Electron transfer between plastocyanin and benzimidazolic coordination compounds in DMSO-H2O

The electron transfer reactions between several chromium(III), iron(III) and cobalt(III) coordination compounds and spinach plastocyanin (PC) were studied. The ligands coordinated to the metal ions are derivatives of benzimidazole. Kinetic studies were carried out in dimethyl sulphoxide-H₂O (25:75%) and the reaction mechanism is discussed. For comparison with previous studies, the reaction of [Co(phen)₃]³⁺ with PC was studied both in aqueous buffer solution and in dimethyl sulphoxide-H₂O (25:75%); the results indicated that the electron transfer is accelerated in reactions carried out in the latter medium. [Fe(2gb)₃](NO₃)₃, 2gb=2-guanidinobenzimidazole, and [Fe(ntb)Cl₂]Cl, ntb=tris(benzimidazolyl)methylamine oxidised PC following a simple second order outer sphere mechanism. The rate constants for electron transfer are 1.4×10⁴±1.1×10² and 706.2±12.7 M⁻¹ s⁻¹, respectively. Cobalt(III) and chromium(III) benzimidazolic compounds behaved as inhibitors to the electron transfer process. NMR studies indicated that the conformation of the protein does not change in DMSO-H₂O (25:75% v/v) when compared with that in aqueous buffer.