad taxonomic patterns of plant naturalizations is based entirely on observations of successful naturalizations. Omission of the failures, however, can introduce bias by conflating the probabilities of introduction and naturalization. Here, we use two comprehensive datasets of successful and failed plant naturalizations in New Zealand and Australia for a unique, flora-wide comparative test of several major invasion hypotheses. First, we show that some taxa are consistently more successful at naturalizing in these two countries, despite their environmental differences. Broad climatic origins helped to explain some of the differences in success rates in the two countries. We further show that species with native relatives were generally more successful in both countries, contrary to Darwin's naturalization hypothesis, but this effect was inconsistent among families across the two countries. Finally, we show that contrary to studies based on successful naturalizations only, islands need not be inherently more invisible than continents.

Complete sequence determination combined with analysis of transposition/site-specific recombination events to explain genetic organization of IncP-7 TOL plasmid pWW53 and related mobile genetic elements.

Recent studies have indicated that the evolutionarily common catabolic gene clusters are loaded on structurally diverse toluene-catabolic (TOL) plasmids and their residing transposons. To elucidate the mechanisms supporting the diversification of catabolic plasmids and transposons, we determined here the complete 107,929 bp sequence of pWW53, a TOL plasmid from Pseudomonas putida MT53. pWW53 was found to belong to the IncP-7 incompatibility group that play important roles in the catabolism of several xenobiotics. pWW53 carried two distinct transposase-resolvase gene clusters (tnpAR modules), five short terminal inverted repeats (IRs), and three site-specific resolution (res) sites that are all typical of class II transposons. This organization of pWW53 suggested the four possible transposable regions, Tn4657 to Tn4660. The largest 86 kb region (Tn4657) spanned the three other regions, and Tn4657 and Tn4660 (62 kb) covered all of the 36 xyl genes for toluene catabolism. Our subsequent transposition experiments clarified that the three transposons, Tn4657 to Tn4659, indeed exhibit their transposability, and that pWW53 also generated another 37 kb toluene-catabolic transposon, Tn4656, which carried the two separated and inversely oriented segments of pWW53: the tnpRA-IR module of Tn4658 and a part of xyl gene clusters on Tn4657. The Tn4658 transposase was able to mediate the transposition of Tn4658, Tn4657, and Tn4656, while the Tn4659 transposase catalyzed only the transposition of Tn4659. Tn4656 was formed by the Tn4658 resolvase-mediated site-specific inversion between the two inversely oriented res sites on pWW53. These findings and comparison with other catabolic plasmids clearly indicate multiple copies of transposition-related genes and sites on one plasmid and their recombination activities contribute greatly to the diversification of plasmid structures as well as wide dissemination of the evolutionary common gene clusters in various plasmids.

Role of protein and ferulic acid in the emulsification properties of sugar beet pectin.

The ability of sugar beet pectin to stabilize 20% w/w limonene oil-in-water emulsions has been investigated. The size of the oil droplets as determined by laser diffraction measurements decreased from about 15 mum to about 6 mum when the pectin concentration increased from 0.05% to 2% w/w but leveled off thereafter, suggesting complete coverage of the oil droplets by the polymer at this optimum concentration. Isotherms for the adsorption of pectin, protein, and ferulic acid were constructed. The adsorption capacities at the oil-water interface of approximately 1.4 and approximately 0.2 mg/m (2) for protein and ferulic acid, respectively, compared to approximately 9.5 mg/m (2) for pectin revealed that the adsorbed fractions of the pectin sample were rich in protein (14.7%) and ferulic acid (2.1%) given that there were only 2.7% protein and 1.06% ferulic acid present in the whole pectin sample. Direct measurements on the adsorbed fraction recovered from the oil droplets via desorption with SDS confirmed that it contained 11.1% protein and 2.16% ferulic acid. The results suggest that one or both of these two functional groups adsorb onto the surface of the oil droplets and stabilize the emulsions. High molecular mass fractions adsorbed preferentially onto oil droplets during emulsification. As compared to those made with gum arabic, the emulsion samples made with sugar beet pectin samples exhibited similar (or even slightly higher) stability.

The influence of the extensional viscosity of very low concentrations of high molecular mass water-soluble polymers on atomisation and droplet impact.

BACKGROUND: Water-soluble polymers are increasingly added to herbicide and pesticide formulations at very low concentrations (100-1000 mg L(-1)) in order to control the spray characteristics, notably to reduce spray drift and influence droplet bounce. The incorporation of polymeric adjuvants improves the efficacy of the spray solutions, thus enabling crop growers to maximise the performance of agrochemical sprays at lower dose rates of active ingredient. It is important to establish a fundamental understanding of how polymers influence the processes involved in droplet deposition. RESULTS: The shear and extensional viscosities of a series of high molecular mass (M(w)) poly(acrylamides) (M(w) approximately 10(6)-10(7)) have been determined at very low concentrations (100-1000 mg L(-1)). The polymer solutions demonstrated typical shear thinning characteristics under shear, and strain hardening behaviour under extension above a critical strain rate. The presence of the polymers was shown to increase the size of droplets produced in atomisation using an agricultural spray nozzle, as measured by laser diffraction. This was attributed to the increase in the extensional viscosity at the strain rates generated under pressure in the spray nozzle and was a function of both polymer concentration and M(w). In addition, the presence of polymer was found to have a significant influence on droplet bounce. CONCLUSIONS: The presence of very low concentrations of high molecular mass poly(acrylamides) significantly influences the size of droplets formed on atomisation and subsequent bounce characteristics. Large extensional viscosities generated above a critical strain rate are responsible for both processes.

A vector holographic optical trap.

The invention of optical tweezers almost forty years ago has triggered applications spanning multiple disciplines and has also found its way into commercial products. A major breakthrough came with the invention of holographic optical tweezers (HOTs), allowing simultaneous manipulation of many particles, traditionally done with arrays of scalar beams. Here we demonstrate a vector HOT with arrays of digitally controlled Higher-Order Poincaré Sphere (HOPS) beams. We employ a simple set-up using a spatial light modulator and show that each beam in the array can be manipulated independently and set to an arbitrary HOPS state, including replicating traditional scalar beam HOTs. We demonstrate trapping and tweezing with customized arrays of HOPS beams comprising scalar orbital angular momentum and cylindrical vector beams, including radially and azimuthally polarized beams simultaneously in the same trap. Our approach is general enough to be easily extended to arbitrary vector beams, could be implemented with fast refresh rates and will be of interest to the structured light and optical manipulation communities alike.

Multiple steps and critical behaviors of the binding of calcium to alginate.

Previous research on the binding and gelation of calcium/alginate in aqueous solution were mostly conducted in the (semi-)concentrated regime, and it did not provide details of the binding process and the formation of egg-box junctions. In the present investigation, the binding of calcium to alginate, of low and high molecular weight and different guluronate/mannuronate ratios, was investigated in dilute solutions using isothermal titration calorimetry (ITC), Ca2+-selective potentiometry, and viscometry techniques. The results reveal three distinct and successive steps in the binding of calcium to alginate with increased concentration of Ca ions. They were assigned to (i) interaction of Ca2+ with a single guluronate unit forming monocomplexes; (ii) propagation and formation of egg-box dimers via pairing of these monocomplexes; and (iii) lateral association of the egg-box dimers, generating multimers. The third step has different association modes depending on the molecular weight of alginate. The boundaries between these steps are reasonably critical, and they closely correlate with the Ca/guluronate stoichiometry expected for egg-box dimers and multimers with 2/1 helical chains. The formation of egg-box dimers and their subsequent association are thermodynamically equivalent processes and can be fitted by a model of independent binding sites. The binding of Ca to alginates of different guluronate contents is controlled by a balance between enthalpy and entropy.

Characterisation and solution properties of a galactomannan from Bauhinia monandra seeds.

This study reports on the chemical and physicochemical properties of the polysaccharide isolated from Bauhinia monandra seeds. The seeds were found to contain 17.8% polysaccharide which consisted predominantly of galactose and mannose. The Man/Gal ratio was found to be approximately 4:1and the average molar mass was 2.54×105g/mol. The extracted material was also found to contain a small amount of protein (5.35%). The galactomannan produced highly viscous solution; the viscosity-shear rate profile was best described by the Williamson model. The mechanical spectrum of a 0.5wt% solution showed that G″ was greater than G' over the frequency range employed while at higher concentrations G' became greater than G″ above a critical frequency. The solutions obeyed the Cox-Merz rule at low concentrations, but there was some deviation at higher concentrations. Viscosity measurements were undertaken over a range of temperatures and the activation energy of viscous flow was found to be 20.75kJ/mol. The rheological properties of solutions of B. monandra galactomannan indicate that it has comparable characteristics to other commercially important galactomannans such as guar gum and locust bean gum and hence has potential as a thickener in the formulation of food and other related products.

Solution properties of targacanthin (water-soluble part of gum tragacanth exudate from Astragalus gossypinus).

Solution properties of tragacanthin (the water-soluble part of gum tragacanth) were studied by gel permeation chromatography (GPC) combined with multi-angle light scattering and viscometry at 25 degrees C. Photon correlation spectroscopy was used to determine the hydrodynamic radius. Ultrasonic degradation was applied to obtain biopolymer fractions of different molecular weights. The dependence of intrinsic viscosity [eta] and radius of gyration (s2)z(1/2) on weight average molecular mass M(w) for this biopolymer were found to be [eta] = 9.077 x 10(-5) M(w)(0.87) (dL g(-1)) and (s2)z(1/2) in the range of M(w) from 1.8 x 10(5) to 1.6 x 10(6). The conformational parameters of tragacanthin were calculated to be 1111 nm for molar mass per unit contour length (M(L)), 26 nm for persistence length (q) and 1.87 ratio of R(g)/R(h). It was found that the Smidsrød parameter B, the empirical stiffness parameter was 0.013, which is lower than that of several polysaccharides indicating the stiff backbone for tragacanthin. The rheological behavior of aqueous solutions of gum tragacanth and its insoluble and soluble fractions (bassorin and tragacanthin, respectively) were studied. For concentrations equal to 1%, at 25 degrees C and in the absence of salt, bassorin solution showed the highest viscosity and shear thinning behaviour. Power law and Williamson models were used to describe the rheological behaviour of bassorin and tragacanthin, respectively. Oscillatory shear experiments showed a gel like structure for the bassorin but for tragacanthin the oscillatory data were as would be expected for semi-dilute to concentrated solution of entangled, random coil polymers. NaCl changed the steady and oscillatory rheological properties of both fractions and in this way the final viscosity of bassorin was even less than tragacanthin. The calculated activation energy for bassorin and tragacanthin indicated a more rapid decrease in viscosity with temperature for tragacanthin. The plot of eta(sp,0) versus C[eta] revealed that the transition from dilute to semi-dilute regime occurs at C*[eta] = 2.82 for tragacanthin.