Reactivity of [Fe4S4(SR)4]2-,3- Clusters with Sulfonium Cations: Analogue Reaction Systems for the Initial Step in Biotin Synthase Catalysis
详细信息 查看全文
- 作者：Christopher J. A. Daley and R. H. Holm
- 刊名：Inorganic Chemistry
- 出版年：2001
- 出版时间：June 4, 2001
- 年：2001
- 卷：40
- 期：12
- 页码：2785 - 2793
- 全文大小：147K
- 年卷期：v.40,no.12(June 4, 2001)
- ISSN：1520-510X
文摘
The first step in catalysis by a class of iron-sulfur enzymes that includes biotin synthase is the one-electronreductive cleavage of the obligatory cofactor S-adenosylmethionine by an [Fe4S4]+ cluster to afford methionineand the deoxyadenosyl radical (DOA
). To provide detailed information about the reactions of sulfonium ionswith [Fe4S4]2+,+ clusters, the analogue reaction systems [Fe4S4(SR')4]2-,3-/[PhMeSCH2R]+ (R' = Et (4, 6), Ph(5, 7); R = H (8), COPh (9), p-C6H4CN (10)) were examined by 1H NMR spectroscopy. Sulfonium ions 8-10react completely with oxidized clusters 4 and 5 to afford PhSMe and R'SCH2R in equimolar amounts as a resultof electrophilic attack by the sulfonium ion on cluster thiolate ligands. Reactions are also complete with reducedclusters 6 and 7 but afford, depending on the substrate, the additional products RCH3 (R = PhCO, p-C6H4CN)and the ylid PhMeS=CHR or (p-NCC6H4CH2)2. Redox potentials of 9 and 10 allow electron transfer from 6 or7. The reaction systems 6/9,10 and 7/9,10 exhibit two reaction pathways, reductive cleavage and electrophilicattack, in an ca. 4:1 ratio inferred from product distribution. Cleavage is a two-electron process and, for examplein the system 6/9, is described by the overall reaction 2[Fe4S4(SR')4]3- + 2[PhMeSCH2R]+ 
2[Fe4S4(SR')4]2-+ PhSMe + RCH3 + PhMeS=CHR. This and other reactions may be summarized as [PhMeSCH2R]+ + 2e- +H+ PhSMe + RCH3; proposed reaction sequences parallel those for electrochemical reduction of sulfoniumions. This work demonstrates the intrinsic ability of [Fe4S4]+ clusters with appropriate redox potentials to reductivelycleave sulfonium substrates in overall two-electron reactions. The analogue systems differ from the enzymes inthat DOA is generated in a one-electron reduction and is sufficiently stabilized within the protein matrix toabstract a hydrogen atom from substrate or an amino acid residue in a succeeding step. In the present systems,the radical produced in the initial step of the reaction sequence, [Fe4S4(SR')4]3- + [PhMeSCH2R]+ [Fe4S4(SR')4]2- + PhSMe + RCH2, is not stabilized and is quenched by reduction and protonation.
). To provide detailed information about the reactions of sulfonium ionswith [Fe4S4]2+,+ clusters, the analogue reaction systems [Fe4S4(SR')4]2-,3-/[PhMeSCH2R]+ (R' = Et (4, 6), Ph(5, 7); R = H (8), COPh (9), p-C6H4CN (10)) were examined by 1H NMR spectroscopy. Sulfonium ions 8-10react completely with oxidized clusters 4 and 5 to afford PhSMe and R'SCH2R in equimolar amounts as a resultof electrophilic attack by the sulfonium ion on cluster thiolate ligands. Reactions are also complete with reducedclusters 6 and 7 but afford, depending on the substrate, the additional products RCH3 (R = PhCO, p-C6H4CN)and the ylid PhMeS=CHR or (p-NCC6H4CH2)2. Redox potentials of 9 and 10 allow electron transfer from 6 or7. The reaction systems 6/9,10 and 7/9,10 exhibit two reaction pathways, reductive cleavage and electrophilicattack, in an ca. 4:1 ratio inferred from product distribution. Cleavage is a two-electron process and, for examplein the system 6/9, is described by the overall reaction 2[Fe4S4(SR')4]3- + 2[PhMeSCH2R]+ 2[Fe4S4(SR')4]2-+ PhSMe + RCH3 + PhMeS=CHR. This and other reactions may be summarized as [PhMeSCH2R]+ + 2e- +H+ PhSMe + RCH3; proposed reaction sequences parallel those for electrochemical reduction of sulfoniumions. This work demonstrates the intrinsic ability of [Fe4S4]+ clusters with appropriate redox potentials to reductivelycleave sulfonium substrates in overall two-electron reactions. The analogue systems differ from the enzymes inthat DOA is generated in a one-electron reduction and is sufficiently stabilized within the protein matrix toabstract a hydrogen atom from substrate or an amino acid residue in a succeeding step. In the present systems,the radical produced in the initial step of the reaction sequence, [Fe4S4(SR')4]3- + [PhMeSCH2R]+ [Fe4S4(SR')4]2- + PhSMe + RCH2, is not stabilized and is quenched by reduction and protonation.