C O M M U N I C A T I O N S
Brønsted correlation, considering the difference in pKa value
between the imidazole in the aggregate and monomeric catalysts.
In conclusion we have presented the first example of peptide-
functionalized gold nanoparticles hydrolytically active against
carboxylate esters. The confinement of the catalytic units in the
monolayer covering the nanoparticles triggers a cooperative hy-
drolytic mechanism operative at pH < 7 in which a carboxylate
and an imidazolium ion act as general base and general acid,
respectively. Such a mechanism is absent with an analogous
monomeric dipeptide, and this results in a more than 300-fold rate
acceleration of the hydrolytic process at low pH in the presence of
the functional nanoparticles. Previously,13 we had observed coop-
erativity between N-methylimidazoles in functionalized nanopar-
ticles although the system was far less active than the present one.
Thus, cooperativity seems to be a rule rather than an exception in
functional nanoparticles.
Figure 1. Log of the apparent second-order rate constant against pH for
the hydrolysis of DNPB catalyzed by nanoparticles 3 (O) or monomer 4
(b). (Inset) Ratio between the rate constants at the different pHs.
Conditions: [buffer] ) 10 mM, 25 °C, 10% CH3OH in H2O (v/v).
Acknowledgment. MIUR support (Contracts 2003054199 to
L.P. and 2002031238 to P.S.) is gratefully acknowledged.
following expression for the second-order rate constant as a function
of pH, eq 1:
Supporting Information Available: Synthetic details, TEM analy-
sis of 3, and inhibition experiments. This material is available free of
2
kapp2 ) kcoop2RCOO-RImH+ + kIm RIm
(1)
References
where RCOO-, RImH+, and RIm represent the fraction of ionized
carboxylic groups, protonated and unprotonated imidazoles, re-
spectively, at a given pH. kcoop2 and kIm2 are the (pH independent)
hydrolytic, second-order rate constants for the cooperative mech-
anism and for that catalyzed by imidazole acting as a general base
or nucleophile, respectively. The solid curve in Figure 1 represents
the best fitting of the experimental points using eq 1. The rate
constants obtained are 4.2 M-1 s-1 for kcoop2 and 38.8 M-1 s-1 for
kIm2. An independent proof of the involvement of the imidazole in
the mechanism dominant below pH 7 comes from the inhibition
(up to 50%, see Supporting Information) of the process at pH 4
and 5.5 by pretreatment of gold nanoparticles 3 with diethylpyro-
carbonate and iodoacetamide, both known22 to suppress the activity
of imidazole as a nucleophile and proton donor. Such an inhibition
is not observed with the monomeric catalyst 4. Thus, it is the
confinement of the dipeptide on the nanoparticle protecting mono-
layer that triggers this cooperative mechanism at low pH.
To address the question whether the cooperating units come from
the same thiol or from two neighboring thiols we have prepared
nanoparticles with different 1:2 compositions. Strikingly enough,
going from 27 to 18% 2 we found the same activity profile. These
results may indicate that cooperativity occurs between carboxylate
and imidazolium ions residing on the same thiol or that there is
clustering of 2 on the monolayer.23 This clustering would offset
the dilution of 2 in 1. Clustering of thiols on the monolayer has
been reported by Rotello24 and Stellacci.25 The reactivity profile
observed with the more lipophilic substrate Z-Leu-PNP is similar
to that observed with DNPB with the important difference that
Z-Leu-PNP binds strongly to the monolayer. Thus, with Z-Leu-
PNP we could run kinetics with excess substrate under turnover
conditions and obtain a saturation profile analogous to that found
in enzymatic catalysis. Analysis of the curve gave KM ) 50 µM
and kcat )1.5 × 10-4 s-1 at pH 7. At this pH 66% of the activity
is due to the cooperative mechanism involving carboxylate and
imidazolium ions, while the remaining 33% is that due to the
imidazole. The cooperative mechanism is not operative with the
monomeric system, and hence no comparison can be made with
the nanoparticles. A comparison can, however, be made as for the
imidazole catalysis which is present in both systems. It reveals that
the reactivity is enhanced 120-fold with the functional MPCs. This
reactivity enhancement can be explained fairly well in terms of
(1) Rowan, S. J.; Sanders, J. K. M. Curr. Opin. Chem. Biol. 1997, 1, 483-
490.
(2) Reichwein, A. M.; Verboom, W.; Reinhoudt, D. N. Recl. TraV. Chim.
Pays-Bas 1994, 113, 343-349.
(3) Suh, J. Acc. Chem. Res. 2003, 36, 562-570.
(4) Wulff, G. Chem. ReV. 2002, 102, 1-28.
(5) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res.
1991, 24, 357-364.
(6) Murakami, Y.; Kikuchi, J.-I.; Hisaeda, Y.; Hayashida, O. Chem. ReV. 1996,
96, 721-758.
(7) Murakami, Y.; Hisaeda, Y.; Song, X.-M.; Ohno, T. J. Chem. Soc., Perkin
Trans. 2 1992, 1527-1528. Groves, J. T.; Neumann, R. J. Am. Chem.
Soc. 1989, 111, 2900-2909. Groves, J. T.; Ungashe, S. B. J. Am. Chem.
Soc. 1990, 112, 7796-7797. Ueoka, R.; Matsumoto, Y.; Moss, R. A.;
Swarup, S.; Sugii, A.; Harada, K.; Kikuchi, J.; Murakami, Y. J. Am. Chem.
Soc. 1988, 110, 1588-1595.
(8) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem.
Soc., Chem. Commun. 1994, 801-802.
(9) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293-346.
(10) Shenhar, R.; Rotello, V. M. Acc. Chem. Res. 2003, 36, 549-561.
Drechsler, U.; Erdogan, B.; Rotello, V. M. Chem. Eur. J. 2004, 10, 5570-
5579.
(11) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000,
33, 27-36. Badia, A.; Lennox, R. B.; Reven, L. Acc. Chem. Res. 2000,
33, 475-481.
(12) Lucarini, M.; Franchi, P.; Pedulli, G. F.; Pengo, P.; Scrimin, P.; Pasquato,
L. J. Am. Chem. Soc. 2004, 126, 9326-9329.
(13) Pasquato, L.; Rancan, F.; Scrimin, P.; Mancin, F.; Frigeri, C. Chem.
Commun. 2000, 2253-2254.
(14) Le´vy, R.; Thanh, N. K. T.; Doty, R. C.; Hussain, I.; Nichols, R. J.;
Schiffrin, D. J.; Brust, M.; Fernig, D. G. J. Am. Chem. Soc. 2004, 126,
10076-10084. Pengo, P.; Broxterman, Q. B.; Kaptein, B.; Pasquato, L.;
Scrimin, P. Langmuir 2003, 19, 2521-2524. Schaaff T. G.; Knight G.;
Shafigullin M. N.; Borkman R. F., Whetten R. L. J. Phys. Chem. B 1998,
102, 10643-10646.
(15) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn,
C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am.
Chem. Soc. 1998, 120, 4845-4849. Fan, J.; Chen, S.; Gao, Y. Colloid
Surf., B 2003, 28, 199-207.
(16) Pengo, P.; Polizzi, S.; Battagliarin, M.; Pasquato, L.; Scrimin, P. J. Mater.
Chem. 2003, 13, 2471-2478.
(17) Containing 10% (v/v) methanol. Solubility up to 6 mg/mL.
(18) Northrop, D. B. Acc. Chem. Res. 2001, 34, 790-797.
(19) This means that kinetics can be followed at 400 nm also at this low pH.
At a lower wavelength the absorbtivity of the nanoparticles is too high.
(20) Kinetics have been run under first-order conditions. Typical catalyst
concentrations were 2.3 × 10-5 M for 3 and 2.3 × 10-4 M for 4.
(21) Bruice, T. C.; Lapinski, R. J. Am. Chem. Soc. 1958, 80, 2265-2267.
(22) For a discussion on inhibitors: Kuhn, R. W.; Walsh, K. A.; Neurath, H.
Biochemistry 1974, 13, 3871-3877.
(23) As suggested by a reviewer, the study of mixtures of monofunctional thiols
would have provided a far more clear-cut demonstration. Regrettably, these
systems proved insoluble under the present conditions.
(24) Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 2000, 122, 734-735. Boal,
A. K.; Rotello, V. M. J. Am. Chem. Soc. 1999, 121, 4914-4915. Boal,
A. K.; Rotello, V. M. J. Am. Chem. Soc. 2002, 124, 5019-5024.
(25) Jackson, A. M.; Myerson, J. W.; Stellacci, F. Nat. Mat. 2004, 3, 330-336.
JA043547C
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1617