José M. Igartuburu

Simplification of the DPPH assay for estimating the antioxidant activity of wine and wine by-products.

The DPPH assay is one of the most commonly employed methods for measuring antioxidant activity. Even though this method is considered very simple and efficient, it does present various limitations which make it complicated to perform. The range of linearity between the DPPH inhibition percentage and sample concentration has been studied with a view to simplifying the method for characterising samples of wine origin. It has been concluded that all the samples are linear in a range of inhibition below 40%, which allows the analysis to be simplified. A new parameter more appropriate for the simplification, the EC20, has been proposed to express the assay results. Additionally, the reaction time was analysed with the object of avoiding the need for kinetic studies in the method. The simplifications considered offer a more functional method, without significant errors, which could be used for routine analysis.

Structural features of an arabinogalactan gum exudates from Spondias dulsis (Anacardiaceae)

The tree Spondias dulcis, located in Venezuela, exudes a light-brown gum. The polysaccharide, isolated from the original gum, contains galactose, arabinose, mannose, rhamnose, glucuronic acid, and its 4-O-methyl derivative. Application of chemical methods, in combination with 1D and 2D NMR spectroscopy afforded interesting structural features of the gum polysaccharide. The unequivocal presence of rhamnose in the polymer structure was confirmed by chemical and spectral data [1H (1.03 ppm); 13C (16.92 ppm)]. Also confirmed was the existence of 3-O- and 6-O-substitutes galactose residues by the spectral data correlations observed in Heteronuclear Multiple Quantum Coherence (HMQC) and Heteronuclear Multiple Bond Correlation (HMBC). Also observed were unequivocal resonances for beta-D-glucuronic acid and its 4-O-methyl derivative, and the presence of 3-O-alpha-L-arabinofuranose and 3-O-beta-L-arabinopyranose residues.

Structural investigation of the polysaccharide of Spondias mombin gum.

The polysaccharide, isolated from Spondias mombin gum exudate, was subjected to acid hydrolysis and Smith degradation processes in order to investigate its relevant structural features. Chemical and spectral evidence suggested the existence of small blocks of (1→3)-linked β-d-galactosyl residues, interspersed with 6-O substituted d-galactosyl residues, in the backbone of the investigated structure; these residues are also in the side-chains. In addition, there was arabinose, up to three units long, predominantly in the form of furanosyl residues (terminal, 2-O- and 3-O-linked). Arabinopyranosyl residues (terminal and 2-O-linked) as well as rhamnose and mannose are also present in minor amounts. Uronic acids, β-d-glucuronic acid and its 4-O-methyl analogue, are attached at the C-6 and C-4 positions of the galactose moieties in the backbone. These sugar acid residues were difficult to remove from the core. 13C-NMR spectroscopy confirmed the results obtained by chemical methods, and showed the presence of reducing terminal sugar residues and internal acetyl groups.

Structure of a hemicellulose A fraction in dietary fiber from the seed of grape variety Palomino (Vitis vinifera cv. palomino).

The structure of one of the hemicellulose A fractions (HA-1) extracted from the seeds of the grape variety Palomino (Vitis vinifera cv. Palomino) has been studied by means of methylation, 1H NMR, 13C NMR, and partial hydrolysis. This hemicellulose seems to be a homogeneous polysaccharide with an apparent molecular weight of 68 500. Its structure is that of an acidic xylan, a linear chain of beta-D-xylopyranosyl units, bonded together by (1-->4) glycosidic links, containing single alpha-D-xylopyranosyl and 4-O-methyl-alpha-D-glucopyranuronosyl residues joined by glycosidic links to position 2 of the xylose units of the main chain, in proportions of one residue to every 25 units of xylose.

New structural features of Acacia tortuosa gum exudate.

Acacia tortuosa produces a clear gum, very soluble in water. Previous reports showed that it was constituted by four fractions, one of them an arabinogalactan-protein complex. The elucidation of the A. tortuosa gum structure by the combination of classical chemical methods, size exclusion chromatography and NMR spectroscopy, was the objective of this investigation. The data obtained show that the heteropolysaccharide is an arabinogalactan type II, highly ramified, with lateral chains at C-2 as well as at C-6 of the galactose 3-O residues; mono-O-substituted galactoses were not detected. There are residues of mannose, the arabinose, pyranose predominantly, is terminal and 2-O-linked. The abundance of the 4-O-methyl-α-d-glucuronic acid was not previously reported. The proteic fraction is probably represented by an arabinogalactan-protein complex that binds poorly with β-glucosyl Yariv reagent, and two glycoproteins. The NMR spectra suggest that the carbohydrate links to hydroxyproline through the galactose (galactosylation).