Xanthan electron micrographs

In this study we use electron microscopy (EM) to study xanthan strandedness and topology both in the ordered and disordered conformation. Correlation of data obtained from electron micrographs to physical properties of dilute aqueous solution on the same sample will be used to provide a working hypothesis of the solution configuration of xanthan. Electron micrographs obtained from xanthan of different origins will be compared to assess similarities and differences in secondary structure at the level of resolution in the used EM technique. 

Ordered conformation. Figure 1. shows representative electron micrographs of samples A, B, C, and D vacuum dried from xanthan solutions under ordering conditions as specified in the legend. Xanthan samples A -D appear as unbranched, uniformly thick, convoluted chains. The contour length varies from molecule to molecule as expected for a polydisperse polymer. The electron micrographic... 

Figure 1. Electron micrographs of xanthan sample A (A), sample B (B) sample C (C) and sample D (D). The electron micrographs were obtained from replicas of vacuum dried solutions containing 100 mM NH4Ac, 50% glycerol and 3 -10 ]ig /ml polymer. Scale bar = 200 nm.

Figure 3 shows an electron micrograph of a representative replica region of "native" xanthan E. This native xanthan (Fig. 3) contain a large number of aggregates, whereas after exposure to 80 °C for 2 months in synthetic brine (27) this sample reveals the general features of a polydisperse, uniformly thick worm-like polymer. 

Figure 3. Electron micrographs of native xanthan sample E. The electron micrographs were obtained from replicas prepared as described in Figure 1. Scale bar = 200 nm.

Figure 4. Average end-to-end distance 1/2 versus contour distance L for xanthan from samples D ( V ), F ( ) and G ( ) calculated from electron micrographs obtained as described in Figure 1. was averaged over a total contour distance of 123 )im, 162 Jim, and 124 )im for samples D, F and G respectively.

Disordered conformation. Figure 5 shows electron micrographs of xanthan D and F obtained from xanthan vacuum-dried from solutions yielding the disordered conformation. The various molecular assemblies are assigned as follow I = single -stranded, II = perfectly matched double stranded, III = branched from double - to single stranded. This assignment will be discussed below. 

Figure 5. Electron micrographs of xanthan sample F (A), and D (B) obtained from replicas of xanthan vacuum dried from solutions containing 0.1 mM ammonium acetate, 50 % glycerol and 3-10 ug /ml polymer. Specie designation I = single-stranded, II = perfectly matched doublestranded, III = branching from double- to single-stranded. Scale bar = 200 nm.

Figure 1. Electron micrograph of xanthan from a fermentation broth (Pfizer Flocon 4800) diluted in 100 mM ammonium acetate (A), and powdered xanthan (Kelco Kelzan XCD) in 2 mM ammonium acetate (B). Scale bar = 200 nm. 

Figure 2.3. Electron micrographs of xanthan from Holzwarth and Prestridge (1977) showing (a) a section of a native xanthan molecule (the frame shows about one-sixth of the entire molecule) and (b) shorter, denaturated xanthan molecules.

Figure 2.4. Electron micrographs of xanthan from Stokke et al (1988) showing (a) a xanthan produced as a fermentation broth (Pfizer, Flocon 4800) and (b) a powdered xanthan (Kelco, Kelzan XCD). Scale bar = 200 nm.

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