Macromolecular distribution of Acacia senegal gum (gum arabic) by size-exclusion chromatography

Fractionation of Acacia senegal gum has been carried out on Sephacryl gels S-400 and S-500. The shape and relative height of the chromatograms are sample dependent.Physicochemical data including circular dichroism and the weight-average molecular weights distribution show that about 70% of the material is composed of homogeneous polysaccharide chains with a very low nitrogen content. The remaining material is a combination of polysaccharide and nitrogen moieties (probably an arabinogalactan-protein complex).Our data are consistent with the new and simplified structural models proposed recently for this polysaccharide.     

Induction of somatic embryogenesis in gum arabic tree [Acacia senegal (L.) Willd.]

Factors affecting somatic embryogenesis from immature cotyledon of gum arabic tree [Acacia senegal (L.) Willd.] were investigated. Induction of somatic embryogenesis was influenced by plant growth regulator concentrations and addition of amino acids in medium. Best induction of somatic embryogenesis was obtained on MS medium supplemented with 0.45 μM 2, 4-D, 2.32 μM Kin and 15 mM L-glutamine. L-glutamine plays a significant role in the maturation of somatic embryos and most of embryos attained maturity only on L-glutamine (15 mM) containing medium. Maximum percent (75.0 ± 2.5) germination of somatic embryos was recorded on medium containing 0.22 μM BAP.     

Mechanical degradation and changes in conformation of hydrophobically modified starch

In this paper, we study the mechanical degradation and changes in conformation of a branched ultrahigh molar mass biomacromolecule, hydrophobically modified starch, as caused by high-pressure homogenization. The characterization was performed with asymmetrical flow field-flow fractionation (AsFlFFF) with multiangle light scattering (MALS) and refractive index detection. The starch which had been chemically modified with octenyl succinate anhydride (OSA) proved to be very large and polydisperse. Upon high-pressure homogenization, the molar mass and rms radius (r(rms)) decreased, and the extent of these changes was related to the turbulent flow conditions during homogenization. The treatment also induced an increase and scaling with size in the apparent density of the macromolecules. To further study the changes in conformation, it was necessary to calculate the hydrodynamic radii (r(h)). This can be determined numerically from the elution times in the analysis and the flow conditions in the AsFlFFF channel. The results showed that the treatment can cause a dramatic decrease in the quotient between r(rms) and r(h), suggesting major conformational changes. These results together could be interpreted as degradation and "crumpling" of the macromolecule, which would give a decrease in r(rms) and an increase in apparent density, together with a "fraying" of more outer parts of the macromolecule, which could give rise to the increase in r(h).     

Pollination biology and breeding system of Acacia senegal

Phenology, pollination biology and breeding system were investigated in three populations of Acacia senegal located in Delhi and Rajasthan. Flowers emit a mild fragrance and produce a minute quantity of nectar. The stigma is wet non-papillate, cup-shaped and generally accommodates one polyad with 16 pollen grains. The style is solid. The mass effect created by the brush type of blossoms attracts a wide variety of insects, of which the giant Asian honeybee, Apis dorsata , is the effective pollinator. Manual in vivo pollination studies have shown that the species is self-incompatible. Self-incompatibility appears to operate inside the embryo sac. Under natural conditions fruit set is as low as 0.36%. Insufficient pollination is the main cause of low fruit set. Manual xenogamous pollinations substantially improve fruit set to 30%     

Acacia senegal gum: continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges

The main chemical and physical features of the Acacia senegal exudate gum and its molecular fractions isolated by chromatographies were determined using a wide variety of methods. Three main molecular fractions were isolated after hydrophobic interaction chromatography (HIC) and biochemical analyses confirmed the presence of an arabinogalactan-peptide (FI), an arabinogalactan-protein (FII), and a glycoprotein (FIII) fraction as described commonly in the literature. Further purification of FIII using size exclusion chromatography revealed three distinct populations. A wide molecular weight distribution within each population with the presence of at least two distinct molecular species per population was identified by high performance size exclusion chromatography coupled to on line multi-angle laser light scattering (HPSEC-MALLS). In addition, both sugars content (neutral and uronic acids) and UV profiles revealed that FIII was composed of a continuum of molecular species differing both by their protein-to-sugar ratio and molecular weight. FI and FII had average molecular weight M(w) of 2.86 x 10(5) and 1.86 x 10(6) g.mol(-1), respectively, and a low polydispersity index (M(w)()/M(n) approximately 1.3). The three populations identified in FIII after HIC separation had M(w) of 2.67 x 10(6), 7.76 x 10(5), and 2.95 x 10(5) g.mol(-1) and very low polydispersity indexes (1.13, 1.04, and 1.01). Estimation of the polypeptide backbone length in the three fractions gave 43, 2253, and 4443 amino acid residues, respectively, hydroxyproline (Hyp) and serine being the most prominent residues within FI and FII, Hyp and Asx (asparagine + aspartic acid) within FIII. Secondary structure prediction from circular dichroism data resulted in polyproline II, beta-sheet, and random coil structures for FII and FIII, whereas no secondary structure was identified in FI. The existence of exposed tryptophanyl residues to the solvent was noticed by fluorescence in FII and FIII, tryptophan residues being absent from FI. In addition, 8-5' non cyclic diferulic acid was identified to be covalently linked to carbohydrate moieties of FII. Infrared spectroscopy identified the different vibrations of saccharidic and peptidic bonds with absorbance amplitudes in agreement with sugar and protein elementary analyses. Titration measurements in order to evaluate the number of charges on total Acacia gum and its molecular fractions revealed that 100% of charges came from polysaccharidic moieties (i.e., glucuronic acids) in FI. Charges coming from polysaccharidic moieties were of 91.3% and 37.9% for FII and FIII, respectively, the remaining 8.7% and 62.1% charges in FII and FIII molecular fractions coming from the polypeptidic backbone.     

Callus formation and induction of a cell suspension culture from Acacia senegal

Cell proliferation has been induced from the cambial zone of a branch of Acacia senegal, kept on the basal Knop and Ball medium in the presence of auxin. Transferred on the more complete nutrient medium of Schenk and Hildebrandt, the colonies gave rise to several cell lines. One of the friable lines, consisting of aggregates of parenchymatous cells, gave a cell suspension culture in an agitated liquid medium which is maintained as a strain of illimited growth. The heterogeneous suspension did not undergo noticeable changes after eight transfers. Aggregates collected on a 1000-m nylon seive were able to grow on a solid medium and gave back a friable callus similar to the initial colonies.     

Predicting the rheology of food biopolymers using constitutive models

Several constitutive models have been discussed to explain data for some foods in diluted and concentrated systems. Firstly, the theories of Rouse and Zimm, as well as rod-like theory, were used to study the conformation of the pectins in dilute solution. Among the dilute theories, the random coil theory of Zimm best explained the experimental data and suggested a certain level of intermolecular interaction present in the dilute pectin solution.

The Bird-Carreau constitutive theory with four empirical constants and zero shear limiting viscosity was used to describe the viscoelastic properties of the solutions of the guar, CMC/guar, glutenin, gluten and wheat flour doughs. The Bird-Carreau model was able to predict η and η′ in the high and low frequency regions for 1% guar solution. In the case of CMC/guar blend, the Bird-Carreau model explained steady shear and dynamic properties very well in the higher shear rate or frequency region of 1–100 s⁻¹. However, η″/ω does not tend to a zero shear constant value. The Bird-Carreau model also gave good predictions on the rheological properties of gluten and glutenin biopolymers in the free-flow region.

The polydisperse type, Doi-Edwards model, fits the experimental G′ and G″ better than the monodisperse model for 5% apple pectin dispersion. However, there is still a discrepancy between experimental and predicted values.     

Nanoparticle-induced assembly of hydrophobically modified chitosan

Hydrophobically modified chitosan (HMC) self-assembles in solution to form gels, making it suitable for applications in oil dispersion, hydrogel design and wound dressing. The self-assembly of HMC is driven by the association of hydrophobic moieties that are attached to chitosan monomers along the polymer chain. We present the results of discontinuous molecular dynamics simulations aimed at understanding how the length and density of the hydrophobic modification chains attached to HMC affect self-assembly and the structure of the resulting network. Long modification chains are required to promote the formation of a stable network in solution at a modification density of 5%; the networks form more readily at a modification density of 10%. The pore size distribution of the resulting HMC network is relatively independent of the modification chain length and density. Insertion of different sized hydrophobic nanoparticles into HMC has a significant impact on network formation, with the particles acting as junction points that promote the association of several HMC chains. The networks form faster in the presence of many small nanoparticles than in the presence of few large nanoparticles. We conclude that HMC could be a viable candidate to form hydrogels in solution.     

Growth and nitrogen acquisition strategies of Acacia senegal seedlings under exponential phosphorus additions

There remains conflicting evidence on the relationship between P supply and biological N₂-fixation rates, particularly N₂-fixing plant adaptive strategies under P limitation. This is important, as edaphic conditions inherent to many economically and ecologically important semi-arid leguminous tree species, such as Acacia senegal, are P deficient. Our research objective was to verify N acquisition strategies under phosphorus limitations using isotopic techniques. Acacia senegal var. senegal was cultivated in sand culture with three levels of exponentially supplied phosphorus [low (200 μmol of P seedling⁻¹ over 12 weeks), mid (400 μmol) and high (600 μmol)] to achieve steady-state nutrition over the growth period. Uniform additions of N were also supplied. Plant growth and nutrition were evaluated. Seedlings exhibited significantly greater total biomass under high P supply compared to low P supply. Both P and N content significantly increased with increasing P supply. Similarly, N derived from solution increased with elevated P availability. However, both the number of nodules and the N derived from atmosphere, determined by the ¹⁵N natural abundance method, did not increase along the P gradient. Phosphorus stimulated growth and increased mineral N uptake from solution without affecting the amount of N derived from the atmosphere. We conclude that, under non-limiting N conditions, A. senegal N acquisition strategies change with P supply, with less reliance on N₂-fixation when the rhizosphere achieves a sufficient N uptake zone.     

Some Acacia gum exudates of the section Phyllodineae

Australian gum specimens from Acacia saliciformis, A. xanthina, A. rostellifera, A. murrayana (two specimens differing in the mode of initiation of gum exudation), A. georginae, A. cyclops, A. implexa, and an un-named species (Maslin ‘P31’) have been analysed. The first four of these are placed within Bentham's Series 1, subseries 6F, A. georginae within subseries 7E, and the remainder within subseries 7F. These data extend considerably the ranges of the analytical parameters reported previously for phyllodine species. The molecular weights of the gums from A. cyclops and A. implexa are much higher than those reported earlier for South African specimens; this may affect some taxonomic deductions based on their examination. The gum composition of A. saligna can no longer be regarded as atypical of a phyllodinous species; a suggestion that A. saligna should be transferred to the section Juliflorae may require reconsideration. The major difference between the specimens of gum from A. murrayana lies in their nitrogenous content. Data are reported for the amino acid compositions of the gums from A. saliciformis and A. xanthina.     

Acacia gum exudates from species of the series gummiferae

An analytical study of the gum exudates from the African species Acacia ehrenbergiana (three specimens), A.xanthophloea (two specimens), A.hockii and A.sieberana var.villosa, and of the Australian species A.calcigera, has been made. There are now 19 species within the series Gummiferae Benth. for which gum parameters are available; of these, only A. ehrenbergiana gum displays a slightly negative optical rotation. The data for the three gum specimens from A. ehrenbergiana give a further example of the extent to which the gum from different trees of one particular species can vary in composition. The data for A.sieberana var. villosa gum are compared with the values established previously for subsp. sieberana; the differences between varieties of one species are similar in extent to those established for some subspecies. Although A. xanthophloea, A. hockii, A. ehrenbergiana, A. seyal and A. karroo are regarded as being very closely related botanically, the values for some of their analytical parameters differ considerably and strongly support the view that it is correct to retain them as distinct species.     

Layered structures of hydrophobically modified chitosan derivatives

Two series of chitosan derivatives, N-aliphatic acyl chitosans (C_n-CS) and N-aliphatic-O-dicinnamoyl-chitosans (C_n-CinCS) with n=2, 4, 8, 12 and 18, were prepared through regioselective reactions. The solid state structures were studied by FTIR and X-ray diffraction techniques. Two different types of layered structures were found to exist in the powder samples of C_n-CS and C_n-CinCS. The C_n-CS series crystallized into a sheet-type structure, in which all the flexible side chains lied down in the sheet in a direction normal to the backbones and partially interdigitated with each other. For the C_n-CinCS series, the polar backbones were stacked into sheets, and the flexible side chains occupied the space between sheets in a direction inclined to the backbones. The relationship between the structures and the solubilities are discussed.     

Stability of cumin oleoresin microencapsulated in different combination of gum arabic, maltodextrin and modified starch

Microencapsulations of cumin oleoresin by spray drying using gum arabic, maltodextrin, and modified starch (HiCap^® 100) and their ternary blends as wall materials were studied for its encapsulation efficiency and stability under storage. The microcapsules were evaluated for the content and stability of volatiles, and total cuminaldehyde, γ-terpinene and p-cymene content for six weeks. Gum arabic offered greater protection than maltodextrin and modified starch, in general, although the order of protection offered was volatiles > cuminaldehyde > p-cymene > γ-terpinene. A 4/6:1/6:1/6 blend of gum arabic/maltodextrin/modified starch offered a protection, better than gum arabic as seen from the t_1/2, i.e. time required for a constituent to reduce to 50% of its initial value. However protective effect of ternary blend was not similar for the all the constituents, and followed an order of volatiles > p-cymene > cuminaldehyde > γ-terpinene.     

Characterization and application of a new aqueous two phase system based on gum acacia (ex.Acacia senegal), a naturally occuring polysaccharide,and polyethylene glycol

Summary Gum acacia has been used with polyethylene glycol to form a two phase aqueous polymeric system. The system followed Newtonian fluid behaviour and the acacia rich phase supported a high concentration of protein (>1.3 mg/ml) in solution. The data on partitioning of various proteins in this system indicates that it can be used for selective separation of a particular protein from a protein mixture.     

Genetic Diversity and Population Structure of Acacia senegal (L) Willd. in Kenya

The level of genetic diversity and population structure of Acacia senegal variety kerensis in Kenya was examined using seven polymorphic nuclear microsatellite loci and two chloroplast microsatellite loci. In both chloroplast and nuclear datasets, high levels of genetic diversity were found within all populations and genetic differentiation among populations was low, indicating extensive gene flow. Analysis of population structure provided support for the presence of two groups of populations, although all individuals had mixed ancestry. Groups reflected the influence of geography on gene flow, with one representing Rift Valley populations whilst the other represented populations from Eastern Kenya. The similarities between estimates derived from nuclear and chloroplast data suggest highly effective gene dispersal by both pollen and seed in this species, although population structure appears to have been influenced by distributional changes in the past. The few contrasts between the spatial patterns for nuclear and chloroplast data provided additional support for the idea that, having fragmented in the past, groups are now thoroughly mixed as a result of extensive gene flow. For the purposes of conservation and in situ management of genetic resources, sampling could target a few, large populations ideally distributed among the spatial groups identified. This should ensure the majority of extant variation is preserved, and facilitate the investigation of variation in important phenotypic traits and development of breeding populations.     

Aqueous solutions of native and hydrophobically modified polysaccharides: temperature effect

Amphiphilic polysaccharides, obtained by the attachment of various hydrocarbon groups onto dextran, are studied in aqueous solutions. The viscosity of their aqueous solutions is examined as a function of concentration and temperature in the range 25-65 degrees C. Varying polymer concentration, viscosity follows a polynomial development of Huggins equation in which the coefficients can be calculated from the Huggins constant determined in the dilute domain (Matsuoka-Cowman equation). For all polymers, the solution viscosity follows an Arrhenius-like variation with temperature. The activation energy of the aqueous solutions is determined as a function of polymer concentration and structural characteristics (nature and amount of grafted hydrocarbon groups). The variation of activation energy with polymer concentration is shown to be consistent with predictions based on the Matsuoka-Cowman equation combined with the equation of Andrade. This conclusion is extended to other polysaccharides using data from the literature.     

Characterisation of a highly hydrophobically modified lactate dehydrogenase

1. Lysine residues of porcine H4 lactate dehydrogenase (L-lactate:NAD+ oxidoreductase EC 1.1.1.27) were modified with methyl-epsilon-(N-2,4-dinitrophenyl)aminocaproimidate - HCl. With increasing incorporation of the reagent a linear decrease of enzymatic activity was noticed. No essential lysyl group with an extraordinary reactivity was modified. 2. The active forms of the modified enzyme with different incorporation values were separated from denatured material by fractional precipitation and gel chromatography. An epsilon-(N-2,4-dinitrophenyl)aminocaproamidinate lactate dehydrogenase was obtained with an average incorporation of 38 groups per tetramer and a residual activity of 42%. This material proved to be homogenous in cellulose electrophoresis. 3. The epsilon-(N-2,4-dinitrophenyl)aminocaproamidinate lactate dehydrogenase is soluble only in glycine buffer at pH 8 and can be stabilized as ternary complex with NAD+ and sodium sulfite. Gel chromatography and ORD measurements show no strong conformational change. 4. epsilon-(N-2,4-dinitrophenyl)aminocaproamidinate lactate dehydrogenase has similar Km values for pyruvate, NADH, lactate and NAD+ as the native enzyme, and shows a lower thermostability due to a diminished stabilization by the hydrate layer on the surface.     

Ionic liquids as solvents for biopolymers: Acylation of starch and zein protein

Biopolymers such as starch and zein protein were found to be soluble at 80 °C in ionic liquids such as 1-butyl-3-methylimidazolium chloride (BMIMCl) and 1-butyl-3-methylimidazolium dicyanamide (BMIMdca) in concentrations up to 10% (w/w). Higher concentrations of biopolymers in these novel solvents resulted in solutions with too high viscosity to stir. Solutions of both starch and zein in BMIMCl were acylated with anhydrides in presence of pyridine to give acetyl starch and benzoyl zein with various degrees of substitution. Without pyridine the acylation reaction did not proceed. ¹H NMR and IR spectroscopies were used to determine the degree of substitution of starch. Viscosity studies indicated that the starch underwent slight reduction in molecular weight during the course of acylation. Starch was also soluble in other non-conventional solvents such as choline chloride/oxalic acid and choline chloride/ZnCl₂. However, zein was insoluble in these solvents.