Albumin-conjugated corrole metal complexes: Extremely simple yet very efficient biomimetic oxidation systems

Article abstract of DOI:10.1021/ja045372c

An extremely simple biomimetic oxidation system, consisting of mixing metal complexes of amphiphilic corroies with serum albumins, utilizes hydrogen peroxide for asymmetric sulfoxidation in up to 74% ee. The albumin-conjugated manganese corroies also disp

Full text of DOI:10.1021/ja045372c

Published on Web 02/11/2005
Albumin-Conjugated Corrole Metal Complexes: Extremely
Simple Yet Very Efficient Biomimetic Oxidation Systems
Atif Mahammed and Zeev Gross*
Contribution from the Department of Chemistry and Institute of Catalysis Science and
Technology, Technion-Israel Institute of Technology, Haifa 32000, Israel
Received August 1, 2004; Revised Manuscript Received December 23, 2004; E-mail: chr10zg@tx.technion.ac.il
Abstract: An extremely simple biomimetic oxidation system, consisting of mixing metal complexes of
amphiphilic corroles with serum albumins, utilizes hydrogen peroxide for asymmetric sulfoxidation in up to
74% ee. The albumin-conjugated manganese corroles also display catalase-like activity, and mechanistic
evidence points toward oxidant-coordinated manganese(III) as the prime reaction intermediate.
Introduction
strong binding site was identified for the 1:1 conjugates, and
the corresponding dissociation constants were found to be in
The chemistry of corroles has remained quite undeveloped
for decades, which may be appreciated by the very limited
number of reported derivatives and lack of corrole-based
applications.1 Recent years have evidenced three major advances
in the field: new synthetic routes for facile preparation of
triarylcorroles,2 efficient catalysis by the corresponding metal
complexes,3 and unique methodologies for preparation of
amphiphilic corroles.4 Regarding oxidation catalysis, manganese
corroles have received a large amount of attention for three
reasons: (a) the facile isolation of (oxo)manganese(V) corroles;5,6a
(b) the low reactivity of the above and mechanistic puzzles
regarding the identity of other oxygen-transferring intermedi-
ate;5,7 (c) the large increase of catalytic activity upon halogen
substitution of the â-pyrroles.6 In parallel, the amphiphilic bis-
sulfonated corrole 1 (Scheme 1) and its metal complexes were
shown to spontaneously form tightly bound noncovalent con-
jugates with human serum albumin (HSA).8 One particularly
the nanomolar range. These developments suggest that conjuga-
tion of metal complexes of corrole 1 with serum albumins might
be useful for inducing asymmetric catalysis in a biomimetic
fashion: the metal complex being responsible for catalysis and
the protein for a chiral environment. This hypothesis was proven
true: the albumin-conjugated iron and manganese complexes
1-Fe and 1-Mn catalyze the enantioselective oxidation of
prochiral sulfides by hydrogen peroxide to sulfoxides, synthons
of prime importance for asymmetric syntheses (Scheme 1).9
Compared to related biomimetic systems,10,11 this one has the
advantage of relying on the most accessible and cheapest
proteins and on extremely simple working procedures. An
additional outcome of the investigations is concerned with the
mechanism of action, primarily dealing with the identity of the
oxygen-atom-transferring intermediate.
Experimental Section
(1) (a) Paolesse, R. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 2, Chapter
11. (b) Erben, C.; Will, S.; Kadish, K. M. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New
York, 2000; Vol. 2, Chapter 12. (c) Sessler, J. L.; Weghorn, S. J. Expanded,
Contacted, and Isomeric Porphyrins; Pergamon: Oxford, 1997.
(2) For the original breakthroughs, see: (a) Gross, Z.; Galili, N.; Saltsman, I.
Angew. Chem., Int. Ed. Engl. 1999, 38, 1427. (b) Paolesse, R.; Jaquinod,
L.; Nurco, D. J.; Mini, S.; Sagone, F.; Boschi, T.; Smith, K. M. Chem.
Commun. 1999, 1307. For recent reviews, see: (c) Gryko, D. T. Eur. J.
Org. Chem. 2002, 1735. (d) Ghosh, A. Angew. Chem., Int. Ed. 2004, 43,
1918.
(3) For a recent review about oxidation catalysis, see: (a) Gross, Z.; Gray, H.
B. AdV. Synth. Catal. 2004, 346, 165. For catalytic cyclopropanation and
aziridination, see: (b) Simkhovich, L.; Mahammed, A.; Goldberg, I.; Gross,
Z. Chem. Eur. J. 2001, 7, 1041. (c) Simkhovich, L.; Gross, Z. Tetrahedron
Lett. 2001, 42, 8089.
Materials. Human serum albumin (essentially fatty acid free), pig
serum albumin (essentially fatty acid free, essentially globulin free),
sheep serum albumin (essentially fatty acid free), bovine serum albumin,
and rabbit serum albumin (essentially fatty acid free) were purchased
from Sigma. All the sulfides and H2O2 (30% in water) were obtained
from Aldrich. The preparation of compound 1 and its manganese(III)
complex 1-Mn was described in previous publications.4
Preparation of 1-Fe: One portion of FeCl2 (30 mg, 23.7 mmol)
was added at once to a pyridine solution (10 mL) of 1 (30 mg, 31.4
µmol), and the mixture was heated immediately to reflux for 5 min
under argon, followed by evaporation of the solvent. The product was
purified by two subsequent silica gel columns (the eluent was methanol
for the first column and ethanol/CH2Cl2 1:1 for the second column),
affording 31 mg (30.7 µmol, 98% yield) of the iron(III) complex
of 1. 19F NMR (CD3OD): δ ) -106.2 (brs, ortho-F), -115.4
(4) (a) Mahammed, A.; Goldberg, I.; Gross, Z. Org. Lett. 2001, 3, 3443. (b)
Saltsman, I.; Mahammed, A.; Goldberg, I.; Tkachenko, E.; Botoshansky,
M.; Gross, Z. J. Am. Chem. Soc. 2002, 124, 7411.
(5) Gross, Z.; Golubkov, G.; Simkhovich, L. Angew. Chem., Int. Ed. 2000,
39, 4045.
(6) (a) Liu, H.-Y.; Lai, T.-S.; Yeung, L.-L.; Chang, C. K. Org. Lett. 2003,
617. (b) Golubkov, G.; Bendix, J.; Gray, H. B.; Mahammed, A.; Goldberg,
I.; DiBilio, A. J.; Gross, Z. Angew. Chem., Int. Ed. 2001, 40, 2132.
(7) (a) Collman, J. P.; Zeng, L.; Decre´au, R. A. Chem. Commun. 2003, 2974.
(b) Wang, S. H.; Mandimutsira, B. S.; Todd, R.; Ramadhanie, B.; Fox, J.
P.; Goldberg, D. P. J. Am. Chem. Soc. 2004, 126, 18.
(9) Ferna´ndez, I.; Khair, N. Chem. ReV. 2003, 103, 3651.
(10) Dembitsky, V. M. Tetrahedron 2003, 59, 4701.
(11) (a) Yang, H. J.; Matsui, T.; Ozaki, S.; Kato, S.; Ueno, T.; Phillips, G. N.,
Jr.; Fukuzumi, S.; Watanabe, Y. Biochemistry 2003, 42, 10174. (b) Hayashi,
T.; Matsuda, T.; Hisaeda, Y. Chem. Lett. 2003, 32, 496. (c) Coulter, E. D.;
Cheek, J.; Ledbetter, A. P.; Chang, C. K.; Dawson, J. H. Biochem. Biophys.
Res. Commun. 2000, 279, 1011.
(8) Mahammed, A.; Gray, H. B.; Weaver, J. J.; Sorasaenee, K.; Gross, Z.
Bioconjugate Chem. 2004, 15, 738.
9
10.1021/ja045372c CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 2883-2887
2883

A R T I C L E S
Mahammed and Gross
Scheme 1. Albumin-Conjugated Corrole Metal Complexes as Catalysts for Asymmetric Sulfoxidation
(brs, ortho-F), -116.5 (brs, ortho-F), -149.8 (s, para-F), -150.5 (s,
para-F), -154.5 (s, para-F), -156.2 (s, meta-F), -157.2 (s, meta-F),
-160.2 (s, meta-F). UV-vis (buffer solution, pH 7.00) λmax (ꢀ(M-1
cm-1)) ) 404 (34 000), 552 (12 000), 738 (2300). MS (electrospray):
m/z 503.5 [(M - 2H)/2]-.
(1-Mn and 1-MnV(O), respectively) conjugated BSA on the asym-
metric oxidation of thioanisole. The catalytic reactions that started with
Mn(III) as catalyst and H2O2 or iodosylbenzene as oxidant were
performed as follows: subsequent addition of substrate (11 µL as a
10% v/v solution in CH3CN) and oxidant (15 µL of 3% H2O2 or 2.2
mg of PhIO) to a 1 mL aqueous solution (phosphate buffer, pH 7.00)
of BSA (0.3 mM) and 1-Mn (0.2 mM) at 24 °C. The reaction was
stirred for 1.5 h prior to the regular workup. For the catalytic reaction
that started with MnV(O) as catalyst, the subsequent addition of substrate
and oxidant was performed only after the BSA/1-Mn solution was
treated by a small amount of iodosylbenzene and filtrated after full
development of the red-brown color due to 1-MnV(O) (90 s). The
electronic spectrum of the solution of 1-Mn and BSA was recorded
at three times: before formation of 1-MnV(O), immediately after the
addition of substrate and oxidant to 1-MnV(O), and at the end of the
catalytic reaction. This revealed that 7% catalyst decomposition occurred
at the second stage (formation of 1-MnV(O) in the absence of
substrate), while no more catalyst was bleached after addition of H2O2,
i.e., 0% catalyst decomposition after both substrate and H2O2 were
added.
Analysis of Reaction Products. All the sulfoxides, except of
2-fluorophenyl sulfoxide, were analyzed by a Merck Hitachi HPLC
on a Daicel chiral column OD (0.46φ cm × 25 cm). The enantiomeric
sulfoxides from ethylphenylsulfide, p-tolylsulfide, 4-fluorothioanisole,
3-bromothioanisole, and 3-chlorothioanisole were eluted with 90%
hexane and 10% isopropyl alcohol with retention times of 20 and 25,
22 and 25, 24 and 26, 26 and 28, and 26.8 and 27.5 min, respectively.
Methyl phenyl sulfoxide was eluted with 80% hexane and 20%
isopropyl alcohol with retention times of 16 and 19 min. The enan-
tiomeric sulfoxides from 4-bromothioanisole and 2-chlorothioanisole
were eluted with 98% hexane and 2% isopropyl alcohol with retention
times of 78 and 83 and 47 and 51 min, respectively. The enantiomeric
sulfoxides from 4-chlorothioanisole and 2-bromothioanisole were eluted
with 95% hexane and 5% isopropyl alcohol with retention times of 40
and 43 and 33 and 37 min, respectively. The eluent was eluted at a
flow rate of 0.5 mL/min and monitored at 250 nm. For all the sulfoxides
the S enantiomer always eluted from the column first.12 Reaction yields
were measured by comparing the peak area ratios of the HPLC
chromatograms of unreacted sulfide and produced sulfoxide. The
response factor for every pair of sulfide/sulfoxides was calculated by
a standard curve that was constructed from the peak area ratios of the
HPLC chromatograms of mixtures of sulfide/sulfoxide obtained after
Quantitation of the Amount of Catalyst Bleaching. The amount
of catalyst bleaching was appreciated by comparing the electronic
spectra before and after catalytic reactions. For 1-Mn, 0.1 mL from
the catalytic reaction mixture (before adding the oxidant) was diluted
into 1.9 mL of phosphate buffer solution (pH 7.0), and the electronic
spectrum was measured with particular emphasis on the optical densities
at λ ) 418, 492, and 618 nm. The same procedure was repeated at the
end of the catalytic reactions. At every wavelength the final absorbance
was reduced from the initial one and the result was divided by the
initial absorption. The average of the three results at the three
wavelengths provided the percentage of 1-Mn bleaching. The same
procedure was performed for 1-Fe, with emphasis on λ ) 418, 558,
and 622 nm.
1
analogous workup by H NMR.
2-Fluorophenyl methyl sulfoxide was resolved by HP-54890 GC with
a J&W chiral cyclodex-B capillary column and FID detector, linked
to the HP Chem-Station (HP-3365). The GC retention times of the
sulfoxides were 96 and 99 min under the following conditions: 100
°C at 7 psi. Nitrobenzene was used as an internal standard. On the
basis of the relative retention times of other sulfoxides with known
absolute configurations on the same GC column, the R isomer was
assumed to be the faster eluting.
Biomimetic Sulfoxidation. Reactions were performed at 24 °C by
subsequent feeding of the reaction vessel with sulfide, 1 mL of aqueous
phosphate buffer solution pH 7.00 of albumin and catalyst, and
hydrogen peroxide (3%) or iodosylbenzene. The ratios of oxidant:
substrate:BSA:catalyst (0.2 mM) were 75:50:1.5:1 and 50:50:1:1 for
the H2O2 and iodosylbenzene reactions, respectively. After 1.5 h the
solution was extracted with CH2Cl2, and the extracts were concentrated
under a steam of argon. The residue was taken up in the HPLC solvent
and analyzed by HPLC. For the catalytic reaction with 2-flurorothio-
anisole, nitrobenzene was added as an internal standard to the CH2Cl2
extract prior to GC analysis.
Catalytic Oxidation with Larger Concentrations of Thioanisole
and H2O2. A 1 mL amount of an aqueous phosphate buffer solution
(pH 7.00) that contained 0.3 mM BSA and 0.2 mM 1-Mn was added
to a vessel that contained 11 µL of thioanisole (100 mM). A 15 µL
amount of 30% H2O2 (150 mM) was added, and the reaction was stirred
for 1.5 h. The solution was extracted with CH2Cl2, and the extracts
were concentrated under a stream of argon. The residue was taken up
in the HPLC solvent and analyzed by HPLC, which revealed a 30%
yield (150 catalytic turnovers) and 46% ee (S) of methyl phenyl
sulfoxide. Seventeen percent of the catalyst was bleached. When an
identical catalytic reaction was performed but with a reaction time of
15 h, 97% yield with 29% ee (S) of methyl phenyl sulfoxide were
obtained and 50% of the catalyst was bleached.
Catalytic Disproportionation of H2O2. The catalase-like activities
of 1-Fe/BSA and 1-Mn/BSA were assessed by volumetric determi-
nation of the amount of O2 released upon addition of H2O2. The reaction
mixture contained pH 7.00 phosphate buffer, 0.3 mM BSA, and 0.2
mM 1-Fe or 1-Mn in a total volume of 4 mL. The reaction was started
by addition of 0.3 mL of H2O2 (30%), i.e., an initial concentration of
743 mM. The amount of O2 that was released within 1.5 h, at 1 atm
and 24 °C, was 4 (11% yield) and 3.7 mL (10% yield) for 1-Fe/BSA
Four different reactions were performed for examining the effects
of starting with either manganese(III) or (oxo)manganese(V) corrole
(12) (a) Harris, R. Z.; Newmyer, S. L.; Ortiz de Montellano, P. R. J. Biol. Chem.
1993, 268, 1637. (b) Yang, H. J.; Matsui, T.; Ozaki, S.; Kato, S.; Ueno,
T.; Phillips, G. N., Jr.; Fukuzumi, S.; Watanabe, Y. Biochemistry 2003,
42, 10174. (c) Ozaki, S.; Matsui, T.; Watanabe, Y. J. Am. Chem. Soc. 1996,
118, 9784. (d) Ozaki, S.; Ortiz de Montellano, P. R. J. Am. Chem. Soc.
1995, 117, 7056.
9
2884 J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005

Albumin-Conjugated Corrole Metal Complexes
A R T I C L E S
Table 1. Effects of Catalyst, Albumin Source, and Substrate on Asymmetric Oxidation of Aryl Methyl Sulfides (R ) Phenyl Substituent in
Column 1) and Ethylphenylsulfide (PhSEt)a (Yields of g90% and Enantiomeric Excesses (ee) of g60% Are Emphasized)
catalyst
)
1
−
Fe:
catalyst
) 1−Mn:
% yield, major enantiomer, % ee
% yield, major enantiomer, % ee
R
HSA
BSA
PSA
RSA
SSA
HSA
BSA
PSA
RSA
SSA
H
76, S, 10
30, S, 15
42, S, 7
88, S, 7
11, R, 2
40, S, 6
50, S, 11
<1, S, 12
8, R, 2
87, S, 38
23, S, 22
67, S, 34
86, S, 23
42, S, 39
89, S, 26
50, S, 10
17, S, 24
25, S, 5
52, S, 13
66, S, 16
92, S, 33
95, S, 24
23, S, 25
5, S, 35
88, S, 16
1, S, 27
9, S, 27
29, S, 14
75, R, 7
76, S, 18
95, R, 7
12, S, 26
5, S, 17
75, R, 15
7, S, 32
6, S, 3
91, S, 20
60, R, 3
76, S, 25
93, R, 7
33, S, 41
61, R, 2
40, S, 6
5, S, 26
11, R, 11
36, R, 13
69, S, 6
69, R, 17
82, R, 6
39, R, 14
74, R, 9
38, R, 1
30, R, 4
27, S, 5
1, S, 6
83, S, 52
73, S, 50
76, S, 68
82, S, 50
4, S, 70
60, S, 33
94, S, 20
98, S, 65
95, S, 32
30, S, 60
18, S, 19
35, S, 23
3, S, 59
90, S, 55
48, S, 32
72, S, 52
90, S, 26
73, S, 59
27, S, 64
63, S, 23
24, S, 56
28, S, 61
29, S, 24
90, S, 49
90, S, 32
81, S, 16
27, S, 54
98, S, 17
40, S, 61
52, S, 38
74, S, 24
30, S, 60
22, S, 10
25, S, 13
88, S, 29
4-CH3
2-Fb
4-F
2-Cl
3-Cl
4-Cl
2-Br
3-Brc
4-Brc
PhSEt
2, S, 44
68, S, 51
16, S, 74
13, S, 32
23, S, 48
95, S, 26
6, S, 4
35, S, 5
36, R, 1
38, S, 33
51, S, 29
86, S, 26
61, S, 5
34, S, 11
55, S, 2
79, S, 17
65, S, 6
86, S, 14
59, R, 2
80, S, 16
a H2O2:sulfide:albumin:catalyst (0.2 mM) ) 75:50:1.5:1; pH 7.0; T ) 24 °C; reaction time ) 1.5 h. % ee determined by “chiral” HPLC, and enantiomers
(R vs S) identified by reference to published works.12 b The identities of the enantiomers of 2-fluoro-thionanisole oxide were deduced from the BSA- and
PSA-catalyzed reactions that provided an excess amount of the S enantiomer for all other substrates. c Identical to reaction conditions in a, but H2O2 was
added in five portions (10 mM per hour).
Table 2. Effects of Albumin Source and Substrate on Asymmetric
Oxidation of Aryl Methyl Sulfides (R ) Phenyl Substituent in
Column 1) and Ethylphenylsulfide (PhSEt) in the Absence of Metal
Catalyst at the Same Reaction Conditions as In Table 1
no catalyst: % yield, major enantiomer, % ee
R
HSA
BSA
PSA
RSA
SSA
H
4-CH3
2-F
13, R, 2
12, R, 4
18, R, 8
0, -, -
<1, -, 0
0, -, -
0, -, -
0, -, -
0, -, -
5, R, 24
7, R, 1
5, R, 2
9, S, 3
3, R, 21
3, S, 1
6, R, 1
2, R, 3
2, S, 5
6, S, 1
<1, -, 0
2, S, 6
0, -, -
0, -, -
0, -, -
1, R, 10
4, R, <1
4, S, 1
10, R, 3
0, -, -
17, R, 1
<1, -, 0
1, S, 5
2, -, 0
0, -, -
0, -, -
0, -, -
27, R, 1
1, R, 1
0, -, -
10, S, 1
0, -, -
1, -, 0
<1, -, 0
0, -, -
1, R, 5
8, R, 4
8, S, 9
4-F
1, S, 4
2-Cl
3-Cl
4-Cl
2-Br
3-Br
4-Br
PhSEt
0, -, -
0, -, -
4, R, 4
0, -, -
6, S, 2
Figure 1. Electronic spectra of 1-Fe (50 µM) in the absence and presence
of HSA (50 µM).
9, R, 22
1, S, 7
and 1-Mn/BSA, respectively. For 1-Fe/BSA most of the O2 was
released within the first 20 min, and examination of the reaction mixture
after 1.5 h revealed that the catalyst was completely bleached. For
1-Mn/BSA the reaction was slower, and only 30% of the catalyst was
bleached after 1.5 h.
contributions to the enantiomeric excesses (ee’s) due to the
existence of protein binding sites for either the substrates or
the oxidant.14
Results and Discussion
The comparison between the results of Table 1 and the control
reactions (Table 2, negligible yields and ee’s) clearly demon-
strates that catalysis is due to the metallocorrole and that the
enantiomeric excesses reflect the chiral environment surrounding
the albumin-conjugated catalyst. The albumin source had a
highly significant effect on the ee’s with the values for all
substrates averaging 51.4%, 45.5%, 36.3%, 32.2%, and 6.5%
for conjugates of 1-Mn with BSA, RSA, PSA, SSA, and HSA,
respectively (the corresponding numbers without taking PhSEt
into account were 53.2%, 45.2%, 37.3%, 32.5%, and 7.1%). A
similar effect was obtained for the conjugates of 1-Fe with
the same albumins, but the ee’s were significantly lower: 21.8%,
14.3%, 21.5%, 14.5%, and 8.0% (22.3%, 14.1%, 22.2%, 15.4%,
and 7.7% without taking PhSEt into account). BSA and PSA
provided an excess of the S enantiomer for all substrates in
conjugation with both catalysts, and the same holds for RSA
and SSA conjugates with 1-Mn, while significant variations
in the identity of the major enantiomer were obtained in
reactions catalyzed by conjugates of 1-Fe with HSA, RSA,
and SSA and the 1-Mn/HSA system. Another reflection of the
metal effect is that the identity of the major enantiomer obtained
One emphasis of the investigations was on simplicity,
compared to other much more sophisticated enzyme-mimicking
systems.10,11 We already demonstrated the spontaneous and very
strong association of corrole 1 and its manganese(III) complex
1-Mn with HSA,8 which is further exemplified by the
pronounced changes in the electronic spectrum of 1-Fe upon
addition of 1 mol equiv of HSA (Figure 1). This allowed for
the examination of these extremely simple noncovalent bio-
conjugates as catalysts for asymmetric oxidations.
Reactions were performed at 24 °C by subsequent feeding
of the reaction vessel with substrate, an aqueous pH 7 buffer
solution of albumin and catalyst, and hydrogen peroxide (3%).13
The workup procedure was also very simple: washing with CH2-
Cl2 provides a colorless solution that contains only product and
unreacted substrate, while the catalyst and protein remain in
the aqueous phase. The other focus was on variability, as may
be appreciated by the results obtained for 11 substrates and 5
different albumin sources (Table 1). Control reactions without
catalyst (Table 2) were performed as well to rule out two
possible interferences: (a) direct oxidation of the sulfides by
H2O2 as an important route for the high yields; (b) significant
(14) For an example of catalyst-free asymmetric sulfoxidation due to a binding
site for the oxidant, see: Colonna, S.; Banfi, S.; Annunziata, R. J. Org.
Chem. 1986, 51, 891.
(13) HSA, BSA, PSA, RSA, SSA: Human, bovine, pig, rabbit, and sheep serum
albumin.
9
J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005 2885

A R T I C L E S
Mahammed and Gross
Table 3. Effects of Oxidant on Asymmetric Oxidation of Selected
Table 4. Effects of Starting with Either Manganese(III) or
Aryl Methyl Sulfides under Catalysis by the 1-Mn/BSA-Conjugatea
(Oxo)manganese(V) Corrole (1-Mn and 1-MnV(O), respectively)
Conjugated BSA on Asymmetric Oxidation of Thioanisole
catalyst
color
% catalyst
oxidant
substrate
% yield
% ee
bleaching
changes
oxidant
catalyst
% yield
% ee
bleaching
H2O2
PhIO
H2O2
PhIO
H2O2
PhIO
thioanisole
83
92
4
24
16
28
52; S
32; S
70; S
42; S
74; S
45; S
0%
20%
0%
30%
0%
nob
yesc
nob
H2O2 (15 mM)
H2O2 (15 mM)
PhIO (10 mM)
PhIO (10 mM)
1-Mn
83
76
92
81
52; S
44; S
32; S
17; S
0
1-MnV(O)
1-Mn
7 (0)a
20
2-chloro-thioanisole
2-bromo-thioanisole
yesc
nob
1-MnV(O)
22
30%
yesc
a The 7% bleaching occurred during the preparation of 1-MnV(O), i.e.,
prior to addition of substrate and H2O2.
a All reactions were performed at pH 7.0, 24 °C, for 1.5 h. The ratios of
oxidant:substrate:BSA:catalyst (0.2 mM) were 75:50:1.5:1 and 50:50:1:1
for the H2O2 and iodosylbenzene reactions, respectively. b The color of the
reaction mixture remained green (due to 1-Mn) during catalysis, and very
few bubbles of gas (oxygen) were released during the reaction. c The color
of the reaction mixture turned from green to brown-red upon addition of
the oxidant, signaling the formation of 1-MnV(O), and returned to green
after 20, 30, and 40 min for thioanisole, 2-chlorothioanisole, and 2-bro-
mothioanisole, respectively.
oxygen were clearly seen. Although not fully quantified, the
extent of O2 formation was more significant in the low-yielding
reactions.
One obvious difference between PhIO and H2O2 is that only
the latter can play a dual role, oxidant and reducible substrate.
The formation of O2 indicates that H2O2 is oxidized in the
presence of less reactive substrates, thus preventing catalyst
bleaching and protein oxidation by the reactive intermediate.
This route does not exist when PhIO is the oxidant, hence the
more extensive catalyst bleaching with less reactive sulfides.
On the other hand, larger chemical yields of sulfoxides are
obtained with PhIO vs H2O2 as oxidants due to competition of
H2O2 with the sulfides for the same reactive intermediates. These
conclusions were substantiated by treating two 1-Mn/BSA
solutions by concentrated (30%) H2O2 in the presence and
absence of substrate. Examination of the reaction products after
1.5 h revealed a 30% yield (150 catalytic turnovers) of
thioanisole oxide (46% ee of the S enantiomer) with 17%
catalyst bleaching in the former case, while a 10% yield
production of O2 with 30% catalyst bleaching was obtained in
latter reaction. 1-Fe also affected the disproportionation of H2O2
(11% yield of O2) but was completely bleached quite fast.
Another difference (Table 3, last column) between the
oxidants is that the color of reaction mixtures turned from green
to red upon addition of PhIO and returned to green only toward
the end of the reactions, while no color changes were noticed
with H2O2 as oxidant. As the colors of manganese(III) and
(oxo)manganese(V) corroles are green and red, respectively,5,6
the above results indicate the presence and absence of substantial
amounts of (oxo)manganese(V) corrole in reactions performed
with PhIO and H2O2, respectively. This and the very different
ee’s (Table 3, fourth column) clearly indicate that different
reactive intermediates are responsible for oxygen-atom transfer
to the sulfides in the two systems. It is also consistent with
examinations in protein-free systems that uncovered the very
low reactivity of (oxo)manganese(V) corroles and suggested that
oxygen-atom transfer could take place from either its precursor
or its successor (oxidant-coordinated manganese(III) and (oxo)-
manganese(V) corroles, respectively).3,5,7,18
with the same albumin conjugated to either 1-Fe or 1-Mn
was frequently opposite: for four, five, and seven of the
substrates with RSA, SSA, and HSA, respectively. The last
variations are concerned with the organic substrates. The ee’s
obtained for methylphenylsulfide were either similar or superior
to ethylphenylsulfide, and the effect of para-aryl substitution
in the methylarylsulfides was similar for the similar in size
methyl and chloro substituents. However, regardless of the
albumin source and the catalyst, the highest ee’s in each
combination (except of HSA) were obtained for meta and/or
ortho substitution. This suggests that the substrate’s effect is
mostly of steric rather than electronic origin.
Throughout the series of substrates and albumins the results
obtained with 1-Mn were superior to those with 1-Fe
regarding all three important subjects: enantioselectivity, chemi-
cal yield, and stability of the catalyst. The first two aspects may
be easily deduced from Table 1 (note numbers in bold): nine
cases of g90% yield with 1-Mn as catalyst vs only four with
1-Fe as catalyst; nine cases of g60% ee with 1-Mn conjugates
vs null with albumin-conjugated 1-Fe (the largest ee under
1-Fe catalysis was 41% for 2-chloro-thioanisole/SSA). The
least obvious benefit of the 1-Mn-catalyzed reactions is the
stability of the system, reflected in the absence of catalyst
bleaching and/or protein oxidation even when less reactive
substrates (such a the 2-halogeno-thioanisoles) were employed.
To elucidate the origin of this intriguing and unique phenom-
enon, which is not shared by the related porphyrin-based systems
whose main drawback is catalyst bleaching,15 the sulfoxidation
of three substrates was performed with iodosylbenzene (PhIO),
the most commonly used oxidant in metal-catalyzed oxida-
tions.15,16
The results shown in Table 3 revealed that reactions
performed with PhIO provide larger chemical yields but smaller
ee’s than those with H2O2. The comparison also revealed
pronounced differences with regard to catalyst bleaching: none
for H2O2 with any of the substrates and more for PhIO oxidation
of the 2-halogeno-substituted than for the more reactive non-
substituted thioanisole.17 An important clue for the uniqueness
of H2O2 is that bubbles due to the formation of molecular
To address the mechanistic issues a comparison was made
between reactions performed as shown in eqs 1 and 2 (results
are shown in Table 4). The normal reaction conditions (eq 1)
consisted of subsequent addition of substrate and oxidant to
BSA/1-Mn solutions, while in those described by eq 2
thioanisole and the full amount of oxidant were only added after
the reaction mixture was first transformed to BSA/1-Mn(O)
by a small amount of PhIO (signaled by a color change from
(15) Meunier, B. Chem. ReV. 1992, 92, 1411.
(16) Katsuki, T. Coord. Chem. ReV. 1995, 140, 189.
(17) Similar results were obtained for 1-Fe, but the differences were smaller:
87% yield, 38% ee, and 15% bleaching vs 98% yield, 16% ee, and 40%
bleaching for oxidation of thioanisole by H2O2 and PhIO, respectively.
(18) For similar proposals in porphyrin chemistry, see: Nam, W.; Jin, S. W.;
Lim, M. H.; Ryu, J. Y.; Kim, C. Inorg. Chem. 2002, 41, 3647.
9
2886 J. AM. CHEM. SOC. VOL. 127, NO. 9, 2005

Albumin-Conjugated Corrole Metal Complexes
A R T I C L E S
green to red-brown). The responses to the latter modification
were quite different for the two oxidants. The color remained
red until full consumption of the oxidant, and the ee decreased
from 32% to 17% when PhIO was the final oxidant. In contrast,
addition of H2O2 to the BSA/1-Mn(O) solution induced an
immediate color change back to that of 1-Mn, and the ee only
decreased from 52% to 44%. Importantly, no fast reaction took
place when thioanisole was added to the BSA/1-Mn(O)
solution. Together with the earlier described results, these
observations provide strong evidence for the reaction mecha-
nisms shown in Scheme 2.
of 1-Mn(O) (step 4a), and it should further depend on the
identity of OX being PhIO or H2O2. The other differences
between results obtained with PhIO and H2O2 may also be
explained similarly. Sulfoxides are almost exclusively produced
by route 3a when X-O ) H2O2, since route 4a is depressed by
the large efficiency of route 4b (oxidation of H2O2). As route
4b does not exist when X-O ) PhIO (1-Mn(O) does not
decompose PhIO efficiently), sulfoxides may be obtained via
both routes 3a and 4a with this oxidant. This hypothesis also
explains the much larger extent of catalyst bleaching with PhIO
(route 5 of Scheme 2) than with H2O2 and that it is especially
pronounced for low-reactivity substrates. The larger the amount
of 1-Mn(O) that is formed via step 2, because of either the
absence of H2O2 (routes 3b and 4b not being available) or the
low reactivity of the sulfide (routes 3a and 4a not being
efficient), the more catalyst bleaching is observed. In fact, when
1-Mn/BSA mixtures were treated with either PhIO or 3% H2O2
in the absence of substrate, complete bleaching and complete
survival of the catalyst were obtained, respectively.19
Conclusions
We demonstrated the efficiency of an extremely simple
biomimetic oxidation system that consists of mixing metal
complexes of amphiphilic corroles with serum albumins and
utilizes hydrogen peroxide for asymmetric sulfoxidation of
sulfides. The albumin-conjugated manganese corroles act exactly
as the various heme enzymes which all display catalase-like
activity when treated by hydrogen peroxide in the absence of
more oxidizable substrates.20 Mechanistic investigations revealed
the importance of the hydrogen-peroxide-coordinated man-
ganese(III) corrole as the prime intermediate for efficient and
enantioselective oxygen-atom transfer. One issue that remains
to be investigated is if the metal or the corrole is responsible
for the larger reactivity of (corrole)Mn-O-X than (corrole)-
Mn(O). In hemes and synthetic iron porphyrins [(porphyrin)-
Fe(O)]+ is significantly more reactive than (porphyrin)Fe-OX,21
but both the reactivity and enantioselectivety of oxidant-
coordinated (porphyrin)Ru(O) are larger than of (porphyrin)-
Ru(O)2,22 while the very large emphasis on isolation of the
extremely reactive [(porphyrin)Mn(O)]+ might have had a
shading effect on the search for other reactive intermediates of
possible importance.23
According to Scheme 2 addition of oxidant X-O to 1-Mn
first affords the oxidant-coordinated 1-Mn(OX) (step 1), which
may either release X to afford 1-Mn(O) (step 2) or react with
sulfide (step 3a) to produce sulfoxide and regenerate 1-Mn.
The results with preformed 1-Mn(O) [immediately reduced to
1-Mn by H2O2 (producing O2) and not by substrate or PhIO]
clearly demonstrate its low reactivity toward sulfides (step 4a)
and its large reactivity toward H2O2 (step 4b). As sulfoxides
are nevertheless formed in high yields in the reactions with H2O2
as oxidant, the presence of another intermediate that is much
less discriminatory than 1-Mn(O) for oxidation of H2O2 relative
to sulfide is clearly required. The most reasonable candidate is
1-Mn(OX): its enantioselectivity with regard to sulfide oxida-
tion (step 3a) may safely be expected to be different than that
Scheme 2. Plausible Reaction Sequences and Plausible
Intermediates in the Catalytic Oxidation of Thioanisoles by Either
PhIO or Hydrogen Peroxidea
Acknowledgment. This research was supported by the
German-Israeli Project Cooperation, DIP-F.6.2. Partial support
by the Fund for the Promotion of Research at the Technion is
acknowledged as well.
JA045372C
(19) Similar results were obtained in protein-free solutions, but the lifetime of
nonconjugated 1-Mn(O) (seen only with PhIO as oxidant) was significantly
shorter than that of 1-Mn(O)/BSA.
(20) Watanabe, Y.; Ueno, T. Bull. Chem. Soc. Jpn. 2003, 76, 1309.
(21) For a most recent series of commentaries about this subject in both heme
and nonheme systems, see: (a) Jin, S.; Bryson, T. A.; Dawson J. H. J.
Biol. Inorg. Chem. 2004, 9, 644. (b) Nam, W.; Ryu, Y. O.; Song, W. J. J.
Biol. Inorg. Chem. 2004, 9, 654. (c) Shaik, S.; de Visser, S. P.; Kumar, D.
J. Biol. Inorg. Chem. 2004, 9, 661. (d) Hatcher, L. Q.; Karlin, K. D. J.
Biol. Inorg. Chem. 2004, 9, 669. (e) Que, L., Jr. J. Biol. Inorg. Chem.
2004, 9, 684.
(22) Gross, Z.; Ini, S. Inorg. Chem. 1999, 38, 1446.
(23) (a) Jin, N.; Groves, J. T. J. Am. Chem. Soc. 1999, 121, 2923. (b) Smegal,
J. A.; Hill, C. L. J. Am. Chem. Soc. 1983, 105, 2920.
a Note that according to MO theory the manganese(V) ion and oxygen
atom in 1-Mn(O) are connected via a triple bond.3
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