Cross-linked dendrimer hosts containing reporter groups for amine guests

Article abstract of DOI:10.1021/ja0294515

The integration of a chromogenic reporter group into the recently reported (Zimmerman, S. C.; Wendland, M. S.; Rakow, N. A.; Zharov, I.; Suslick, K. S. Nature 2002, 418, 399-403) monomolecular imprinting approach is described. The resulting highly cross-linked, macromolecular hosts show rapid, selective, high affinity, two-point binding of straight-chain diamine guests. Over longer times, the hosts are more promiscuous, binding a broader range of diamines. A rigorous test of imprinting was performed wherein the cross-reactivities of two dendrimers derived from different templates are compared. The test reveals a guest-dependent kinetic binding effect masquerading as evidence of a highly selective two-point imprinting process. Copyright

Full text of DOI:10.1021/ja0294515

Published on Web 02/28/2003
Cross-Linked Dendrimer Hosts Containing Reporter Groups for Amine Guests
Eric Mertz and Steven C. Zimmerman*
Department of Chemistry, UniVersity of Illinois at Urbana-Champaign, 600 South Matthews AVenue,
Urbana, Illinois 61801
Received November 22, 2002 ; E-mail: sczimmer@uiuc.edu
Scheme 1
Considerable attention has been focused on improving the
properties of molecularly imprinted polymers (MIPs).1,2 Recently,
we disclosed3 a strategy for imprinting a single molecular template
within a protein-sized dendritic polymer.4 This proof of principle
study showed that a porphyrin could template the formation of a
macromolecular host whose eight carboxylic acid groups within a
cross-linked dendrimer selectively bound porphyrin guests present-
ing four or eight hydrogen bond acceptor sites.
Although porphyrins serve as spectrophotometric indicators of
binding, an ideal host would itself signal the complexation.
Additionally, one would like to know whether the same approach
would succeed with a smaller template and dendrimer presenting
fewer binding contacts. Herein we report the integration of a
chromogenic reporter group into our monomolecular imprinting
approach. We also apply a rigorous test of imprinting by comparing
the cross-reactivities of two dendrimers derived from different
templates, a method occasionally applied to MIPs.5 In the current
work, the test reveals a guest-dependent kinetic binding effect
masquerading as evidence of a highly selective two-point imprinting
process.
An appealing chromogenic reporter for amines is (trifluoro-
acetyl)azobenzene dye 1,6 studied by Mohr in the context of
polymer-based amine sensors.7 Dye 1 reversibly forms hemiaminal
2 resulting in a ∼50 nm blue shift that is readily seen as a change
in solution color from red-orange to yellow. In THF, the measured
association constant (Kassoc) for the complexation of butylamine by
1 (R ) Me) was ca. 150-300 M-1
.
cross-linked dendrimers.3,8 The MALDI-MS spectrum of 11 showed
formation of 13-16 out of a possible 16 cross-links.9 Bis(imine)
11 was readily hydrolyzed to give 12, whose MALDI-MS spectrum
was consistent with loss of the core. Likewise, the λmax shift from
436 to 480 nm indicated full conversion of the imine to the ketone.
Qualitative binding studies of 12 with 8 and butylamine in THF
indicated much stronger binding of diamine 8 than butylamine.
Thus, 8 equiv of 8 changed the color of a 30 µM solution of 12 in
THF from red-orange (λmax ) 480 nm) to yellow (λmax ) 431 nm),
whereas under the same conditions butylamine showed a negligible
color change. A 10.7 ppm upfield shift in the 19F NMR signal due
to the trifluoromethyl group of 12 confirmed that the binding
involved formation of a hemiaminal with 8.13 A Job plot indicated
1:1 stoichiometry (12‚8), and the binding reversibility was shown
by diluting a solution of complex whereupon the color reverted to
orange-red and gave spectra of mostly unbound 12.
The dendron used in this study was the same as that used for
the porphyrin imprinting3 and for the synthesis of cored dendrimers.8
Compound 3, synthesized in four steps from N,N-dimethylaniline
and 4-fluorobromobenzene (Scheme 1), was used to link the
dendron to the dye.9 Thus, boronic acid 4, made in two steps from
5-iodo resorcinol, underwent Suzuki coupling with 3 to give 5,
which, in turn, was deprotected and trapped in situ with third-
generation dendritic bromide 6 (structure not shown) to afford the
dendron-dye conjugate 7 in good yield.
Two-point covalent imprinting was examined by using 1,4-
diaminobutane (putrescine, 8) as a template. Although 8 formed a
2:1 complex with 7 (see 2), it was not stable under the high-dilution
conditions needed for the unimolecular cross-linking reaction. The
two dendrons were instead linked to the diamine core through the
bis-imine (i.e., 9).10 Thus, 1,4-diazidobutane was refluxed with
triphenylphosphine to form the bis(iminophosphorane) intermediate
which was treated with dye 7 to provide 9.11 RCM reaction of 9
using Grubbs’ catalyst 1012 provided 11 as a mixture of isomeric
Complexation studies in THF afforded an apparent association
constant (Kapp ) 2.7 × 104 M-1) for 12‚8 that was ca. 200-fold
higher than that for butylamine‚7 (Kapp ≈ 140 M-1). The advantage
of two-point binding is less when compared to the stepwise Kapp
for formation of a 2:1 complex between butylamine and 12 (Kapp(11)
9
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10.1021/ja0294515 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
intramolecular general base catalyzed addition mechanism. The
latter is a type of bifunctional catalysis that has been documented
extensively by Hine,14 and specifically in the formation of carbinol
amines by Page and Jencks.15 Whatever the origin, the selectivity
shown by 12 was clearly not maintained over time. In longer-term
(12-24 h) screening assays, there were smaller differences between
diamines 8 and 13-17 as each exhibited a higher degree of complex
formation. Several other diamines, including the cyclohexyl di-
amines and N-methylated diamines, achieved 30-60% of the signal
changes seen with 8 and 13-17.
Described herein is an approach to macromolecular hosts that
show rapid, selective, high affinity, two-point binding of straight-
chain diamine guests. Over longer times, the hosts are more
promiscuous, binding a broader range of diamines. However, the
binding site, made by cross-linking the two template-linked
dendrons, carrying 16 alkene groups each, does not arise from
template-mediated imprinting. This is in contrast to our previous
study wherein a porphyrin template holding 8 dendrons each with
8 alkenes produced an effective imprint. Current efforts are focused
on further defining the design rules for this monomolecular
imprinting approach and looking at new scaffolds that might be
generally applicable. It is clear that the integration of reporter units
as binding groups is a compelling advance because it allows rapid
screening of guests and small guest libraries. Reporter group-cored
dendrons for other functional groups are currently under develop-
ment.
Figure 1. Binding selectivity profile for 12.
) 500 M-1, Kapp(12) ) 1000 M-1), but this comparison is
complicated by the interaction between the dye sites.
The chromogenic response shown by 12 allowed its selectivity
profile to be established readily using a small library of amino and
alcoholic targets (Figure 1). A simple, qualitative screening assay
involved adding 1 equiv of each guest to a 20 µM solution of 12
in THF and measuring the increase in absorbance at λ ) 425 nm
(∆A425) 30 s after mixing.
Acknowledgment. Funding of this work by the National
Institutes of Health (GM61067) is gratefully acknowledged.
Supporting Information Available: Synthetic details and ad-
ditional binding and characterization data (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
The remarkable level of selectivity for the straight-chain diamines
(e.g., 13-17, Figure 1), in particular for 8 and 14, suggests a cross-
linked dendrimer with an effectively imprinted binding site showing
a high degree of shape and functional group discrimination.
However, a key control experiment would be to show that the same
chemistry outlined in Scheme 1, but using a different diamine
template, ideally one weakly bound by 12, gives an imprinted
dendrimer with a different binding profile, in other words, to
demonstrate different cross-reactiVities for two dendrimers im-
printed with different templates. To apply this stringent test, a
second cross-linked dendrimer was synthesized from 1,3-bis(2-
azidoethyl)benzene.9 Rather than showing selective binding of its
template (i.e., 18), a diamine that was weakly bound by 12, this
new dendrimer showed a guest binding profile very similar to that
seen for 12 (Figure 1).9 In another control study, a third dendrimer
was prepared by treating 7 with 10 at a concentration that favors
formation of a cross-linked dimer. The dimer, purified by size
exclusion chromatography (SEC) and appearing identical to 12 by
1H NMR, MALDI-MS, and analytical SEC, also showed guest
binding properties similar to those of 12.
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