Atomic force microscopy of tomato and sugar beet pectin molecules

Atomic force microscopy has been used to image the structure of pectin molecules isolated from unripe tomato and sugar beet tissue. The tomato pectin molecules were found to be extended stiff chains with a weight average contour length of LW = 174 nm and a number average contour length of LN = 132 nm (LW/LN = 1.32). A proportion of the pectin molecules (∼30%) were found to be branched structures. Chemical analysis of the sugar beet pectin extracts showed that the samples contained protein (8.6%). This protein proved difficult to remove and is believed to be covalently attached to the polysaccharide. Imaging of the extracted pectin revealed largely un-aggregated chains: a small fraction (33%) of which were extended stiff polysaccharide chains and a major fraction (67%) of which were of polysaccharide–protein complexes containing a single protein molecule attached to one end of the polysaccharide chains (‘tadpoles’). In addition the sample contained a small number of aggregated structures. The un-aggregated pectin molecules were found to be predominately linear structures with a small fraction (∼17%) of branched structures. The branched structures were all in the free polysaccharide fraction and no branched pectin chains were observed in the protein–polysaccharide complexes. Alkali treatment was found to remove the protein. For the alkali-treated, un-aggregated structures the average contour lengths were found to be LW = 137 nm, LN = 108 nm with LW/LN = 1.27. It is proposed that the ‘tadpole’ structures contribute to the unusual emulsifying properties of sugar beet pectin.