Much useful work on pectin characterization can be performed on whole isolated cell walls or alcohol-insoluble residue (AlR) (see Section 1.6). However, if necessary, pectins can be extracted from some tissues (especially ripe fruits) with chelating agents at neutral pH and 20 °C. This does not apply to most other plant tissues, especially actively growing ones. Extraction can be increased by heating, but with partial depolymerization. Heating at around pH4 is least detrimental in this respect; at this pH, oxalate is a more effective chelator than EDTA or EGTA. Oxalate is also easier than EDTA, EGTA and hexametaphosphate to remove (e.g. by dialysis) after pectin extraction. Another agent increasing some pectins' extractability, and maintaining their solubility, is 0.5 M imidazole (pH 7) (Zhang et al. 2007 ).

The ' pectins ' described below (homogalacturonan, RG - I, RG - II, etc.) are probably conjoined domains, linked end-to-end by glycosidic bonds into a single polysaccharide; in muro each 'pectin' is not a separate polysaccharide in its own right. For example, high-Mr pectin extracted from sugar-beet roots with aqueous imidazole was degraded to RG- I , RG- I I and free GalA by polygalacturonases (which hydrolyse homogalacturonan but not rhamnoga-lacturonans; Ishii & Matsunaga 2001). The simplest explanation is that the pectin was a long chain with RG-I and RG-II domains arranged like beads along a homogalacturonan 'string'. Further support for the domain concept came from the characterization of oligosaccharides released by mild acid hydrolysis, including GalA-GalA-GalA-Rha-GalA-Rha, which appears to be a fragment straddling part of a homogalacturonan domain and part of an RG-I domain (Coenen et al. 2007 ).

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