Anation of laccase type 2 copper by azide and thiocyanate ions

A mechanistic study of the anation of type 2 Cu(II) in fully oxidized laccase by azide and thiocyanate ions (X-) is reported. The rate data support a mechanism involving rapid formation of an outer-sphere complex (laccase . X-) followed by rate-limiting dissociative interchange to give the inner-sphere complex (laccase-X-) product: (formula; see text) Rate parameters for the laccase-azide reaction are k'2 = 1.25 X 10(-1) s-1 and k'-2 = 1.50 X 10(-2) s-1; those for the laccase-thiocyanate reaction are k'2 = 2.0 X 10(-2) s-1 and k'-2 = 1.1 X 10(-2) s-1 (25 degrees C, pH 6.1 phosphate buffer, I = 0.5 M). Although the rate law for anation of type 2 Cu(II) by N3- in 2-(N-morpholino)ethanesulfonic acid (Mes) or acetate medium (kobsd = k3 + k4[N3-]) differs from that observed in phosphate buffer, the data may still be accounted for in terms of the above mechanism, with Kos[N3-] much less than 1 [in Mes, k3 = 2.2 X 10(-2) s-1 and k4 = 3.3 X 10(-1) M-1 s-1; in acetate, k3 = 2.1 X 10(-2) s-1 and k4 = 3.4 X 10(-1) M-1 s-1 (25 degrees C, pH 6.0, I = 0.25 M)]. The equilibrium and spectroscopic characteristics of the laccase type 2 Cu(II)-N3- complex are compared with those of low molecular weight copper(II)-azide species, and the factors responsible for the very low substitutional reactivity of the type 2 cupric ion are discussed.