Desulfurization pathway

The course of the photolysis of a number of cyclic sulfoxides, however, has been shown not to involve simple photoextrusion processes. In fact, the work of Schultz and Schlessinger19,20 and Still and coworkers21 has shown the existence of a novel desulfurization pathway leading to cyclic ethers or to carbonyl compounds by formal loss of the sulfur atom only, by certain cyclic sulfoxides. 

A variation of the pathway in which 2 -hydroxybiphenyl-2-sulfinate is converted to 2 -hydroxybiphenyl-2-sulfonate (HBPSo) has been reported [26,62], In this alternate desulfurization pathway, the HBPSo further spontaneously cyclizes to sultone , BPSo. The latter is a substrate for DszA (Fig. 2), and is desulfurized to 2, 2 -dihydroxybiphenyl (DHBP) and sulfite. 

Figure 2. Alternate (minor) desulfurization pathway in IGTS8, with a branch point at HBPSi.

Further, although the first two enzymes in the pathway seem similar to the DBT desulfurization pathway of IGTS8, they have exclusive substrate specificity, since they cannot convert DBT to HBPSi. The Gordonia sp. Strain 213 and a related species 213F were classified as new species named as G. desulfuricans, based on their unique 16S rRNA sequence [18],... 

Figure 5. BT desulfurization pathway of G. rubropertinctus strain T08. The dotted line indicates another possible pathway for coumaranone formation.

The desulfurization pathway was proposed to be BT -> BT sulfone -> benzo[e][l,2]oxanthiin S-oxide -> o-hydroxystyrene. Additionally, formation of the intermediate benzo(e)(l,2)oxathiin S,S dioxide was inferred to a side pathway resulting in formation of benzofuran as shown in Fig. 7. This pathway is similar to that reported for Sinorhizobium KT55, Paenibacillus sp. strain All-2 and R. erythropolis KT462. 

Engineered biocatalysts with altered specificity have been developed. A Rhodococcus strain capable of DBT as well as BT desulfurization has been developed by cloning dsz genes into a strain-containing BT desulfurization pathway. The variety of sulfur compounds in petroleum feedstocks and complexity of the problem may require use of a consortium rather than a single bacterial strain. Alternately, use of multiple bioreactors each with a single dominant strain may be employed to achieve maximum desulfurization [299],... 

Further work at EniTecnologies was conducted with Rhodococcus strains. Rhodococ-cus was selected for its metabolical versatility, easy availability in soils and water, and remarkable solvent tolerance. Its capabilities for catalyzing diverse transformation reactions of crude oils, such as sulfur removal, alkanes and aromatics oxidation and catabolism caught their attention. Hence, genetic tools for the engineering of Rhodococcus strains have been applied to improve its biotransformation performance and its tolerance to certain common contaminants of the crude oil, such as cadmium. The development of active biomolecules led to the isolation and characterization of plasmid vectors and promoters. Strains have been constructed in which the careful over-expression of selected components of the desulfurization pathway leads to the enhancement of the sulfur removal activity in model systems. Rhodococcus, Gordona, and Nocardia were transformed in this way trying to improve their catalytic performance in BDS. In a... 

In a patent on biological desulfurization [100] of petroleum/coal, only the use of whole cell biocatalysts was claimed. The biocatalysts included microorganisms belonging to the genus Pseudomonas, Flavobacterium, Enterobacter, Aeromonas, Bacillus or Corynebacterium. The desulfurization pathway (sulfur-specific vs. destructive) was not specified. The Japanese patents No. JP2071936C and JP7103379B seem to be equivalent patents. 

With the exception of quinoline HDN, all catalysts are active for the reactions studied. All three catalysts are found to attain complete sulfur removal in the form of H2S (100% HDS) from benzothiophene after 1 h of reaction time (Figure 27.6(a)). The desulfurization pathway is found to go through the rapid hydrogenation of benzothiophene to dihydrobenzo-thiophene, followed by cleavage of the C-S bond in dihydrobenzo-thiophene, to yield ethyl benzene as the main product of the reaction. As seen in Table 27.3, the oxynitride and the oxycarbide are almost twice as active for HDS than the sulfated hematite. In terms of the turnover frequency, the oxycarbide is more active than the oxynitride for all reactions tested. 

Determined at 30 % of total 4,6-DMDBT conversion. DDS, direct desulfurization pathway ... 

Oxidation of DBT using the recombinant DszC enzyme produced DBTO2 in 79% yield no DBTO was observed. In the absence of reductase, there was no detectable conversion. When DBTO was used as a substrate, it was quantitatively converted to DBTO2, indicating that DBTO is indeed an intermediate in the desulfurization pathway. Assays using benzyl sulfide as the substrate produced 71% benzyl sulfoxide, 6% benzyl sulfone, and the remainder unreacted substrate, demonstrating that sulfides are also substrates for the enzyme. Under identical conditions, using benzyl sulfoxide as the substrate, 11% was converted to the sulfone. 

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