Models for terminal Ni(S-cysteine) modification in [NiFe]hydrogenases by iodoacetamide and iodoacetate

The cis-dithiolate complex [N,N' -bis(2-mercaptoethyl-2-methylpropyl)-1,5-d iazacyclooctane]nickel(II) (Ni-1*) reacts with stoichiometric amounts of io doacetamide to yield S-alkylated, mono- and diacetamide complexes, [(AA)Ni- 1*][I] and [(AA)(2)Ni-1*][I](2). Their molecular structures are established by X-ray crystallography and find the former in pseudo-square planar geome try with no additional coordination of the amide functionality, while the l atter is an octahedral N2S2O2NiII complex. The assignment of the axial coor dination ligands as amide oxygen atoms is consistent with IR-spectroscopic v(C=O) results both in the solid and solution states. The complexes are fur ther characterized by UV/Vis spectroscopy, conductance measurements, and el ectrochemical studies. Comparisons are drawn between the alkylation of thes e simple dithiolate complexes and the loss of [NiFe]hydrogenase activity up on addition of alkylating agents.