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176 Wilfred E. Kleinjan et al. globules is not crystalline orthorhombic sulfur, but rather a liquid or amor- phous form of sulfur, which converts into crystalline orthorhombic sulfur on drying or aging. After this, it was shown that the density of the sulfur globules produced by two Allochromatium species is 1.31 g cm3 [31, 32]. This is considerably lower than the density of liquid sulfur (1.89 g cm3) or that of any form of crystalline sulfur (1.9–2.2 g cm3) [33], indicating that the globules did not consist of pure liquid sulfur either. The suggestion was made that sulfur globules consist of sulfur and an unidentified moiety, which is probably water. In a model proposed by Steudel et al. [34] the glob- ules consist of a nucleus of sulfur rings (S8) with water around it and long- chain sulfur compounds such as polysulfides or polythionates as amphiphi- lic compounds at the interface. Polythionate chains were however not detect- ed in cultures of Al. vinosum storing sulfur globules inside the bacterial cell, thereby limiting the possible amphiphilic compound in the model to long- chain polysulfides or sulfanemonosulfonates [32]. Polythionates in sulfur globules of Al. vinosum were also not detected by Prange et al. [28] who used X-ray absorption near-edge spectroscopy (XANES) [27]. Moreover, they ruled out the presence of sulfur rings and found the globules to consist of long sulfur chains terminated by organic groups (R-Sn-R), indicating the inapplicability of Steudels model of a core of sulfur rings surrounded with water and polythionates or polysulfides for Al. vinosum sulfur globules. Some controversy has however arisen around these XANES measurements since Pickering et al. [25] claimed that the experimental results were misin- terpreted due to artifacts and that sulfur globules of Al. vinosum consisted of simple, solid sulfur. The discussion around this topic focuses on the measurement mode, with each mode having its own problem (electron yield mode with a sample charging effect; fluorescence mode with a self-absorp- tion effect; transmittance mode with a pinhole effect) [35, 36]. The model for sulfur globules of Al. vinosum that corresponds best with the available experimental data consists of long sulfur chains terminated by organic groups as was suggested by Prange et al. [28]. This can account for the ob- served low density and does not need to assume that the observed density is erroneously low. XANES seems to be a useful technique because it can give information on the structure of sulfur globules in situ. Prange et al. showed that isolation of sulfur globules from the bacteria affects the structure of the sulfur [28], thereby limiting the value of studies with experimental techniques which do require isolation of the sulfur globules (e.g., XRD). There are however also certain limitations of the technique concerning interpretation of the data and availability of reference compounds, such as long chain organic sulfur compounds. The sulfur globules of the chemotrophic bacteria B. alba have also been studied quite extensively. The density of the globules has not been deter- mined but energy-dispersive X-ray microanalysis showed that the globules consisted almost entirely of sulfur [9]. XANES [28] and Raman spectroscopy [26] confirmed that the sulfur globules of B. alba consist of S8 sulfur rings.PDF Image | Topics in Current Chemistry
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