The discovery of an enantioselective receptor for (-)-adenosine from a racemic dynamic combinatorial library

Article abstract of DOI:10.1021/ja0647699

The use of laser polarimetry detection coupled with HPLC is demonstrated to enable the discovery of enantioselective receptors from racemic dynamic combinatorial libraries. Templating with an enantiopure analyte, such as (-)-adenosine, leads to amplificat

Full text of DOI:10.1021/ja0647699

Published on Web 08/30/2006
The Discovery of an Enantioselective Receptor for (-)-Adenosine from a
Racemic Dynamic Combinatorial Library
Sharon M. Voshell,‡ Stephen J. Lee,*,† and Michel R. Gagne´*,‡
U.S. Army Research Office, P.O. Box 12211, Research Triangle Park, North Carolina 27709, and Department of
Chemistry, UniVersity of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290
Received July 5, 2006; E-mail: mgagne@unc.edu; stephen.lee2@us.army.mil
Dynamic combinatorial chemistry (DCC)1 has developed into a
powerful tool for the amplification and identification of new hosts
for a broad range of guests, ultimately yielding such remarkably
complex structures as the catenated dodecapeptide receptor for
acetylcholine.2 DCC experiments aimed at the discovery of enan-
tioselective receptors have, however, not been reported, though
diastereoselective receptor amplifications are known.2,3
We report herein two tools (laser polarimetry (LP) detection and
pseudo-enantiomers) for the discovery and identification of a
peptide-like enantioselective receptor for (-)-adenosine,4,5 whose
biological receptors represent important medicinal targets.6 The
receptor was elicited from a racemic dynamic combinatorial library
(DCL), which has advantages of enhanced stereochemical diversity
over enantiopure versions (Scheme 1) and ease of synthesis.
Scheme 1. Templating a rac-1 DCL with (-)-adenosine
Figure 1. (a) UV absorbance (289 nm) of untemplated DCL (1 mM in
monomer). (b) LP of untemplated DCL. (c) UV absorbance (289 nm) of
DCL + (-)-adenosine (1 mM rac-1, 5 mM (-)-adenosine). (d) LP of DCL
+ (-)-adenosine.
Table 1. Oligomer Quantities (from rac-1) (HPLC UV area %) in
the Untemplated and (-)-Adenosine Templated Casesa
Enantioselective receptors for (-)-adenosine could support the
HPLC area %
search for adenosine nucleotide receptor inhibitors.6
library member
untemplated
templated
∆
One difficulty with identifying enantioselective receptors from
a DCL is that two conditions are required: (1) a binding propensity,
and (2) a binding differential for the two enantiomers (diastereo-
meric host-guest complexes). While some of these data should be
available by comparing the templating efficiency of an (R)- and an
(S)-analyte for an enantiopure DCL, we sought a more direct single
experiment protocol. We reasoned that templating a racemic DCL
with an enantiopure guest would amplify the best host (condition
1), and if that receptor also bound the analyte enantioselectively,
then the best matched enantiomer would be amplified over the
mismatched antipode (condition 2).7 The two methods described
herein were designed to detect this enantio-amplification in the best
host.
dimers
trimers
tetramers
hexamers
44
29
24
3
56
21
23
0
+12
-8
-1
-3
a 1 mM rac-1, 5 mM (-)-adenosine, 50 mM TFA, 1% 2-methoxyethanol
in CH2Cl2, 24 h equilibration.
unit and an achiral peptide. The addition of TFA deprotects the
acetal and initiates reversible hydrazone exchange to form the cyclic
oligomers (Scheme 1). Figure 1a shows the HPLC chromatogram
(UV absorbance at 289 nm) for a nontemplated DCL (diastereomer
resolution is not achieved).
When the DCL was templated with (-)-adenosine (5 equiv with
respect to rac-1), the dimer was amplified at the expense of the
higher order oligomers (Figure 1c). As shown in Table 1, the shift
toward dimer and loss of the higher oligomers, while modest, is
easily quantified by HPLC/UV. The increase from 44 to 56% dimer
(∆ ) +12%) and concomitant decrease in the higher oligomers
suggests a dimer of modest binding ability. More interesting,
however, was the appearance of a signal in the LP trace at a
retention time corresponding to the dimer (Figure 1d). This signal
clearly indicates that the dimer has had one of its homochiral
diastereomers selectively enhanced over the other (the heterochiral
dimer RS is achiral, Chart 1).
The classic method for detecting enantiomeric excess is via
polarimetry, a powerful tool when matched with an HPLC
separation (laser polarimeter (LP) detector).8 Since achiral com-
pounds and racemates are polarimetry silent, only receptors enriched
in one enantiomer (i.e., from a diastereoselective host-guest
complex) will give an LP signal and thus be detected. Creating a
signal from an otherwise null background should, in principle, also
enable the analysis of more complex libraries than previously
possible.
For proof of principle studies, we chose the cyclic hydrazone
family of DCC dipeptides; rac-12,9 is composed of a racemic proline
Additional evidence for an enantioselective amplification of 2
was obtained by examining the templating effect of (-)-adenosine
† U.S. Army Research Office.
‡ University of North Carolina at Chapel Hill.
9
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10.1021/ja0647699 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Chart 1. Diastereomers of 2
These results demonstrate that it is indeed possible to elicit an
enantioselective receptor from a racemic DCL. The demonstrated
applicability of laser polarimetric detection and pseudo-racemic
monomers to first identify and then pinpoint the stereochemical
identity of interesting receptors from complex libraries suggests
an effective strategy for continued efforts to discover enantiose-
lective receptors. Enantioselectivity in the recognition of a nucle-
otide bodes well for the feasibility of future endeavors aimed at
discovering selective receptors for important biological ligands,
especially considering the ease with which the racemic library
members may be altered and screened.
with an (S)-1 and an (R)-1 DCL. As shown in Figure 2a, the increase
in dimer was significantly larger for the (S)-DCL than for the (R)-
DCL, and this was especially true at low monomer concentrations
(1 mM). We thus concluded that (S,S)-2 was the enantioselective
receptor for (-)-adenosine and that it was amplified selectively in
the racemic DCL (Chart 1).10
Acknowledgment. We thank Dr. Matthew Crowe (UNC Mass
Spec Facility), in addition to Prof. Jim Jorgenson and Diana
Scheerbaum for UHPLC analyses and helpful discussions. For
financial support, we gratefully acknowledge the Defense Threat
Reduction Agency Basic Research program administered by the
Army Research Office (W911NF04D0004).
Supporting Information Available: Experimental details for
monomer synthesis and DCL conditions. This material is available free
of charge via the Internet at http://pubs.acs.org.
References
Figure 2. (a) Amplification of 2 (% dimertemplated - % dimeruntemplated) by
(-)-adenosine in an (S)-1 DCL (9), (R)-1 DCL (2), and rac-1 DCL (b)
versus initial monomer concentration. (b) Ratio of LP peak height to UV
peak height for the dimer in an untemplated DCL versus % ee of 1.
(1) For reviews, see: (a) Ramstro¨m, O.; Lehn, J.-M. Nat. ReV. Drug DiscoVery
2002, 1, 26-36. (b) Rowan, S. J.; Cantrill, S. J.; Cousins, G. R. L.;
Sanders, J. K. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2002, 41, 898-
952. (c) Otto, S.; Furlan, R. L. E.; Sander, J. K. M. Drug DiscoVery Today
2002, 7, 117-125. (d) See also: Zhang, W.; Moore, J. S. Angew. Chem.,
Int. Ed. 2006, 45, 4416-4439.
To estimate the enantioenrichment of (S,S)-2 over (R,R)-2 in the
racemic DCL, new DCLs (nontemplated) were synthesized using
variable quantities of (S)-1 and (R)-1. Plotting the ratio of the
dimer’s LP and UV signals versus the enantiomeric excess of 1
generated a crude calibration curve for the dimer’s enantiomeric
excess (Figure 2b). This calibration curve yielded an ee of ∼21%
for the dimer in the templated racemic DCL (1 mM, Figure 1c and
1d).11 This value compares favorably with some of the best peptide-
based receptors for chiral ammonium ions.4a,12
More convenient for this and future studies would be an in situ
method for directly measuring the amount of (S,S)-2, (R,R)-2, and
(S,R)-2 under the templating conditions. To this end, d7-(S)-1 was
prepared (from d7-proline) and mixed with (R)-1 to generate a
pseudo-racemic monomer. The resulting unique mass signature of
the diastereomeric library components was expected to enable MS
analysis for diastereo- and enantio-composition assessment ((R,R)-2
) M+, (S,R)-2 ) M++7, and (S,S)-2 ) M++14).
(2) Lam, R. T. S.; Belenguer, A.; Roberts, S. L.; Naumann, C.; Jarrosson,
T.; Otto, S.; Sanders, J. K. M. Science 2005, 308, 667-669.
(3) (a) Gonza´lez-AÄ lvarez, A.; Alfonso, I.; Gotor, V. Chem. Commun. 2006
2224-2226. (b) Corbett, P. T.; Tong, L. H.; Sander, J. K. M.; Otto, S. J.
Am. Chem. Soc. 2005, 127, 8902-8903.
(4) For the use of chiral receptors for enantioselective binding of amines,
see: (a) Zhang, X. X.; Bradshaw, J. S.; Izatt, R. M. Chem. ReV. 1997,
97, 3313-3361. (b) Naemura, K.; Tobe, Y.; Kaneda, T. Coord. Chem.
ReV. 1996, 148, 199-219. (c) Stoddart, J. F. Top. Stereochem. 1987, 17,
207-288.
(5) Amide-based receptors incorporating π-stacking and H-bond donor/
acceptor moieties bind nucleobases, though enantio-recognition must
necessarily come from the sugar portion of the analyte. See for example:
(a) Conn, M. M.; Deslongchamps, G.; de Mendoza, J.; Rebek, J., Jr. J.
Am. Chem. Soc. 1993, 115, 3548-3557. (b) Muehldorf, A. V.; Van Engen,
D.; Warner, J. C.; Hamilton, A. D. J. Am. Chem. Soc. 1988, 110, 6561-
6562.
(6) Adenosine nucleotide receptors are ubiquitous and important medicinal
targets, influencing such conditions as hypertension, inflammation, asthma,
memory, immune responses, etc. See for example: Jacobson, K. A.; Gao,
Z.-G. Nat. ReV. Drug DiscoVery 1996, 5, 247-264.
(7) Concentrating a single enantiomer into the amplified receptor causes a
concomitant buildup of its mirror image into the other members of the
DCL. This effect reduces the LP signal from these enantioenriched
components and dilutes the entropic cost of deracemization.
(8) (a) Goodall, D. M. Trends Anal. Chem. 1993, 12, 177-184. (b) Bobbitt,
D. R.; Linder, S. W. Trends Anal. Chem. 2001, 20, 111-123. (c) Halls,
S. C.; Lewis, N. G. Tetrahedron: Asymmetry 2003, 14, 649-658.
(9) Roberts, S. L.; Furlan, R. L. E.; Otto, S.; Sanders, J. K. M. Org. Biomol.
Chem. 2003, 1, 1625-1633.
(10) (S,S)-2 gives a levorotatory (negative phase) peak in the LP (Figure 1d).
(11) This analysis accounts for statistical quantities of the achiral heterodimer,
but may not account for templating induced changes in its concentration.
(12) Cyclic peptides selectively bind chiral quaternary ammonium ions. See:
(a) Heinrichs, G.; Vial, L.; Lacour, J.; Kubik, S. Chem. Commun. 2003,
1252-1253. For achiral binding, see: (b) Madison, V.; Deber, C. M.;
Blout, E. R. J. Am. Chem. Soc. 1977, 99, 4788-4798 and references
therein.
LC-MS analysis of the untemplated library indicated a roughly
statistical mixture of dimers: d14-(S,S)-2:(R,R)-2:d7-(R,S)-2 was 19:
15:66%.13 Under templating conditions, however, the amount of
heterodimer drops and the ratio of d14-(S,S)-2 to (R,R)-2 increases
to 1.7:1 (34:20:45%), an enantio-ratio that is similar to that obtained
from the calibration curve in Figure 2 (22% ee ) 1.5:1). The
pseudo-enantiomer methodology thus enables the direct measure-
ment of the amplified receptor’s enantiomeric and diastereomeric
ratios.14
(13) The ratio of (R,R)-2 to (S,S)-2 was not exactly 1:1 in the pseudo-racemate.
(14) Direct characterization of the differential binding properties of 2 and (-)-
adenosine was hindered by poor solubility in nonacidic templating
solutions (where library exchange does not occur), perhaps reasonably
suggesting that the actual analyte is protonated. See: Kampf, G.; Kapinos,
L. E.; Griesser, R.; Lippert, B.; Sigel, H. J. Chem. Soc., Perkin Trans. 2
2002, 1320-1327.
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