Molecular recognition by fluorescent imprinted polymers

Article abstract of DOI:10.1016/S0040-4039(99)02027-4

New fluorescent imprinted polymers have been prepared for guest-host selectivity studies. Significant differences were found in the fluorescence response between the bound and unbound states.

Full text of DOI:10.1016/S0040-4039(99)02027-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 123–126
Molecular recognition by fluorescent imprinted polymers
Daniel L. Rathbone,∗ Danqing Su, Yongfeng Wang and David C. Billington
Pharmaceutical Sciences Research Institute, Aston University, Aston Triangle, Birmingham B4 7ET, UK
Received 3 September 1999; accepted 26 October 1999
Abstract
New fluorescent imprinted polymers have been prepared for guest–host selectivity studies. Significant differen-
ces were found in the fluorescence response between the bound and unbound states. © 1999 Elsevier Science Ltd.
All rights reserved.
Keywords: fluorescence; hydrogen bonding; molecular recognition; polymers.
As part of a programme of automated synthesis and biological screening we decided to investigate the
phenomenon of molecular recognition in polymer networks with a view to constructing a high-throughput
fluorescence-based assay. If a test compound hydrogen bonds to a fluorescent group in an imprinted
polymer cavity then there should be a change in the fluorescence properties of the polymer which may be
detected by fluorimetry. In this case the test compound presents a similar hydrogen bond donor/acceptor
pharmacophore arrangement to that of the imprinting molecule. With an appropriate choice of imprinting
molecule this may be used in an automated assay to screen libraries of potential drug candidates. Thus,
compounds similar in size, shape and hydrogen-bonding profiles to that of a lead compound may be
discovered. Other potential applications may be envisaged, for example, in screening out molecules
which bind in the same manner as existing drugs having microbial resistance problems from libraries
of active anti-microbial agents.
In recent years imprinted polymers have been developed having recognition sites that rely upon both
the shape of the cavity and the presence of hydrogen bond donors and acceptors for their selectivity
in guest–host interactions.1–5 Imprinted polymers have been used successfully as stationary phases for
HPLC enantiomeric separations and have been found to be an effective means for the selective removal of
molecules from bulk materials such as in the decaffeination of coffee.6 As a step on from this, molecular
imprinting has been combined with fluorescence to produce a polymer-based fluorescent chemosensor
for the detection of aqueous cyclic adenosine monophosphate.7
This paper describes our initial results from four fluorescent hydrogen-bonding monomers used in the
preparation of new imprinted polymers which exhibit binding-dependent fluorescence.
∗
Corresponding author. Tel: +44 (0)121 3593611, ext 4770; fax: +44 (0)121 3590733; e-mail: d.l.rathbone@aston.ac.uk
0040-4039/00/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02027-4
tetl 15955

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2-Aminopyridine, 7-hydroxy-4-methylcoumarin (themselves both fluorescent) and 3-aminorhodanine
were chosen as suitable hydrogen bond donor/acceptor scaffolds from which to construct the desired
monomers. Thus, 2-acrylamidopyridine (1), two fluorescent benzylidine 3-acrylamidorhodanines (2a,b;
presumed to be the Z-isomers by analogy to Z-benzylidene rhodanines prepared in the same manner8)
and 7-hydroxy-4-methylcoumarin acrylate (3) were prepared as in Scheme 1.
Scheme 1. Preparation of fluorescent monomers
Polymers were prepared from trimethylolpropane triacrylate7 or ethylene dimethacrylate1–3 containing
5 µmol g-1 each of cyclododecylidene pyridine-2-carboxamidrazone 49 (the imprinting molecule, Fig. 1)
and a fluorescent monomer (CHCl3/AIBN/Ar/60°C/24 h). Compound 4 was chosen as the imprinting
molecule by virtue of its obvious hydrogen bond donor/acceptor capabilities together with the fact
that we have libraries of several hundred heteroarylcarboxamidrazones at our disposal with which to
probe the selectivity of the imprinted polymers. The crude polymeric materials were washed free of
starting materials, ground to a fine powder and extracted rigorously to remove the imprinting molecule.
Each polymer was then re-exposed to the imprinting molecule and the fluorescence response of the
polymer was obtained with and without the imprinting molecule using a Wallac Victor2 1420 multilabel
counter in standard 96-well format. Three samples of each polymer were weighed into the 96-well plate,
suspended in methanol and the fluorescence response was measured in quintuplet along with controls
comprising methanol-filled and blank wells. The greatest differences in fluorescence between polymer-
bound imprinting molecule and free polymer were found when trimethylolpropane triacrylate was used
as the crosslinking agent rather than ethylene dimethacrylate. The results for the former are shown in Fig.
2 and are normalised for fluorescence counts/mg of polymer.
Fig. 1. Imprinting molecule
Of the four polymers, 1 and 2a had the larger fluorescence output and the latter exhibited the
greatest fluorescence count ratios between the bound and unbound states with an approximate two-

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Fig. 2. Fluorescence response of the imprinted polymers in the presence and absence of bound imprinting molecule. The pairs
of numbers on the horizontal axis denote the combinations of excitation and emission wavelengths used (nm)
fold difference across the entire wavelength range in this study. These results confirm the molecular
recognition and fluorescence potential of these polymers. The selectivity of these polymers towards
binding molecules other than the imprinting molecule is currently being investigated using libraries of
benzylidene heteroarylcarboxamidrazones.
Acknowledgements
The authors are grateful to Dr. H. O. Alpar (Aston University) for use of the Wallac Victor2 1420
multilabel counter.
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