Highly stereospecific generation of helical chirality by imprinting with amino acids: A universal sensor for amino acid enantiopurity

Article abstract of DOI:10.1002/anie.200803116

(Chemical Equation Presented) Helical chirality can be imprinted onto a 2,2′-dihydroxybenzophenone derivative (see picture) in a highly stereospecific manner. A single amino acid combines with the receptor to form an imine with two internal hydrogen bonds. The azo group allows sensing of amino acid enantiopurity by circular dichroism spectroscopy.

Full text of DOI:10.1002/anie.200803116

Angewandte
Chemie
DOI: 10.1002/anie.200803116
Chirality Sensors
Highly Stereospecific Generation of Helical Chirality by Imprinting
with Amino Acids: A Universal Sensor for Amino Acid
Enantiopurity**
Hyunwoo Kim, Soon Mog So, Cindy Pai-Hui Yen, Elisângela Vinhato, Alan J. Lough,
Jong-In Hong,* Hae-Jo Kim,* and Jik Chin*
Generation of helical and axial chirality has been the topic of
much interest in biology and chemistry. Controlling axial
chirality with stereogenic-center-based chirality has shown to
be useful for determining the absolute configurations or
enantiopurities of chiral acids,[1] alcohols,[2] and natural[3] or
unnatural amino acids,[4] and also for stereoselective synthesis
of unnatural amino acids.[5] However, it has been a challenge
to control axial chirality from stereogenic-center-based chir-
ality with a high degree of stereospecificity. In biology, amino
acid chirality (the l form) is used for the stereospecific
generation of helical chirality (for example, the right-handed
a helix of proteins). The energy difference between the right-
and left-handed a-helical peptides[6] is small per amino acid
residue,[7] and more than twenty l amino acids are required to
favor the right-handed a helix. Finding the minimal structural
requirement of a receptor for generating helical chirality from
amino acid chirality may provide interesting insights into
stereospecific folding[8] and stereoselective recognition[9] of
molecules. Herein we report how helical chirality can be
imprinted onto 2,2’-dihydroxybenzophenone (1) in a highly
stereospecific manner with a single amino acid (Scheme 1). A
signaling group can also be attached to the receptor 2 for
general sensing of amino acid enantiopurity.
Scheme 1. Generating helical chirality in 2,2’-dihydroxybenzophenone 1
with a single amino acid. AL =l-alanine, G=global minimum, L=local
minimum. See text for details.
Compound 1 is readily available commercially. It has axial
or helical chirality and exists as an equal mixture of rapidly
interconverting P and M forms (P-1 and M-1 in Scheme 1a).
Receptor 1 resembles [4]helicene[10] in that they are both
helical with four consecutive six-membered rings (including
hydrogen bonds in 1). Amines react with 1 to form imines
within minutes at ambient temperatures (Scheme 1b). Under
the same conditions, it takes weeks to form imines with
benzophenone. Thus the two hydroxy groups in 1 greatly
activate the carbonyl group towards nucleophilic attack
through double H bonding. If the tetramethylammonium
salt of alanine (0.1m) is added to a solution of 1 (0.1m) in a
protic solvent, such as CD3OD (Scheme 1c), two sets of
[*] Prof. Dr. J.-I. Hong
Department of Chemistry, Seoul National University
Seoul 151-747 (Korea)
E-mail: jihong@snu.ac.kr
Prof. Dr. H.-J. Kim
Department of Chemistry, Kyonggi University
Suwon 443-760 (Korea)
1
signals are detected in the H NMR spectrum (Figure 1a).
E-mail: haejkim@kgu.ac.kr
The two compounds in Scheme 1c are diastereomeric and are
expected to give distinct NMR signals. However, if an aprotic
solvent, such as CD3CN is used (Scheme 1b), a remarkably
clean H NMR spectrum results with just one set of signals
(Figure 1b).
H. Kim, Dr. S. M. So, C. P.-H. Yen, Dr. E. Vinhato, Dr. A. J. Lough,
Prof. Dr. J. Chin
Department of Chemistry, University of Toronto
80 St. George Street, Toronto M5S 3H6 (Canada)
Fax: (+1)416-978-7113
1
Computation can be used to help explain the high
stereospecificity for imine formation in an aprotic solvent
such as CD3CN. Density functional theory computation
(DFT; B3LYP at the 6-31G* level)[11] shows that the global
minimum-energy structure of the imine 1-AL-G formed
between anionic l-alanine and 1 has two internal H bonds
(Figure 2a). One internal H bond in 1-AL-G is a resonance-
E-mail: jchin@chem.utoronto.ca
[**] We thank the Natural Sciences and Engineering Research Council of
Canada, Korea Research Foundation (Grant No. 2007-314-C00172),
and Seoul R&BD for financial support. E.V. thanks CNPq of Brazil
for a postdoctoral fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803116.
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shows that 1-AL-G is more stable than 1-AL-L by about
5.2 kcalmolÀ1. It is interesting that the energy difference
between 1-AL-G and 1-AL-L is so large for such closely
related and simple diastereomers with just two H bonds. This
energy difference translates to an equilibrium ratio of about
5.5 ” 103 for [1-AL-G]/[1-AL-L] at 258C. Thus if both internal
H bonds are maintained in aprotic solvents, only one imine
should form to any observable extent, as confirmed by
1H NMR spectroscopy (Figure 1b).
Figure 1. The doublet signals in the 1H NMR spectrum of the alanine
methyl groupupon formation of imine(s) with 1 in a) CD3OD and
b) CD3CN.
It is apparent from the computed structures that 1-AL-G is
more stable than 1-AL-L owing to the relative positioning of
the alanine methyl groups (Scheme 1b). The methyl group in
1-AL-G is positioned in an unhindered area, whereas the
methyl group in 1-AL-L is in a sterically crowded area close to
one of the phenol groups. Thus l-alanine generates axial
chirality in the P form stereospecifically upon formation of
the imine complex in aprotic solvents (i.e., the equilibrium in
Scheme 1b favors 1-AL-G over 1-AL-L).
Protic solvents appear to disrupt the charged internal
hydrogen bonds in 1-AL-G and 1-AL-L to give 1-AL-G* and 1-
AL-L*, respectively (Scheme 1c). Indeed, crystal structures of
1-AL-G*[15] and 1-AL-L*[16] reveal that the internal charged H
bonds can be broken while maintaining the resonance-
assisted H bonds (Figure 2c and d). It appears that the
weaker intramolecular H bond breaks to form intermolecular
H bonds at the high concentrations required for crystalliza-
tion. Computation shows that 1-AL-G* and 1-AL-L* are of
comparable energy, which is in agreement with the integra-
tion ratio of the two doublet signals in the 1H NMR spectrum
(Figure 1a). Apart from using protic solvents, a base can be
used in aprotic solvents to eliminate the stereospecificity by
breaking the charged H bond. Amino acid esters cannot form
the charged H bond and do not control the helical chirality
(see Supporting Information).
Figure 2. a),b) Computed structures of 1-AL-G (a) and 1-AL-L (b). c),d)
X-ray crystal structures of 1-AL-G* (c) and 1-AL-L* (d). The structures in
(a),(c) have helical chirality in the P form; those in (b),(d) in the M
form.
Helical contents of protein molecules are often measured
with circular dichroism (CD) spectroscopy.[17] The CD signals
are weak for amino acids bound to receptor 1. We therefore
covalently attached signal-amplifying diazo functional groups
to the receptor 2 (Figure 3b). Figure 3a shows the CD and
UV spectra of the imines 2-AL-G and 2-AD-G formed
between l-alanine and 2 and between d-alanine and 2,
respectively.
It is evident from Scheme 1 and the computational studies
(Figure 2a,b) that other natural or unnatural a-amino acids
should behave similarly to alanine. Indeed, a wide variety of
amino acids, including asparagine, phenylalanine, serine, and
valine, all give one imine diastereomer each with 1 or 2 in
aprotic solvents, as shown by 1H NMR spectroscopy (see
Supporting Information). Computation shows that the ste-
reospecificity increases with increase in the size of the amino
acid side chain. Thus, alanine is expected to give the lowest
stereospecificity (5500:1). Such a high degree of stereospeci-
ficity for generating helical chirality from stereogenic-center-
based chirality has been observed with large biomolecules,
such as proteins and nucleic acids, but not with small
molecules. CD spectra of the imines formed with different
l-amino acids not only have the same bisignate sign (P
helicity) but they are also remarkably close in intensity
(Table 1). If the CDs were identical for different amino acids,
assisted hydrogen bond (RAHB)[12,13] between the protonat-
ed imine and the phenolate oxyanion. The other internal
H bond in 1-AL-G is between the alanine carboxylate anion
and the remaining phenolic hydrogen atom. Such charged H
bonds are generally stronger than neutral H bonds.[13] The
1H NMR signals for the two H bonds determined in DMSO
are shifted far downfield and are independent of concen-
tration (Supporting information, Figure S1), as would be
expected for strong intramolecular H bonds.[14] Comparing
the structure of 1-AL-G and the structure of the receptor 1, it
is evident that l-alanine generates axial chirality in the P form
upon formation of the imine.
The second most stable minimum energy structure of the
imine formed between anionic l-alanine and 1 is 1-AL-L
(Scheme 1b and Figure 2b). This local minimum energy
structure also has the two strong internal H bonds and is
closely related to the structure of 1-AL-G. However, a
comparison of the structures of the imine complexes and
the receptor (Scheme 1a,b), shows that 1-AL-L is in the M
form whereas 1-AL-G is in the P form. It is likely that 1-AL-G
and 1-AL-L are in equilibrium through rotation of the bond
connecting the imine to one of the phenols. DFT computation
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380 nm followed by the positive Cotton effect at 340 nm is in
agreement with the structure of 2-AD-G (M helical chirality)
formed with d-alanine. This empirical approach has been
used for determining the absolute configuration of diols,[18a]
diamines,[18b] biphenyls,[18c] binols,[18d] and helicenes[18e] (Sup-
porting Information).
1
Our CD and H NMR spectroscopy results indicate that
different a-amino acids form imines with 2 and generate
helical chirality not only with the same sense but also with a
high degree of stereospecificity. Thus, 2 is an excellent
chirality sensor for amino acids. There is excellent correlation
between CD absorption at 360 nm and enantiomeric excess of
valine in the imine complex formed with 2 (see Supporting
Information). The crystal structure of the imine formed
between l-valine and 2 is analogous to that of 1-AL-G*
(Figure 2c). Although a variety of interesting chirality sensors
have been developed for amino acids,[19,20] none to date have
been shown to interact with a high degree of stereospecificity.
Previously reported sensors give different CD signals for
different amino acids. In contrast, CD spectra obtained from
the reactions of 2 and different amino acids are remarkably
close (Table 1).
Generation of helical chirality with unprecedented ste-
reospecificity has been achieved by imprinting of amino acid
chirality[21] onto a small molecule receptor (1). The control of
the helical chirality is accomplished by an imine and two H
bonds between underivatized amino acids and the receptor.
The excellent agreement between DFT computational and
Figure 3. a) Circular dichroism spectra of 2-AL-G and 2-AD-G (50 mm in
acetonitrile, 1 cm cell, at 258C) and their UV/Vis spectra. b) Structure
of 2 and 2-AL-G.
1
experimental data, including CD and H NMR spectrscopy,
Table 1: CD data for imines formed from amino acids and 2 in
acetonitrile.
and X-ray crystallography, provides valuable insight into the
origin of the stereospecificity. Furthermore, a universal
chirality sensor 2 for amino acids has been developed by
attaching a CD signal-amplifying group to 1.
Amino acid
UV/Vis[a]
CD[a]
Ratio
[mdegmax/Amax
]
[b]
Amax
lmax [nm] mdegmax IP [nm][c]
Received: June 29, 2008
Published online: October 8, 2008
l-Ala
1.84
1.84
2.06
2.02
2.11
2.12
2.00
1.88
2.08
360
355
363
363
363
360
358
354
355
64.5
67.8
77.4
66.2
69.6
74.0
67.6
68.3
76.4
361
355
357
363
362
359
358
353
353
35.0
36.8
37.6
32.8
33.0
34.9
33.8
36.3
36.7
l-Asn
l-Gln
l-Ile
l-Leu
L-Met
l-Phe
l-Ser
l-Thr
l-Trp1.88
l-Tyr
Keywords: amino acids · chirality · circular dichroism ·
.
helical structures
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1.83
57.2
360
360
362
66.0
60.7
30.4
359
361
33.7
33.2
l-Val
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8.2201(5),
c = 23.1214(10) ꢀ,
b = 98.134(3)8,
V=
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