Molecular Weight Size And Shape Of Pectins

Over a period spanning more than 50 years of research, the molecular parameters of pectins have been measured extensively by numerous methods. Nevertheless, significant disagreements prevail concerning these parameters. The controversy may arise from the variability of pectin with plant source, whether dissolved pectin exists in solution as individual molecules or as aggregates, and the tendency of pectin to fragment. High methoxy (ca DM = 73) commercial citrus pectins give number-average molecular weights in the range 4-4.9 X 104 (12). A linear plot of reduced osmotic pressure against concentration appears to indicate a well-behaved unaggregated polymer. The second virial coefficient calculated from osmometry indicated a rodlike shape. Nevertheless, briefly heating pectin resulted in a slow concentration-dependent disaggregation that was also shown by osmometry. High-performance size-exclusion chromatography (HPSEC) with on-line vis-cometry detection on a series of commercial citrus pectins gave results consistent with the aggregated rod model ( 13). HPSEC studies of tomato cell wall pectins with computer-aided curve fitting of the ehromatograms revealed that all pectins investigated were composed of a linear combination of five macromolecular subunits (14). The relative sizes of these subunits as obtained from their radii of gyration (i?g) were 1:2:4:8:16, with the smallest subunit having an Rg of about 25 Â. Dissociation of pectin into smaller-size fragments by dialysis against 0.05 M NaCl led to the conclusion that cell-wall pectin acted as if it were an aggregated mosaic, held together at least partially through noncovalent interactions. Superimposed and individual circular microgels in the micrometer size range were isolated from the cell walls of peaches and visualized by electron microscopy after rotary shadowing. Dilute NaCl and 50% aqueous glycerol disaggregated these microgels into rods, segmented rods, and kinked rods that collectively comprised the internal gel network of the microgels (15).

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