mechanisms involves the cooperative catalysis by an acid
and base pair on the side chains of two adjacent amino acids
with the proper folding of the protein polymers. For example,
the His-Asp catalytic dyad contributes to the activities in
many RNase A proteins.5 However, lack of availability of
the hydrolytic enzymes in sufficient quantities, low enzyme
stability, and substrate selectivity have limited the potential
of using hydrolases for broad applications. As a consequence,
novel catalysts that can cleave phosphoester and carboxylic
ester bonds under mild conditions are still urgently needed.
In this paper, we report a novel biomimetic nanocatalyst,
Fe2O3-Asp-His, for the hydrolysis of paraoxon and 4-nitro-
phenyl acetate with high catalytic efficacies in Milli-Q water
(pH 7.0) at 37 °C, swithout employing extremes of pH or
heavy metals (Scheme 1). Fe2O3-Asp-His is a lead selected
Table 1. Cleavage of Paraoxon by 12 nm Maghemite
Nanoparticle-Supported Amino Acidsa
entry
AA1, AA2
conv (%)b entry AA1, AA2 conv (%)b
1
2
3
4
5
6
7
8
9
10
nanoparticlec
Asp
Cys
Glu
His
Lys
Ser
Asp, His
Asp, Lys
Asp, Cys
<1
5
15
<1
6
11
12
13
14
15
16
17
18
19
20
Asp, Ser
Glu, His
Glu, Lys
Glu, Cys
Glu, Ser
His, Cys
His, Ser
Lys, Ser
Lys, Cys
Asp + Hise
28
51
50
44
45
30
40
17
39
< 1
2
4
Scheme 1. Repeated Uses of Fe2O3-Asp-His for the
Hydrolysis of Paraoxon and 4-Nitrophenyl Acetate
77/92d
27
25
a Conditions: paraoxon (0.5 mM) and a magnetic nanocomplex (amino
acid concentration 0.06 mM) in 2 mL of Milli-Q water, 37 °C, 48 h.
b Average of at least two runs; HPLC analyses. c 12 nm maghemite
nanoparticles coated with oleate (no amino acids attached) (ref 8). d Reaction
time: 96 h. e Unsupported Asp (0.14 mM) and His (0.14 mM) and paraoxon
(0.5 mM) in 2 mL of Milli-Q water at 37 °C for 48 h.
The magnetic Fe2O3 cores allow our nanocatalysts to be
facilely concentrated and recovered for repeated uses via
applying a permanent magnet externally.
The nanocomplexes in Table 1 were fabricated via surface-
exchanging presynthesized 12 nm maghemite nanoparticles8
with either a single amino acid analogue or a mixture of
two amino acid dopamine derivatives (molar ratio 1:1)
(Supporting Information). TEM measurements were em-
ployed for examining the iron oxide cores of the nanopar-
ticle-amino acid complexes, and elemental analyses were
utilized for evaluating the amount and composition of amino
acid coatings on the surface of magnetic nanoparticles.
Entries 2-7 in Table 1 are nanoparticles functionalized with
a monad of an amino acid analogue, whereas nanocomplexes
coated with a dyad of amino acid residues are shown in
entries 8-19. The catalytic activities of these nanocomplexes
were investigated by utilizing the nanoparticles to catalyze
the hydrolysis reaction of paraoxon. A typical experiment
employed for our assay involved the introduction of a
nanocomplex (amino acid concentration 0.06 mM) to a
solution of paraoxon (0.5 mM) in 2 mL of Milli-Q water at
37 °C. After 48 h, the nanocomplex was magnetically
concentrated and removed from the solution. The remaining
solution was then subjected to HPLC analyses using an
internal standard for estimating the conversion yield of
paraoxon. To minimize experimental errors, we repeated
every experiment at least two times. The averages of our
repeated assays for all nanocomplexes are listed in Table 1.
from a small library of nanocomposites (Table 1).6 Each
nanocomplex in the library comprised a 12 nm maghemite
(Fe2O3) nanocore wrapped with a shell of either a monad
(AA1 ) AA2) or a dyad of amino acids (AA1 and AA2 molar
ratio 1:1). Only those amino acids with a carboxylate, a basic
or a nucleophilic group on the side chain such as Asp, Glu,
His, and Lys were employed for constructing the library.
Immobilization of these amino acid analogues on the surface
of a nanoparticle allows their acidic and basic side chains to
be positioned in close proximity to each other, potentially
leading to cooperative catalysis from two neighboring amino
acids. Dopamine was utilized as a linker for supporting amino
acid residues since ethenediols such as dopamine have a
strong affinity for undercoordinated surface sites of metal
oxide.7 The R-amino groups of the surface amino acids were
acylated to mimic the amide bonds of the enzyme backbones.
(5) (a) Mohan, C. G.; Boix, E.; Evans, H. R.; Zikolovski, Z.; Nogue´s,
M. V.; Cuchillo, C. M.; Acharya, K. R. Biochemistry 2002, 41, 12100. (b)
Schultz, L. W.; Quirk, D. J.; Raines, R. T. Biochemistry 1998, 37, 8886.
(6) A similar combinatorial approach using a polymer matrix was
previously reported: Menger, F. M.; Eliseev, A. V.; Migulin, V. A. J. Org.
Chem. 1995, 60, 6666.
(7) (a) Rajh, T.; Chen, L. X.; Lukas, K.; Liu, T.; Thurnauer, M. C.; Tiede,
D. M. J. Phys. Chem. B. 2002, 106, 10543. (b) Kohler, N.; Fryxell, G. E.;
Zhang, M. J. Am. Chem. Soc. 2004, 126, 7206.
(8) (a) Park, J.; An, K.; Hwang, Y.; Park, J.-G.; Noh, H.-J.; Kim, J.-Y.;
Park, J.-H.; Hwang, N.-M.; Hyeon, T. Nat. Mater. 2004, 3, 891. (b) Lu, J.;
Fan, J.; Xu, R.; Roy, S.; Ali, N.; Gao, Y. J. Colloid Interface Sci. 2003,
258, 427.
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Org. Lett., Vol. 8, No. 15, 2006