Biomedical Engineering Reference
In-Depth Information
analysis. Notably, the proximity of the Zn(II) atoms in 2 precludes the possibility of
any intramolecularly catalyzed HPNP hydrolysis reaction. In addition, the X-ray
structure of 2 revealed that an acetate counterion bridges the two Zn atoms in a
m
2
-
fashion, further inhibiting the ability of HPNP to bind to the active site, which
prevents intermolecular catalysis. Upon the addition of 2 equivalents of Cl
and
CO (1 atm) to a solution of 2,opencomplex3 rapidly forms as confirmed by
1
Hand
31
P NMR, and ESI-MS. In contrast to 2,opencomplex3 has a suitable geometry for
HPNP binding between the Zn(II) sites to enable catalytic bimolecular hydrolysis.
Reversibility was demonstrated by bubbling N
2
into the solution to reform 2.
The catalytic properties of ligand 1, condensed macrocycle 2, and open com-
pound 3 were evaluated in the context of HPNP hydrolysis under pseudo-aqueous
(mixed solvent) conditions and monitored by UV-vis spectroscopy as a function of
time (Fig.
5c
). Although ligand 1 exhibited slow but measurable catalytic activity,
condensed macrocycle 2 was completely inactive under identical reaction condit-
ions. In contrast, open complex 3 was extremely active and capable of quantita-
tively hydrolyzing all of the HPNP substrate in less than 40 min. The allosteric
effect is generated by adjusting the distance between the catalytic Zn(II) metal
centers using reversible coordination chemistry occurring at the Rh(I) metal
centers. The system can be efficiently interconverted between completely “on”
and “off” catalytic states through the use of small molecule regulators that alter the
accessibility of substrate to the active binuclear Zn(II) sites.
In general, such synthetic allosteric systems represent the first step toward
functional abiotic analyte detection strategies that take advantage of catalytic
amplification. The attractive feature of a completely reversible allosteric catalyst
with good turnover and catalytic rates is that it can become a central amplification
motif used for many future systems where the regulatory sites are designed as
receptors for different analytes with comparable amplification capabilities.
A particularly elegant and powerful extension of this technology is highlighted
by the recent report of a supramolecular allosteric catalyst that exhibits a PCR-like
reaction cascade [
30
]. PCR is an enzyme-mediated process where, in response to
temperature cycling, a single target DNA molecule can be rapidly amplified into
many billions of target molecules. Since its conception in 1985 [
31
], PCR has
become a universal laboratory tool for most scientists involved in biochemical and
molecular biological research. Indeed, effective PCR-based amplification is now
the backbone of modern molecular diagnostics enabling the identification of genetic
markers for disease with unmatched sensitivity and high reliability. A limitation of
PCR, however, is that it only works with nucleic acid targets. Therefore, a signifi-
cant challenge exists to create alternate synthetic systems that amplify the recogni-
tion events of non-nucleic acid targets. Such constructs would advance the
development of ultrasensitive detection systems for a much wider class of analytes,
including ones that are relevant to biosensing and beyond.
The catalyst system used to demonstrate PCR-like target amplification is shown
in Fig.
6
. In previous work [
28
,
32
] catalyst 4 was opened in the presence of Cl
and
CO to generate 5. The open cavity in 5 catalyzed an acyl transfer reaction between
pyridyl carbinol and acetic anhydride to produce acetic acid which was signaled
Search WWH ::
Custom Search