Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 11

species; these systems are more commonly based on the use of O-atom
donors. There are systems; however, these are typically radical-
induced processes and do not involve formation of oxo species. This
extensive literature on Ru/non-porphyrin systems can be traced through refs.
41-43.
Oxidase activity (eqs. 3 and 4). As the complexes
(porp = porphyrin dianion) introduced above do exhibit oxidase activity
(Sections 3.5-3.7), some brief elaboration of the enzymatic oxidase systems
seems appropriate. To our knowledge, there are no oxidases that utilize
metalloporphyrin centres as the active catalytic site. The majority of
oxidases are Cu-containing protein systems 44,45 , and those exemplified in eq.
3 typically convert a primary alcohol moiety in a sugar residue to an
aldehyde group. The “simplest” is probably galactose oxidase, which is one
of a growing class of “free radical” enzymes, and it is only recently that the
nature of the active site has been elucidated 46,47 . There is only a single redox-
active metal centre, which, in the Cu(II) state, is a so-called “normal, non
blue, type-2 Cu”; the second required redox site needed to mediate the
overall 2e-redox reaction is provided by a modified tyrosine radical, present
as an equatorial ligand within an overall square pyramidal Cu site that
contains (at pH 7) 2 histidines and water as the other equatorial ligands and a
tyrosine as the axial ligand 44,47 . The axial tyrosine is thought to play a
mechanistic role by facilitating abstraction of a proton from the substrate
alcohol 47 . An oxidase system as outlined in eq. 4 typically transforms
polyphenols/diols to quinone/diketones, as exemplified by lactase and
ascorbate oxidase (eq. 11). Both these enzymes in the oxidized state contain,
as well as the normal type-2 Cu, the “blue, type-1 Cu” (which exhibits an

intense blue colour, and an EPR signal with small Cu hyperfine coupling due
to delocalization of the spin density towards a cysteine S-atom), and “type-3
Cu” (a centre that is EPR silent because of anti-ferrromagnetic
exchange coupling between the Cu-atoms via a bridging ligand) 48 . The
structure of ascorbate oxidase shows the 4 Cu centres quite close together:
the type-1 Cu is coordinated by 2 histidines, 1 methionine, and 1 cysteine
residue, the type-2 Cu has 3 histidines and a water and the type-3,