BIOCHEMICAL-MECHANISMS FOR THE DESULFURIZATION OF COAL-RELEVANT ORGANIC SULFUR-COMPOUNDS

Two microbial strains (Brevibacterium sp. DO, Pseudomonas aeruginosa O S1) were isolated for their ability to desulfurize dibenzothiophene (D BT) and benzyl methyl sulfide (BMS). Enrichment was achieved by a sulf ur-selective screening system using the model compounds as the sole so urce of sulfur for bacterial growth. Brevibacterium sp. DO utilizes DB T as a sole source of sulfur, carbon and energy for growth, whereas Ps eudomonas aeruginosa OS1 metabolizes BMS to only a small extent under sulfur-selective conditions. Investigations of the regulation of enzym es involved in the desulfurization of coal-relevant sulfur compounds i ndicate that in nature at least two mechanisms exist: 'carbon regulati on' and 'sulfur regulation'. The biochemical mechanisms leading to the desulfurization of BMS and DBT are similar. The sulfur atom of both c ompounds is initially oxidized to the corresponding sulfone, and cleav age of the C-S bond proceeds via the formation of a chemically unstabl e hemimercaptal (S-oxidized form) by oxidation of the carbon atom adja cent to the sulfur atom. These results indicate that oxidation of sulf ur to its highest oxidation state may be the precondition for the oxid ative cleavage of the covalent C-S bonds. By isotope-labelling experim ents using O-18(2), the initial enzymes were identified as sulfoxygena ses that use molecular oxygen. Cleavage of the C-S bond of DBT and BMS leads to the formation of organic sulfinic acids as intermediates. Wi th DBT the sulfinic acid is desulfurized probably by hydrolysis; this results in the formation of sulfite and benzoate. The desulfurization of BMS proceeds by sulfonic acid-oxidation. The applicability of these biochemical mechanisms to the microbial desulfurization of coal is di scussed.