Signal amplification and detection via a supramolecular allosteric catalyst

Article abstract of DOI:10.1021/ja0437306

The design of a supramolecular allosteric catalyst system for catalytic signal amplification and detection is presented. The catalyst was switched on by the introduction of an analyte that also behaves as an allosteric activator. Concentrations of Cl^- ions as low as 800 nM were catalytically amplified and detected. The signal was transduced via a pH-sensitive fluorescent probe and observed visually using a laboratory, handheld UV lamp and by spectrophotometry. Furthermore, the allosteric effect was quantified using gas chromatography for a range of Cl^- concentrations. This three-part detection scheme involving analyte binding, allosteric catalyst activation, and signal transduction represents a new approach to small-molecule detection. Copyright

Full text of DOI:10.1021/ja0437306

Published on Web 01/22/2005
Signal Amplification and Detection via a Supramolecular Allosteric Catalyst
Nathan C. Gianneschi, SonBinh T. Nguyen, and Chad A. Mirkin*
Department of Chemistry and the Institute for Nanotechnology, Northwestern UniVersity, 2145 Sheridan Road,
EVanston, Illinois 60201-3113
Received October 14, 2004; E-mail: chadnano@northwestern.edu
The development of methods for the construction of supramo-
1
lecular architectures offers chemists the ability to synthesize many
potentially useful species. Ultimately, when one has control over
the synthesis of compounds containing large, well-defined cavities,
one can design species that can arrange molecules in a specific
and predictable fashion for catalysis and detection of guest
molecules. Indeed, chemists are now beginning to utilize supramo-
lecular chemistry as a tool toward the realization of catalytic systems
with enzymelike properties.2
Our research has focused on utilizing the concept of allosteric
3
regulation in nonbiological catalytic systems. Allosteric regulation
is a nascent concept in synthetic catalysis despite its widespread
3,4
presence in biology. We have used allosteric regulatory reactions
to control both the rate and selectivity of the asymmetric ring-
3
opening of epoxides. In turn, we have demonstrated the reversible
in situ modulation of catalysis through the introduction and removal
of allosteric effector molecules capable of binding specifically to
a structure control element within a supramolecular catalyst.3b These
studies have prompted us to investigate the plausibility of using
allosteric control in a catalytic detection scheme in which the
analyte, behaving as an allosteric effector, binds to the sensor and
acts as an “on” switch for catalysis. The signal is amplified via the
5
subsequent catalytic cycle, and the products are detected using a
convenient fluorophore probe strategy, reminiscent of biological
6
assays such as ELISA. Herein, we present an initial proof-of-
concept demonstration of the use of a supramolecular allosteric
catalyst in signal amplification and detection (Figure 1).
Complexes 1 and 2, Figure 1, were synthesized via the Weak-
Link approach1
a,1e,7
according to literature procedures. Compound
3a
1
was previously characterized in the solid-state by a single-crystal
Figure 1. Supramolecular allosteric catalytic signal amplifier. A slow
background reaction occurs in the absence of analyte (Cl or CO). Analyte
binding opens the cavity and allows substrate molecules to enter, where
they undergo a fast intramolecular reaction generating acetic acid, which
protonates a pH-sensitive fluorescent probe.
-
3
a
X-ray diffraction study. Each complex contains two structural
domains containing Rh(I) metal centers and a catalytic domain
containing two Zn(II) metal centers. The macrocyclic cavity of
compound 1 can be opened to form 2 by the introduction of CO
gas (1 atm) in the presence of Cl ions (benzyltriethylammonium
chloride) in CH
to break the thioether/Rh(I) bonds, leaving the phosphine/Rh(I)
bonds intact. The result of the selective breaking of these bonds
is a concomitant, significant change in molecular shape. The Weak-
-
To amplify a detection event using an allosteric catalyst, a
suitable reaction is needed in which substrates react at significantly
different rates in the presence of the open and closed states of the
catalyst. Furthermore, it is critical that the chemistry occurring at
the catalytic site be orthogonal to the chemistry occurring at the
regulatory site. The acyl transfer reaction between acetic anhydride
and pyridyl carbinol was employed here as the signal amplification
reaction. This reaction has several features that make it useful for
this purpose. First, the reaction can be catalyzed in a bimetallic
fashion. Second, acetic acid is a product of the reaction, allowing
-
2
Cl
2
. Significantly, both CO and Cl are required
3
Link approach has been demonstrated to be amenable to a host of
metal-coordination environments and ligand combinations.1
a,1e,7
A
variety of functional groups have been shown to initiate the ring-
opening reaction depending on the choice of ligand and metal
combination. These functional groups range from cyanide and halide
ions to pyridyl-based compounds, nitrile-based compounds, amines,
and CO, which provide for a range of potential analytes. Impor-
tantly, these small molecules often react selectively with specific
metal and ligand combinations.1
the utility of this novel detection strategy, data for the detection of
Cl ions is presented.
8
coupling of the amplification step to a pH-sensitive fluorophore.
We hypothesized that this design would provide a switch between
a Lewis-acid-catalyzed monometallic (Zn(II)-salen) reaction and
a bimetallic reaction in which the acetic anhydride is activated by
one Zn(II)-salen moiety while in proximity to a pyridyl carbinol
molecule bound to the other Zn(II) center within the supramolecular
a,1e,7
As a preliminary example of
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10.1021/ja0437306 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
the same concentration of pH-sensitive fluorophore, components
of the catalytic reaction and CO (1 atm).9
This approach brings together three key elements in order to
effect efficient and facile recognition, amplification, and detection
of analyte. First, the analyte acts to switch “on” an allosteric catalyst.
Second, this switch takes the form of a topological change that
results in a significant increase in the rate of acyl transfer from
acetic anhydride to pyridyl carbinol. Third, the production of acetic
acid is easily coupled to a pH-sensitive fluorophore. The essential
component is the allosteric effect that gives rise to a significant
rate difference and allows one to obtain excellent differentiation
between activated and inactive catalyst, by both GC and the easily
implemented fluorophore method.
In light of the generality of the Weak-Link approach to the
construction of structurally related flexible multimetallic supramo-
lecular entities, we are currently expanding this detection strategy
to a range of chemically and biologically relevant analytes.
Acknowledgment. C.A.M. acknowledges NSF and the AFOSR
Figure 2. Product (4-acetoxymethylpyridine) concentration vs time for a
range of Cl ion concentrations. Reactions were monitored by GC.
Conditions: CH2Cl2, rt, 1 mM pyridyl carbinol, 1 mM acetic anhydride,
-
1
.5 mM biphenyl (standard), 1 mM closed catalyst, CO (1 atm), and
appropriate amounts of benzyltriethylammonium chloride.
for support of this research.
Supporting Information Available: Detailed experimental pro-
cedures (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
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ratios. By GC, [mM] quantities of Cl can be detected; however,
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(
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the amplification of the signal and corresponding presence of analyte
-
(
Figure 3). Concentrations of Cl as low as 800 nM could easily
(9) Commercial chloride ion-selective electrodes typically have a micromolar
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be observed using a commercial, handheld UV (365 nm) lamp and
-
differentiated from a system not exposed to Cl , but containing
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