Open sulfur atom insertion

The sulfur compounds in crude oil can be separated into two classes aliphatic and aromatic hydrocarbons. The aliphatic hydrocarbons are characterized by open carbon chains, which can be branched. One or several sulfur atoms are inserted into these carbon chains in different ways, as shown in Table 34.2. These compounds are denoted sulfides, disulfides, and thiols [11]. 

Aconitase was initially crystallized in the [3Fe-4S]" " form, and these crystals could be converted to the [4Fe S] " " form by addition of ferrous ammonium sulfate. Alternatively, anaerobic crystallization methods were used to crystallize the [4Fe-4S] " " form directly." The iron-sulfur cluster of aconitase sits within a solvent-filled cleft in the protein structure and is coordinated by Cys 358, Cys 421, and Cys 424. The [3Fe S]+ cluster shows the typically cuboidal geometry, with the open iron site directed toward the active site cleft. Conversion to the active [4Fe-4S] " " form results in almost no structural perturbation, with the extra iron atom simply being inserted into the vacant site in the cluster. The fourth iron is tetrahedrally coordinated, but with a solvent hydroxide rather than a cysteine as the fourth ligand. (The electron density clearly reveals a bound solvent, and previous ENDOR studies had demonstrated that the bound species was associated with only a single proton.)... 

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