C O M M U N I C A T I O N S
Table 1. Apparent Second-Order Rate Constants (k2) for BNP
Cleavage, DMP Binding Constants (KDMP), and Relative Reactivity
(krel) for Different Zn(II)-Based Agents (pH 7, 40°C unless
otherwise noted)
Thus, the formation of an amount of linear form largely exceeding
the nicked one in the early stages of the reaction indicates that
1
0.4-MPGN-Zn(II) preferentially performs double strand cleavage.
The use of nanoparticles bearing an array of active units implies
entry
catalyst
k2 (s-1 M-1
)
KDMP (M-1
)
krel
ref
trading preorganization and rational design for flexibility and self-
organization. The gain obtained on activity is impressive. The most
effective Zn(II)-based catalyst so far reported provides a 36 000-fold
acceleration for the cleavage of BNP at pH 7 and 3.6 mM complex
concentration (Table 1, entry 8).1c,10 10.4-MPGN-Zn(II) at only 50 µM
Zn(II) complex concentration produces a 300 000-fold rate acceleration
(pH 7, 40 °C). The second-order rate constants measured for the
dimetallic sites in the nanoparticles well compare with those observed
for the most reactive lanthanide and Co(III) complexes.7,9 The activity
against DNA is less impressive but characterized by an unprecedented
ability to perform double strand cleavage. Such behavior is typical of
enzymes but extremely difficult to attain with artificial systems1c and
can be ascribed to the multivalent nature of the nanoparticle-based
catalyst. In fact, in the contact area between DNA and the nanoparticle
surface, several reactive sites are present so that contemporaneous
attack on both strands is possible.
a
1
2
3
4
5
OH-
3-Zn(II)
5-Zn(II)2
2.4 × 10-5
0.0055b
0.11b
-
1
500
230
4583
62 500
9
c
c
c
c
0.012b
1.0 × 102
3.1 × 102
1.3 × 102
2.2 × 103
1
0.1-MPGN-Zn(II)
[10.1-MPGN-Zn(II)]2 1.5b
(4.7 × 104)d
8.3 × 102
3.7 × 103
n. d.
6
7
8
9
1
0.4-MPGN-Zn(II)
0.45b
18 750
58 333
c
c
[10.4-MPGN-Zn(II)]2 1.4b
pseudoHis-Zn(II)
BPAN-Zn(II)2
0.0030e
125 10
11
175 12
1.7 × 10-4 8.6 × 101 d
7
10 QX-TACN-Zn(II)2
0.0042f
n. d.
a 35 °C. b pH 8. c This work. KBNP
.
e 50 °C. f pH 11.
d
plexation of the substrate occurs, both binding and intrinsic
reactivity of the catalytic site contribute to the second-order rate
constant. KDMP values reveal that in the present case most of the
reactivity gain derives from enhanced binding.14 Incidentally, such
a high affinity for DMP further supports the dimetallic nature of
these sites since the KDMP values determined are much larger than
those, lower than 100 M-1, usually measured for monometallic
Zn(II) complexes4 and similar to that of 2.7 × 103 M-1 (pH 7, 25
°C) reported for the dimetallic complex 4-Zn(II)2.4c Moreover, in
the case of 10.1-MPGN, for which the binding constants for both
DMP and BNP could be obtained, a relevant hydrophobic contribu-
tion to binding is evident (KBNP ) 20KDMP).
Acknowledgment. Financial support by MUR (Contracts
2006034123 and 2006039071) is gratefully acknowledged.
Supporting Information Available: Synthesis; characterization of
1, 5, and 1-MPGN; and kinetic experiments. This material is available
References
Remarkably, also the activity of the mononuclear sites on
1-MPGN (entries 4 and 6) is considerably larger than that of
reference 3-Zn(II). In this case too, for both 10.1-MPGN and 10.4
(1) (a) Komiyama, M.; Takeda, N.; Shigekawa, H. Chem. Commun. 1999,
1443–1451. (b) Niittymaki, T.; Lonnberg, H. Org. Biomol. Chem. 2006, 4,
15–25. (c) Mancin, F.; Tecilla, P. New J. Chem. 2007, 31, 800–817.
(2) (a) Pasquato, L.; Rancan, F.; Scrimin, P.; Mancin, F.; Frigeri, C. Chem.
Commun. 2000, 2253–2254. (b) Manea, F.; Houillon, F. B.; Pasquato, L.;
Scrimin, P. Angew. Chem., Int. Ed. 2004, 43, 6165–6169. (c) Pengo, P.;
Baltzer, L.; Pasquato, L.; Scrimin, P. Angew. Chem., Int. Ed. 2007, 46,
400–404. (d) Manea, F.; Bindoli, C.; Polizzi, S.; Lay, L.; Scrimin, P.
Langmuir 2008, 24, 4120–4124.
(3) (a) Martin, M.; Manea, F.; Fiammengo, R.; Prins, L. J.; Pasquato, L.;
Scrimin, P. J. Am. Chem. Soc. 2007, 129, 6982–6983. (b) Zaupa, G.; Prins,
L. J.; Scrimin, P. J. Am. Chem. Soc. 2008, 130, 5699–5709.
(4) (a) Feng, G. Q.; Mareque-Rivas, J. C.; de Rosales, R. T. M.; Williams,
N. H. J. Am. Chem. Soc. 2005, 127, 13470–13471. (b) Feng, G. Q.;
Mareque-Rivas, J. C.; Williams, N. H. Chem. Commun. 2006, 1845–1847.
(c) Feng, G. Q.; Natale, D.; Prabaharan, R.; Mareque-Rivas, J. C.; Williams,
N. H. Angew. Chem., Int. Ed. 2006, 45, 7056–7059.
-
MPGN nanoparticles, the data indicate a relevant contribution to
the activity due to enhanced binding. The more pronounced effect
observed with 10.4-MPGN could be the result of a combined
interaction of the substrate with a monometallic complex and
adjacent ammonium ions of protonated, uncomplexed ligands.
Comparison of the reactivity of 1-MPGN with binuclear complex
5-Zn(II)2 (entry 3) gives full account of the effectiveness of the
nanoparticles-based catalyst. In both cases two BAPA-Zn(II) units are
held in proximity with a poorly preorganized arrangement, but while
nanoparticles show high reactivity, 5-Zn(II)2 is even less efficient than
the mononuclear counterpart 3-Zn(II). Inspection of entries 8-10,
summarizing the reactivity of the most efficient Zn-based catalysts
for BNP so far reported, reveals that such behavior is not uncommon
with binuclear Zn(II) complexes, which usually behave quite poorly
with this substrate even in the case of preorganized systems.1c It is
thus very remarkable that the spontaneous and flexible organization
of the reactive sites on the 1-MPGN monolayer forms binuclear Zn(II)-
based catalytic sites with high activity.
The encouraging results obtained with BNP prompted us to
exploit 1-MPGN multivalency in the cleavage of a multivalent
substrate like DNA. Considerable activity was indeed observed.
Incubation of pBR 322 plasmid DNA with 10.4-MPGN-Zn(II) at
pH 7.0 and 37 °C for 24 h resulted in its significant cleavage (Figure
1B), whereas no reaction at all was observed with monomeric
3-Zn(II) (SI). The pseudo-first-order rate constant estimated at the
maximum 15 µM Zn(II) complex concentration accessible15 is 2
× 10-6 s-1, a 5 orders of magnitude rate acceleration over
uncatalyzed DNA hydrolysis.16 Such a result appears even more
remarkable if one considers that, for simple geometric reasons, only
a fraction of the Zn(II) complexes present on a single nanoparticle
can interact with the bound substrate. Most important, the amount
of linear DNA (form III) formed is 50% larger than that of nicked
DNA (form II). More than 100 single strand cleavage events are
needed to obtain linearization in the case of a random process.17
(5) (a) Livieri, M.; Mancin, F.; Tonellato, U.; Chin, J. Chem. Commun. 2004,
2862–2863. (b) Livieri, M.; Mancin, F.; Saielli, G.; Chin, J.; Tonellato, U.
Chem.sEur. J. 2007, 13, 2246–2256.
(6) pKa of the Zn-bound water molecules in complex 3-Zn(II) are 8.0 and 10.2
at 25 °C (ref 5). Lower values may be expected for 10.4-MPGN-Zn(II) due
to electrostatic effects, as in metallomicelles; see: Bunton, C. A.; Scrimin,
P.; Tecilla, P. J. Chem. Soc., Perkin Trans. 2 1996, 419–213.
(7) The rate estimated for the spontaneous hydrolysis of BNP at pH 7.0 and
40 °C is 1.1 × 10-10 s-1; see Chin, J.; Banaszczyk, M.; Jubian, V.; Zou,
X. J. Am. Chem. Soc. 1989, 111, 186–190.
(8) Apparent Zn(II) binding constant for ligand 3 at pH 8 is 106.5 at 25° C
(data from ref 5), which ensures more than 90% complex formation.
(9) Yatsimirsky, A. K. Coord. Chem. ReV. 2005, 249, 1997–2011.
(10) Ichikawa, K.; Tarnai, M.; Uddin, M. K.; Nakata, K.; Sato, S. J. Inorg.
Biochem. 2002, 91, 437–450.
(11) Kaminskaia, N. V.; He, C.; Lippard, S. J. Inorg. Chem. 2000, 39, 3365.
(12) Arca, M.; Bencini, A.; Berni, E.; Caltagirone, C.; Devillanova, F. A.; Isaia,
F.; Garau, A.; Giorgi, C.; Lippolis, V.; Perra, A.; Tei, L.; Valtancoli, B.
Inorg. Chem. 2003, 42, 6929–6939.
(13) Precipitation of the nanoparticles in the presence of an excess of the
lipophilic BNP was observed with 10.4-MPGN.
(14) For cooperation in substrate binding and catalysis by functional units
arranged on a surface see: (a) Major, R. C.; Zhu, X.-Y. J. Am. Chem. Soc.
2003, 125, 8454–8455. (b) Basabe-Desmonts, L.; Reinhoudt, D. N.; Crego-
Calama, M. Chem. Soc. ReV 2007, 36, 993–1017; see also ref 3.
(15) DNA precipitation occurs when the overall concentration of metal complex
exceeds that of the DNA (24 µM in residues).
(16) Uncatalyzed rate (pH 7.0 and 37 °C): 1 × 10-11 s-1; see: Hettich, R.;
Schneider, H.-J. J. Am. Chem. Soc. 1997, 119, 5638–5647.
(17) Branum, M. E.; Tipton, A. K.; Zhu, S.; Que, L., Jr. J. Am. Chem. Soc.
1996, 123, 1998–1904.
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