to the production of nitro and phenoxy radicals, which then reacted together to give
2- and 4-nitrophenols. In this catalytic antibody, the protein thus appeared to have
two main roles, it protected MP8 against oxidative degradation and induced a
regioselectivity of the reaction towards the formation of 2-nitrophenol as a major
product [52].
The only report to date on oxygen atom insertion into a C–H bond catalyzed by
ArMs was from the group of Hayashi and Hisaeda. They prepared apo-Mb and
mutants and then inserted heme derivatives bearing up to eight anionic carboxylate
moieties into the heme-binding pocket (Fig. 5). The resulting ArMs not only
showed peroxidase activity but also catalyzed the oxidation of catechol with rates
up to 11-fold higher than native Mb, as well as the regioselective hydroxylation of
ethylbenzene to 1-phenylethanol by H 2 O 2 , when Mb was reconstituted with a
manganese-porphycene cofactor (Fig. 5) [36, 42].
2.1.6 Epoxidation
Here also, only a few examples of artificial metalloenzymes catalyzing alkene
epoxidation have been reported so far. In the first papers simultaneously reported by
Soumillion and Kazlauskas et al. the native Zn(II) ion of isoform II of human
carbonic anhydrase (hCAII) was replaced by several metal cations including Co(II),
Cu(II), Ni(II), Mn(II) and Rh(I). The X-ray crystal structures of hCAII substituted
with these metal cations revealed that the primary coordination sphere of each of
these was uniquely perturbed relative to the native Zn(II), although the overall
metal-binding motif constituted by three histidine residues remained intact [53]. In
the particular case when the Zn(II) ion was replaced by Mn(II), the new hCAII-Mn
(II) metalloenzyme was found to catalyze the enantioselective alkene epoxidation
with up to 66.5% ee and 12.5% conversion [54, 55]. To broaden the range of
catalysts capable of catalyzing the epoxidation reaction, Reetz et al. have decided to
covalently modify papain with a manganese-salen complex, using Michael addition
of the thiolate function of cysteine 25 from papain to a maleimide substituent
carried by the salen ligand. The artificial enzymes appeared to catalyze epoxidation
reaction with enantiomeric excesses of about 10% [56, 57].
Finally, the catalytic activity of Xln A-based artificial hemoproteins was also
investigated for the selective oxidation of alkenes. Mn(III)(TpCPP) was
non-covalently inserted into Xln10A and the oxidation of various styrene derivatives by various oxidants such as hydroperoxides (H 2 O 2 and
t
BuOOH), sodium
hypochlorite (NaOCl), sodium periodate (NaIO 4 ), and oxone
® (KHSO 5 ) was tested
in the presence of the Mn(III)(TpCPP)-Xln10A biocatalyst. Positive results were
only observed with KHSO 5 as oxidant, together with the highest catalytic activity
reported so far for the oxidation of styrene catalyzed by artificial metalloproteins.
However, a rather low chemo- and enantio-selectivities (3% < ee < 25% in favor of
the (S)-product) were observed for the epoxidation of styrene and poorly activated
styrenes derivatives. On the contrary, high reverse enantioselectivity (80% in favor
of the (R)-isomer) was observed for the epoxidation of para-methoxystyrene by
KHSO 5 (Fig. 6), which constitutes the highest enantioselectivity ever reported to
date for an epoxidation reaction catalyzed by an ArM [46].
Current Applications of Artificial Metalloenzymes …
375
2- and 4-nitrophenols. In this catalytic antibody, the protein thus appeared to have
two main roles, it protected MP8 against oxidative degradation and induced a
regioselectivity of the reaction towards the formation of 2-nitrophenol as a major
product [52].
The only report to date on oxygen atom insertion into a C–H bond catalyzed by
ArMs was from the group of Hayashi and Hisaeda. They prepared apo-Mb and
mutants and then inserted heme derivatives bearing up to eight anionic carboxylate
moieties into the heme-binding pocket (Fig. 5). The resulting ArMs not only
showed peroxidase activity but also catalyzed the oxidation of catechol with rates
up to 11-fold higher than native Mb, as well as the regioselective hydroxylation of
ethylbenzene to 1-phenylethanol by H 2 O 2 , when Mb was reconstituted with a
manganese-porphycene cofactor (Fig. 5) [36, 42].
2.1.6 Epoxidation
Here also, only a few examples of artificial metalloenzymes catalyzing alkene
epoxidation have been reported so far. In the first papers simultaneously reported by
Soumillion and Kazlauskas et al. the native Zn(II) ion of isoform II of human
carbonic anhydrase (hCAII) was replaced by several metal cations including Co(II),
Cu(II), Ni(II), Mn(II) and Rh(I). The X-ray crystal structures of hCAII substituted
with these metal cations revealed that the primary coordination sphere of each of
these was uniquely perturbed relative to the native Zn(II), although the overall
metal-binding motif constituted by three histidine residues remained intact [53]. In
the particular case when the Zn(II) ion was replaced by Mn(II), the new hCAII-Mn
(II) metalloenzyme was found to catalyze the enantioselective alkene epoxidation
with up to 66.5% ee and 12.5% conversion [54, 55]. To broaden the range of
catalysts capable of catalyzing the epoxidation reaction, Reetz et al. have decided to
covalently modify papain with a manganese-salen complex, using Michael addition
of the thiolate function of cysteine 25 from papain to a maleimide substituent
carried by the salen ligand. The artificial enzymes appeared to catalyze epoxidation
reaction with enantiomeric excesses of about 10% [56, 57].
Finally, the catalytic activity of Xln A-based artificial hemoproteins was also
investigated for the selective oxidation of alkenes. Mn(III)(TpCPP) was
non-covalently inserted into Xln10A and the oxidation of various styrene derivatives by various oxidants such as hydroperoxides (H 2 O 2 and
t
BuOOH), sodium
hypochlorite (NaOCl), sodium periodate (NaIO 4 ), and oxone
® (KHSO 5 ) was tested
in the presence of the Mn(III)(TpCPP)-Xln10A biocatalyst. Positive results were
only observed with KHSO 5 as oxidant, together with the highest catalytic activity
reported so far for the oxidation of styrene catalyzed by artificial metalloproteins.
However, a rather low chemo- and enantio-selectivities (3% < ee < 25% in favor of
the (S)-product) were observed for the epoxidation of styrene and poorly activated
styrenes derivatives. On the contrary, high reverse enantioselectivity (80% in favor
of the (R)-isomer) was observed for the epoxidation of para-methoxystyrene by
KHSO 5 (Fig. 6), which constitutes the highest enantioselectivity ever reported to
date for an epoxidation reaction catalyzed by an ArM [46].
Current Applications of Artificial Metalloenzymes …
375