Pectic polysaccharides from mature orange (Citrus sinensis) fruit albedo cell walls: Sequential extraction and chemical characterization

The orange albedo alcohol insoluble solids (AIS) wall preparation, material prepared from the albedo tissue of mature orange fruit, contained 55 mol% pectic polysaccharides, including homogalacturonan (HG), rhamnogalacturonan I (RG I) and rhamnogalacturonan II (RG II) together with Type I arabinogalactan (AG) and arabinan. It also contained cellulose (22 mol%) and other non-cellulosic polysaccharides (14 mol%), including xyloglucans (XGs) (10 mol%), heteromannans (2 mol%) and heteroxylans (2 mol%). Sequential extraction of the AIS performed with NaOAc, CDTA, Na2CO3, 1 M KOH and 4 M KOH produced soluble fractions (Frs.) with different relative proportions of these polysaccharides. The NaOAc and CDTA Frs. were composed predominantly of pectic polysaccharides (89 and 97 mol%, respectively) with only very low proportions of other non-cellulosic polysaccharides. Na2CO3 (50 mM) released highly ramified pectic polysaccharides, whereas stronger alkali (1 M and 4 M KOH) liberated mostly non-cellulosic polysaccharide, comprised predominantly of XG. The NaOAc, CDTA and Na2CO3 Frs. contain low levels of 2-O-methyl-Fuc and 2-O-methyl-Xyl, suggesting the presence of RG II. To obtain the RG II for structural characterization, the NaOAc fraction was digested with a combination of endo-polygalacturonase (endo-PG; PG I and PG II) and exo-PG, and the degradation product was fractionated by anion exchange and size exclusion chromatography. The isolated product (approx. 0.5% of orange albedo AIS) contained all six diagnostic sugars and has a glycosyl linkage composition consistent with the structural model of RG II.