Acacia senegal vs. Acacia seyal gums - Part 1: Composition and structure of hyperbranched plant exudates (original) (raw)
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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 h w of 2.86 × 10 5 and 1.86 × 10 6 g‚mol-1 , respectively, and a low polydispersity index (M w /M n ∼ 1.3). The three populations identified in FIII after HIC separation had M h w of 2.67 × 10 6 , 7.76 × 10 5 , and 2.95 × 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,-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.
Structural features of the polysaccharide gum from Acacia glomerosa
Food Hydrocolloids, 2001
Acacia glomerosa, Benth. (Vulgares Series) exudates a clear gum which produces gels easily. The physico-chemical data and sugar composition are very close to gum arabic from Acacia senegal, except that A. glomerosa gum contains a high nitrogen content. A series of degraded products was prepared by acid hydrolysis and Smith-degradation. The characterization of the degraded products by partial hydrolysis, sugar composition and by the application of uni-and bidimensional spectroscopy led to know interesting structural features of the polysaccharide isolated from A. glomerosa gum. This polysaccharide, as that from A. senegal gum, consists of 1,3-b-d-galactopyranosyl backbone. There are side-chains of 1,3-b-d-galactopyranosyl oligosaccharides attached to position six of the galactan main chain. Arabinose (furanosyl and pyranosyl) residues may be up to four units long because it was necessary to prepare four polysaccharides to remove them from the gum structure. Uronic acid residues were dif®cult to remove as has been observed in other Acacia gums.
Characterisation and molecular association of Nigerian and Sudanese Acacia gum exudates
Food Hydrocolloids, 2015
The chemical and physicochemical characteristics of gum exudate samples harvested from mature trees of Acacia senegal at two new specific ecolocations in Nigeria, have been investigated together with gum samples harvested from Acacia senegal and Acacia seyal originating from Sudan. The monosaccharide sugar compositions for the Acacia senegal gum samples were found to be similar, but the protein contents for the Nigerian samples were significantly higher than recorded for the Sudanese sample. Gel Permeation Chromatography coupled to light scattering, refractive index and U.V. detectors, has shown the presence of arabinogalactan, arabinogalactanprotein and glycoprotein fractions within the Acacia senegal gums and has also shown the presence of an additional small proportion of very low molar mass proteinaceous material all the samples which has previously been ignored. The plot of radius of gyration, Rg, as a function of elution volume showed a discontinuity for one of the Nigerian samples and for the Acacia seyal gum sample at elution volumes corresponding to the AGP component suggesting a different molecular structure. Plots of Mw-v-Rg confirmed that the molecules had a compact structure. The hydrodynamic size of the molecules was followed using dynamic light scattering as a function of time and it was found that molecular association occurred in solution. The extent of association increased as the protein content in the sample increased and was inhibited in the presence of electrolyte, it was concluded that association was due to electrostatic interaction between the protein moieties and glucuronic acid groups on individual macromolecules.
Carbohydrate Polymers, 2012
The structure of the arabinogalactan-protein (AGP) fraction of the gum exudate of Acacia senegal (gum Arabic) isolated from hydrophobic interaction chromatography was investigated using HPSEC-MALLS, small angle neutron scattering and TEM observations. Literature reported that the AGP structure of gum Arabic adopts a very compact conformation in solution due to the attachment of short arabinoside side chains and much larger blocks of carbohydrate to the polypeptidic backbone. The present study revealed that AGP in solution had a weight average molecular weight M w of 1.86 × 10 6 g mol −1 and a radius of gyration R g of 30 nm. In addition, two exponent values were identified in the R g , [Á], R h and vs. M w relationships highlighting two types of conformations depending on the molecular weight range considered: a low molar mass population with long-chain branching and a compact conformation and a high molar mass population with short-chain branching and an elongated conformation. AGP would behave in solution as a branched or hyper-branched polymer with conformations ranging from globular to elongated shape depending on the size of the carbohydrate branches. Small angle scattering form factor revealed an elongated average conformation corresponding to a triaxial ellipsoid while inverse Fourier transform of the scattering form factor gave a maximum dimension for AGP of 64 nm. Transmission electron microscopy highlighted the existence of two types of flat objects with thicknesses below 3-5 nm, single particles with a more or less anisotropic spheroidal shape and aggregated structures with a more elongated shape. A remarkable feature of all particle morphologies was the presence of an outer structure combined to an inner more or less porous network of interspersed chains or interacting structural blocks, as previously found for the arabinogalactan (AG) main molecular fraction of Acacia gum. However, clear differences were observed in the density and morphology of the inner porous network, probably highlighting differences in the degree of branching. The existence of assembled AG as part of the AGP family was confirmed using TEM micrographs at high resolution. Fused AGP dimers, trimers, tetramers and multimers were also identified. These molecular assemblies questioned about the nature of interactions involved.
Seventy four authenticated A. seyal samples have been studied in order to establish the molecular characteristics of this important species and the differences between the two variants A. seyal var. seyal (ASS) and A. seyal var. fistula (ASF) belonging to the Gummiferae series. Comparison is thus possible with A. senegal from the Vulgares series. The weight average molecular weight of A. seyal is at least three times greater than A. senegal. From average values ASF has a significantly higher molecular weight than ASS, contains less protein and more inorganic ash residue. Yet despite the high molecular weight, the intrinsic viscosities of ASF and ASS are less than for A. senegal, indicating a more compact molecular structure, which is supported by the root mean square radii measurements giving molecular size ratios of A. seyal to A. senegal of 0.77–1. The protein distribution in A. seyal is different, and whereas the protein in A. senegal is mainly associated with the high molecular weight component (AGPw106) it is distributed differently and mainly associated with a lower molecular weight component in A. seyal. The Mark-Houwink plots for A. senegal confirm the differences in shape and size compared with A. seyal. The average molecular weight for ASF is 2.1!106, for ASS 1.7!106 compared with an average value of 0.6!106 for AS.
PRELIMINARY MACROMOLECULAR ANALYSIS OF ACACIA GUM EXUDATES FROM ACACIASENEGALAND ACACIA SEYAL
Acacia senegal and Acacia seyal are important agro forestry cash crops indigenous to several countries of sub-Saharan Africa. The gum exudates produced by these species is termed gum Arabic which is an approved food additive (E414), primarily used as an emulsifier. In the current study, the molecular structure and biophysical properties of gum samples harvested from mature trees of A.senegal and A. seyal have been investigated. Preliminary analyses of the gums using transmission electron microscopy showed the presence of varied macromolecules, ranging in size from ~8 - ~60 nm. Furthermore, it was observed that molecular interaction among the molecules occurred in solution after incubation for five days at room temperature (250C).
Characterization of Two Acacia Gums and Their Fractions Using a Langmuir Film Balance
Journal of Agricultural and Food Chemistry, 2000
The mechanical properties of monolayers from two Acacia gums [Acacia senegal (L.) Willd. and Acacia seyal Del.] and their three fractions isolated by hydrophobic interaction chromatography were studied with a Langmuir film balance to obtain a more complete understanding of their action mode. The analysis of compression isotherms revealed that A. senegal gums globally exhibit better interfacial properties than A. seyal ones. The behavior of the whole gums appeared to be strongly influenced by their arabinogalactan-protein complex.
2012
Polysaccharides (GNF) from Acacia mearnsii de Wild gum exudates, collected from trees growing in the south of Brazil, were characterized ( 13 C and HSQC NMR, GC-MS, colorimetric assays). A commercial gum arabic (GAC) was analyzed similarly and compared with GNF. There were differences, consistent with distinct behavior in tensiometry tests and as emulsion stabilizer. GNF had a higher protein content than GAC, with small differences in the monosaccharide composition, the greater one being the lower uronic acid content of GNF (4%), compared with GAC (17%). GNF had a much broader molecular mass distribution, M w /M n , and a lower M w . GNF was more efficient in lowering the surface tension of water and saline solutions and was more efficient in emulsifying castor oil droplets. Results were discussed taking into account structural and molecular differences between the studied gums. It was concluded that polysaccharides from A. mearnsii de Wild are candidates as substitutes of currently commercialized arabic gums (Acacia senegal and Acacia seyal) having, depending on their application, improved properties.