The oxovanadyl(IV) catalysed oxidation of adrenaline by molecular oxygen

Abstract

The oxidation of adrenaline (H₂LH⁺) by molecular oxygen in the presence of catalytic amounts of VO²⁺ ions has been followed using a Clark-type oxygen electrode. The empirical rate law –d[O₂]/dt=k_obs.[O₂]+p was obtained in which p is a small constant (only observable at low pH) and k_obs. is given by the relationship (i), where [VO]_T and [L]_T are total initial concentrations of k_obs.=A[VO]_T[L]_T//_{B[H⁺]²+C[L]T+D[L]T²}+E//_[H⁺]+F(i) VO²⁺ and adrenaline respectively. It is demonstrated that this behaviour is consistent with the involvement of both VO²⁺ and V²⁺ species and furthermore that it also calls for the presence of a tris(adrenaline) species, V(LH)₃⁺. A kinetically determined equilibrium constant for reaction (ii) is, VO(LH)₂+ H₂LH⁺⇌ V(LH)₃⁺+ H₂O (ii) reported. The necessary proton– and metal–ligand equilibrium constants were obtained by pH- and ¹H n.m.r.-titration techniques. All solutions were made up to a constant ionic strength (I= 0.100 mol dm^–3) with KNO³ at 25.00 ± 0.02 °C.