抄録
The structure of the arabinogalactan protein of gum arabic and its mode of supramolecular association have been investigated by atomic force microscopy. Gum arabic samples and fractions derived by hydrophobic interaction chromatography were dissolved in H2O and deposited on surfaces for imaging by tapping mode atomic force microscopy in air. Two different surfaces were used: mica and graphite. The mica surface is hydrophilic, negatively charged, and acquires a thin layer of water from atmospheric water vapor. Graphite is a hydrophobic surface, and is poorly hydrated. The two surfaces provide complementary information about the samples.
On mica, rounded globules were observed. This reflects weak attraction to the mica surface, and consequent intramolecular condensation of the sample components. The globules appear to fall into two classes differing in size. The diameters at half height, uncorrected for tip broadening effects, were approximately 20-40 nm and approximately 50-60 nm for the two classes. Real diameters would be smaller. The globules are tentatively identified as arabinogalactan (AG) and arabinogalactan protein (AGP), respectively.
On graphite, both globular and extended forms were observed. Resolution was generally superior to that obtained on mica. Small globules less than about 20 nm in diameter were observed as isolated features, and as subunits in assemblies, where close packing allowed a globule diameter of 10-16 nm to be determined. The small globules were attributed to partially dehydrated AG polysaccharide. They were the most prevalent feature imaged, in accord with the gum composition of 89% AG component, and a hydrodynamic diameter measured in solution of approximately 20-30 nm for AG. Extended structures, with both thin linear sections and linear arrays of globules, were observed and attributed to the AGP component. The AGP structure observed was consistent with the "wattle blossom" model.
