Methylation of Iron−Sulfur Complexes by Trimethyl Phosphate

Reaction of [(C(4)H(9))(4)N](2)[Fe(4)S(4)(SR)(4)] (R = C(6)H(5), C(2)H(5)) with (CH(3)O)(3)PO in DMSO-d(6) afforded [(C(4)H(9))N](2){Fe(4)S(4)(SR)(3)[(CH(3)O)(2)PO(2)]} and CH(3)SR as revealed by (1)H and (31)P{(1)H} NMR spectroscopy. The more reduced species [(C(2)H(5))(4)N](3)[Fe(4)S(4)(SC(2)H(5))(4)] gave uncoordinated (CH(3)O)(2)PO(2)(-) and CH(3)SC(2)H(5) in addition to an unidentified iron thiolate species. Stoichiometric methylation of mononuclear [(C(2)H(5))(4)N](2)[Fe(SC(2)H(5))(4)] by (CH(3)O)(3)PO afforded [Fe(2)(SC(2)H(5))(6)](2)(-) as well as free (CH(3)O)(2)PO(2)(-) and CH(3)SC(2)H(5). Kinetic studies revealed the rate constant for methylation of [(C(2)H(5))(4)N](3)[Fe(4)S(4)(SC(2)H(5))(4)] to be more than 200-fold higher than that of the oxidized analogues [(C(4)H(9))(4)N](2)[Fe(4)S(4)(SR)(4)] (R = C(6)H(5), C(2)H(5)). The compound [(C(2)H(5))(4)N](2)[Fe(SC(2)H(5))(4)] had the highest rate constant, >/=5 x 10(-)(3) s(-)(1) at concentrations of 5.0 mM in complex and 1.0 mM in (CH(3)O)(3)PO. Attempts to prepare site-differentiated tetranuclear iron-sulfur complexes by removing one thiolate via methylation and addition of second, capping ligands are described. These results are discussed in the context of protein metal thiolate moieties that transfer methyl cations for substrate synthesis, such as carbon monoxide dehydrogenase/acetyl coenzyme A synthase, and repair of DNA alkylation damage.