Anis Arnous

Quantitative Prediction of Cell Wall Polysaccharide Composition in Grape (Vitis vinifera L.) and Apple (Malus domestica) Skins from Acid Hydrolysis Monosaccharide Profiles

On the basis of monosaccharide analysis after acid hydrolysis of fruit skin samples of three wine grape cultivars, Vitis vinifera L. Cabernet Sauvignon, Merlot, and Shiraz, and of two types of apple, Malus domestica Red Delicious and Golden Delicious, an iterative calculation method is reported for the quantitative allocation of plant cell wall monomers into relevant structural polysaccharide elements. By this method the relative molar distribution (mol %) of the different polysaccharides in the red wine grape skins was estimated as 57-62 mol % homogalacturonan, 6.0-14 mol % cellulose, 10-11 mol % xyloglucan, 7 mol % arabinan, 4.5-5.0 mol % rhamnogalacturonan I, 3.5-4.0 mol % rhamnogalacturonan II, 3 mol % arabinogalactan, and 0.5-1.0 mol % mannans; the ranges indicate minor variations in the skin composition of the three different cultivars. These cell wall polysaccharides made up approximately 43-47% by weight of the skins (dry matter), the rest mainly being lignin. The predicted relative molar levels of the polysaccharide elements in the apple skins, which made up approximately 49-64% by weight of the skins (dry matter), appeared to be similar to those of the grape skins. The apple skins were estimated to be relatively richer than grape skins in arabinan, total levels 10-13 mol %, and relatively lower in mannan content, total levels </=0.3 mol %. The data also demonstrate the superiority of trifluoroacetic acid to hydrochloric acid for hydrolysis of plant cell wall material to monosaccharides, notably with respect to the galacturonic acid levels and, in turn, in relation to predicting the relative contents of structural pectin elements in the plant cell wall substrates.

Kinetics of Enzyme-Catalyzed Cross-Linking of Feruloylated Arabinan from Sugar Beet

Ferulic acid (FA) groups esterified to the arabinan side chains of pectic polysaccharides can be oxidatively cross-linked in vitro by horseradish peroxidase (HRP) catalysis in the presence of hydrogen peroxide (H(2)O(2)) to form ferulic acid dehydrodimers (diFAs). The present work investigated whether the kinetics of HRP catalyzed cross-linking of FA esterified to α-(1,5)-linked arabinans are affected by the length of the arabinan chains carrying the feruloyl substitutions. The kinetics of the HRP-catalyzed cross-linking of four sets of arabinan samples from sugar beet pulp, having different molecular weights and hence different degrees of polymerization, were monitored by the disappearance of FA absorbance at 316 nm. MALDI-TOF/TOF-MS analysis confirmed that the sugar beet arabinans were feruloyl-substituted, and HPLC analysis verified that the amounts of diFAs increased when FA levels decreased as a result of the enzymatic oxidation treatment with HRP and H(2)O(2). At equimolar levels of FA (0.0025-0.05 mM) in the arabinan samples, the initial rates of the HRP-catalyzed cross-linking of the longer chain arabinans were slower than those of the shorter chain arabinans. The lower initial rates may be the result of the slower movement of larger molecules coupled with steric phenomena, making the required initial reaction of two FAs on longer chain arabinans slower than on shorter arabinans.