Roberto Gilli

Polysaccharides from hot water extracts of roasted Coffea arabica beans: isolation and characterization

Abstract A polysaccharide fraction was obtained from the hot water extract of defatted ground dark roasted coffee (pure Coffea arabica blend for espresso brewing technique) by means of classical precipitation methods. This high molecular weight (〈 M w 〉=10u2008900xa0Da) product was shown to be composed of mannose, galactose, arabinose and traces of proteinaceous material. Further fractionation yielded two different carbohydrate polymers, which were structurally characterized. One polysaccharide was identified as a β - d -(1-4) mannan containing small amounts of galactose and arabinose. The second polysaccharide, obtained in low yield after removal of the mannose-containing polymeric material, was identified as an arabinogalactan. The starting fraction was found to be composed of about 80% (mol basis) of mannan and of about 20% of arabinogalactan. 13 C-NMR spectra revealed that the arabinogalactan has a backbone chain of β -(1-3)- d -galactopyranose units. Some of these units were substituted with either terminal β - d -galactose or terminal α - l- arabinofuranose side chains mainly in C-6 position. Both the mannan and the arabinogalactan isolated are structurally related to the polysaccharides originally present in the green coffee beans.

NMR analysis of succinoglycans from different microbial sources: partial assignment of their 1H and 13C NMR spectra and location of the succinate and the acetate groups☆

In order to obtain information on the location of succinate and acetate groups, comparative NMR analyses were carried out on succinoglycans from different microbial sources by using conventional and advanced NMR techniques. In particular, one-dimensional, 1H and 13C NMR spectra were recorded for qualitative and quantitative analysis on native high-molecular-weight succinoglycans (both in the Na+ salt and free-acid forms) from Pseudomonas sp. NCIB 11592, Agrobacterium radiobacter A201-25, Rhizobium meliloti YE-2, and Rhizobium sp. isolated from Vicia faba and compared with those of the deacylated and deacylated-depyruvated, partially depolymerised exopolysaccharides from Rhizobium meliloti YE-2. Moreover, a series of two-dimensional experiments was performed on all the exopolysaccharides aiming at the partial assignment of the NMR spectra. The NMR data showed that succinate is located on O-6 of either one or both of the two side chain 3-linked beta-D-Glc residues, whereas the acetate (when it is present) is located on one of the O-6 of backbone 4-linked beta-D-Glc units, but the specific site could not be determined. In addition, the spectral features of the succinate substituent were found to be sensitive to pH changes.

Structural characterization and solution properties of an acidic branched (1→3)-β-D-glucan from Aureobasidium pullulans☆

An acidic exopolysaccharide was isolated from a selected strain of Aureobasidium pullulans. On the basis of spectroscopic and chromatographic techniques, the polymer was identified as a beta-D-glucan containing a main chain of (1-->3)-linked beta-D-glucopy-ranosyl units substituted at the O-6 position by single beta-D-glucopyranosyl side chains. The ratio of units in the main chain to units in the side chain was found to be 1.4:1. The ionic character of this exopolysaccharide is due to the presence of malate residues which are linked to the polymer through ester bonds. The degree of substitution was estimated to be very low (0.05). In aqueous solution no signals are present in the NMR spectra strongly suggesting that the polymer adopts a rigid ordered conformation as further confirmed by rheological data. A solvent-induced conformational transition was observed in DMSO in which NMR spectra with good signal-to-noise ratio were obtained. The solution behaviour of the polymer is similar to that of other branched (1-->3)-beta-D-glucans in spite of both the degree of branching and the substitution with malate groups.

Effect of monomer composition on proton dissociation of weak polyacids

The study of the proton dissociation process of weak polyacids (eg α carboxylic poly(monoprotic)acid) is based on the knowledge of the change in electrostatic free energy, G(el), as a function of the variation of the number of charges on the polymer chain. The original treatment proposed by Manning can be used to describe the proton dissociation process of weak poly(monoprotic)acids, in the absence of pH-induced conformational transitions. In order to describe the alpha dependence of pKa of weak co-poly(monoprotic)acids containing two different acidic groups in different amounts along the polymer chain, a simple modification of the model is proposed. Abbreviations used: Due to the difficulty of using non-ASCI characters in the main text, we have used the following abbreviations: G(el) Gel 10-pH 10-pH pKa pKa alpha α alphan αn dG(el)/dalpha δGel/δα KA and KB KA and KB CA and CB CA and CB βA and βB βA and βB δpK δpK §max ξmax

SOLUTION PROPERTIES OF THE EXOPOLYSACCHARIDE PRODUCED BY PSEUDOMONAS MARGINALIS STRAIN HT041B

Abstract The solution properties of the exopolysaccharide marginalan produced by Pseudomonas marginalis HT041B were investigated by means of low-angle laser light-scattering, capillary viscometry, and rheology. Potentiometric and viscosimetric data indicated the absence of a cooperative transition of the disorder-to-order type. The experimental findings obtained in dilute solution (Mark-Houwink coefficients, rigidity coefficient, characteristic ratio) suggested that the polymer behaves like a semiflexible chain which adopts a disordered conformation. The rheological behaviour of more concentrated marginalan solutions, as determined by means of both steady shear and oscillatory measurements, further confirmed the disordered conformational state of the polymer in solution. 1. Presented at the XVIIIth International Carbohydrate Symposium, Milan, Italy, July 21-26, 1996.

Thermal stability of succinate groups in the exopolysaccharide marginalan

Abstract An investigation on the possible effect of temperature on the conformation of the exocellular polysaccharide marginalan, produced by Pseudomonas marginalis strain HT041B, in aqueous solutions led to the observation that 30 % of the succinate groups were peeled off from the polymeric backbone when polymeric solutions were heated at 70 °C. This desuccinylation effect was investigated by means of NMR, circular dichroism and viscosity measurements. The experimental findings were compared with those obtained for a sample of chemically desuccinylated marginalan. A possible mechanism for the temperature-induced hydrolysis of succinate groups is discussed. 1. Presented at the XVIIIth International Carbohydrate Symposium, Milan, Italy, July 21-26, 1996.

Peculiar rheological behaviour of tetrabutylammonium hyaluronate in mixture with its n-propyl ester derivative in dimethylsulfoxide

Abstract Steady shear and dynamic oscillatory tests were performed both on tetrabutylammonium hyaluronate and on its n-propyl ester derivative in dimethylsulphoxide (DMSO) in the high polymer concentration regime. The viscoelastic behaviour of the individual polymers is typical of an entangled network of disordered polysaccharides. The supramolecular structure of the tetrabutylammonium hyaluronate in DMSO reflects the occurrence of intermolecular association. A peculiar rheological response of the mixtures in DMSO solution has been found.