Chemistry Reference
In-Depth Information
cofactor and to allow increased access of substrates. The sulfoxidation activity of this
Cr(III)-salophen myoglobin complex was studied and showed a six-fold rate increase
over free Cr(III)-salophen in solution. The enantioselectivity of free salophen was 0%,
while that of the myoglobin catalyst was 13%. While the ee is low for this artificial
metalloprotein, Wantabe and co-workers were successful in demonstrating that asym-
metric reactions can be accomplished using chiral protein cavities.
5.4.3
Dual Anchoring Strategy
To increase the enantioselectivity of these myoglobin metalloenzymes, Lu and
co-workers have successfully utilized a covalent linkage approach [62]. In an earlier
attempt a Mn(III)-salen complex was incorporated into apo-myoglobin by mutating
residue 103 to cysteine, followed by modification with a methane thiosulfonate deri-
vative of Mn(III)(salen) (Figure 5.16). This catalyst showed sulfoxidation activity; how-
ever, the ee was only 12%. As such a low ee might be a result of the ability of the bound
ligand to exist in multiple conformations within the protein cavity, it was hypothesized
that the rotational freedom of the salen complex could be limited if it was anchored at
Figure 5.16 A Mn(III)(salen) derivatizing reagent used to prepare
protein-Mn(III)(salen) conjugates.
Figure 5.17 Structure of myoglobin complexed with a heme group.
The two residues chosen for mutation, Y
103
and L
72
, are shown. Left:
Side view of myoglobin. Right: Top view of myoglobin. Color scheme:
Carbon (white), oxygen (red), nitrogen (blue), iron (pink).
