Development of
Metal-Responsive Fluorescent Chemosensors
Overview
Fluorescent chemosensors-molecules
that change their fluorescence in response to substrate binding-offer an
extremely sensitive optical method for the real-time monitoring of molecular
interactions. Such chemosensors
are finding increased use in fields as diverse as biology, medical analysis,
and environmental monitoring. The
majority of fluorescent chemosensors operate by one
of three mechanisms: (1) suppression of photoinduced
electron transfer or enhancement of heavy-atom quenching; (2) variation of the
distance between two fluorophores, modulating the
efficiency of interchromophore energy transfer; and
(3) alteration of the microenvironment of a solvatochromic
fluorophore (e.g., by displacement from a cyclodextrin cavity).
Recent
Results
We
recently described a new approach to the development of fluorescent chemosensors based on a signal transduction pathway in
which metal binding induces conformational restriction of the fluorophore, resulting in enhanced fluorescence
(illustrated by the titration of A
with Ca2+.
We have also shown that metal ion binding could restrict the
excited-state rotation of a biaryl chromophore, suppressing intersystem crossing and leading to increased emission. We have now applied the restriction of
excited-state dynamics to suppression of the other fundamental nonradiative decay pathway, internal conversion, in biarylacetylenes. This indicates that both nonradiative
decay pathways are subject to conformational control, and that this signaling
pathway should be generally accessible in simple flexible fluorophores.
This, in turn, has implications for fluorosensor
design, in that the majority of previous approaches require the sacrifice of
either architectural simplicity or broad ligand
scope.
We
have also developed dual-signaling fluorescent chemosensors
based on conformational restriction and induced charge transfer. The
combination of two signaling mechanisms - conformational restriction and
induction of charge transfer - allows metal binding to turn two
fluorescence emission bands on independently.
Furthermore, fluorophores such as B emit
visible fluorescence at two different wavelengths, simplifying assessment of fluorescence
enhancement: shown below (left to right) are samples of B, B+Li+,
B+Mg2+, and B+Ca2+.
Current Efforts
In a development
that bodes well for the construction of combinatorial libraries of chemosensors, we have found that analogs
of B immobilized on polystyrene resin exhibit similar emission upon UV
irradiation. This provides strong
potential for the synthesis of combinatorial libraries of chemosensors.
The development and elaboration of these solid-phase fluorescent chemosensors as well as the adaptation of our sensors to an aqueous
environment, are currently underway. In addition we are interested
in the application
of our fluorophores towards the recognition of
protein-surface interactions and oligonucleotide hydbridization.