Reversible dioxygen binding and phenol oxygenation in a tyrosinase model system.

The complex [Cu2(L-66)]2‡
(L-66 ˆ a,a'-bis{bis[2-(1'-methyl-2'-
benzimidazolyl)ethyl]amino}-m-xylene)
undergoes fully reversible oxygenation
at low temperature in acetone. The optical
[lmaxˆ362 (e 15 000), 455 (e 2000), and
550 nm (e 900mÿ1cmÿ1)] and resonance
Raman features (760 cmÿ1, shifted to
719 cmÿ1 with 18O2) of the dioxygen
adduct [Cu2(L-66)(O2)]2‡ indicate that
it is a m-h2 :h2-peroxodicopper(ii) complex.
The kinetics of dioxygen binding,
studied at ÿ78 8C, gave the rate constant
k1ˆ1.1mÿ1 sÿ1, for adduct formation, and
kÿ1ˆ7.8 10ÿ5 sÿ1, for dioxygen release
from the Cu2O2 complex. From these
values, the O2 binding constant Kˆ
1.4104mÿ1 at ÿ78 8C could be determined.
The [Cu2(L-66)(O2)]2‡ complex
performs the regiospecific ortho-hydroxylation
of 4-carbomethoxyphenolate
to the corresponding catecholate
and the oxidation of 3,5-di-tert-butylcatechol
to the quinone at ÿ608C.
Therefore, [Cu2(L-66)]2‡ is the first
synthetic complex to form a stable
dioxygen adduct and exhibit true tyrosinase-
like activity on exogenous phenolic
compounds.