Determination of amino acid enantiopurity and absolute configuration: Synergism between configurationally labile metal-based receptors and dynamic covalent interactions

Article abstract of DOI:10.1002/chem.201302721

Reliable determination of the enantiomeric excess of free amino acids can be obtained by measuring the induced circular dichroism of a multicomponent assembly formed by a modified tris(2-pyridylmethyl)amine ligand, a zinc salt, and the amino acid of inter

Full text of DOI:10.1002/chem.201302721

FULL PAPER
DOI: 10.1002/chem.201302721
Determination of Amino Acid Enantiopurity and Absolute Configuration:
Synergism between Configurationally Labile Metal-Based Receptors
and Dynamic Covalent Interactions
Francesca A. Scaramuzzo, Giulia Licini, and Cristiano Zonta*[a]
Abstract: Reliable determination of the enantiomeric excess of free amino acids
can be obtained by measuring the induced circular dichroism of a multicomponent
assembly formed by a modified tris(2-pyridylmethyl)amine ligand, a zinc salt, and
the amino acid of interest. The systems furnish reliable information for all natural
Keywords: amino acids · configura-
tion determination · circular dichro-
ism · chirality · ligands · molecular
recognition
amino acids.
Introduction
Ever growing interest in new and efficient stereoselective
reactions requires the development of effective methods
that allow accurate determination of the enantiomeric
excess (ee) of reagents and products.[1] For this purpose, the
use of supramolecular sensors is increasing and is usually
combined with optical signaling techniques because they are
rapid and often inexpensive.[2] In this context, particular in-
terest has been directed toward the use of circular dichroism
(CD).[3] This is due to the high information content con-
veyed by this technique and to the wide availability of sensi-
tive CD instruments. Because, in several cases, the chiral
compounds of unknown enantiopurity lack strong chromo-
phores, intermolecular interactions with stereodynamic chro-
mophoric probes are used to generate distinct Cotton ef-
fects.[4] Moreover, the induced CD can allow determination
of the absolute configuration of the compound of interest.
The molecular recognition event is ruled by interactions
such as inclusion complexes,[5] chiral ion pairs,[6] imine bond
formation,[7] or metal–ligand interactions.[8] The latter have
been extensively examined in tetradentate metal complexes
that are characterized by a propeller-like arrangement of
the ligand around the metal (Figure 1).[9–15]
Figure 1. Clockwise (P) and counter-clockwise (M) arrangement of tris(2-
pyridylmethyl)amine (Tpy) complexes.
genic element. This particular feature has been extensively
studied by the group of Canary.[9] In particular, tripodal li-
gands derived from quinoline or pyridine that are able to
form propeller-like complexes with ZnII and CuII ions have
been used as molecular sensors or as redox-triggered chirop-
tical molecular switches.[10] More recently, Anslyn and
Canary reported that the association between achiral quino-
line-based metal complexes and chiral carboxylate, or N-
Boc protected amino acids,[11] resulted in exciton-coupled
CD, as well as the association between tris(2-pyridylmethyl)-
amine (Tpy) metal complexes and chiral secondary alco-
hols.[12]
In the past years, we have been interested in the synthesis
of tripodal ligands and their relative metal complexes for
their application in catalysis and in supramolecular chemis-
try.[13–15] As an example, enantiopure TiIV complexes bearing
pseudo-C3 amino triphenolate ligands have been shown to
act as NMR chiral solvating agents for the stereochemical
analysis of a series of sulfoxides,[14] or to control the dimeri-
zation process to the corresponding m-oxo dinuclear spe-
cies.[15] More recently, we have extended our interest to Tpy
ligands and, in this article, we describe the synthesis and ap-
plication of a new Tpy derivative, bearing an aldehydic
moiety on one of the three arms, and its corresponding zinc
complex. We envisioned that this new system could offer the
possibility of combining the recognition capability of a con-
figurationally labile Tpy complex, which has already been
Such peculiar geometry gives raise to the formation of
two enantiomeric complexes in solution that are character-
ized by a clockwise (P) or counter-clockwise (M) arrange-
ment of the ligand. The handedness of the molecular system
can be controlled by the presence of a stereogenic center in
the backbone ligand, or by a chiral guest carrying a stereo-
[a] Dr. F. A. Scaramuzzo, Prof. G. Licini, Dr. C. Zonta
Dipartimento di Scienze Chimiche
Universitꢀ degli Studi di Padova
Via Marzolo 1, 35131 Padova (Italy)
E-mail: cristiano.zonta@unipd.it
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302721.
Chem. Eur. J. 2013, 19, 16809 – 16813
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shown to be efficient for the coordination of carboxylate
functionalities, with imine condensation chemistry, which is,
in our case, directed to amine functions. This synergism is
efficiently used for the determination of the ee of all natural
a-amino acids.
disappearance of the formyl proton at 10.03 ppm. The for-
mation of imine 3a-d-Phe was also confirmed by FTIR
measurements by the disappearance of the characteristic
carbonyl stretching of the aldehyde (1700 cmÀ1) and by the
appearance of the stretching peak of the C=N bond
(1635 cmÀ1). In the UV/Vis spectra, the new species was
characterized by the appearance of two peaks at 260 and
287 nm, respectively, which are negligible in the parent com-
plex 2a under the same experimental conditions. Interest-
ingly, the complex 3a-d-Phe shows a strong positive Cotton
effect in the region between 220 and 320 nm (Figure 2). No
Results and Discussion
We first synthesized the new Tpy derivative 1a, consisting
of the Tpy core bearing a 3-formyl-phenyl substituent on
one of the three arms (Scheme 1). Compound 1a was ob-
Scheme 1. Synthesis of the ligands 1a and 1b. Reagents and conditions:
i) NaBH
ACHTUNGTRENNUNG
phenylboronic acid, Pd
AHCTUNGTRENNUNG
12 h, 908C (97%).
tained in two steps from reductive amination of commercial-
ly available 6-bromo-2-pyridinecarboxaldehyde and di(2-pi-
colyl)amine, followed by Suzuki coupling with 3-formyl-
phenyl boronic acid.[16–17] The synthesis was optimized to
prepare ligand 1a on a gram scale without requiring chro-
matographic purification. The corresponding zinc complex
2a was synthesized by addition of ZnACTHUNRGTNEG(UN ClO4)2 6H2O to ligand
1a in acetonitrile (Scheme 2). Condensation of 2a with an
amino acid could be monitored by 1H NMR, UV and IR
spectroscopy (Scheme 2).
As an example, upon mixing 2a and d-phenylalanine (d-
Phe) in deuteromethanol solution, 1H NMR indicated the
Figure 2. Comparison between the CD spectra obtained upon reaction of
2a respectively with: a) Ala, b) Arg, c) Phe, and d) Ser. Curves obtained
with d-form are in solid lines, whereas those obtained with l-form are in
dashed lines.
meaningful CD signals were observed in this region by using
compound 2a, whereas the free amino acid d-Phe gave
a weaker signal from 200 to 230 nm. Moreover, the presence
of an excess of a single component did not affect the intensi-
ty of the signal. As expected, the specular CD spectrum was
observed when l-Phe was used. On the basis of similar sys-
tems previously studied by other groups, we can hypothesize
that the enhancement of the dichroic signal could originate
from the preferential formation of a single propeller-like ar-
rangement of the pseudo-C3 metal complex. The simultane-
Scheme 2. Different routes for the preparation of the multiassembly 3a-
Phe.
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Enantiopurity and Absolute Configuration of Amino Acids
FULL PAPER
ous coordination of the carboxylate to the metal center and
the condensation reaction play a major role in fixing the
handedness of the complex.
To confirm the crucial role of the aldehyde–amine con-
densation, we synthesized compound 1b, an analogue of 1a
the plot of the corresponding d-amino acid. The configura-
tion of the stereocenter could be assigned based on the sign
of the first Cotton effect, because all l-stereocenters gave
a negative signal and the d-stereocenters a positive signal.
Having obtained CD spectra that differed in shape and in-
tensity depending on the amino acid used, we wanted to dis-
criminate between them on the basis of their identities and
absolute configuration. For this purpose, we employed Prin-
cipal Component Analysis (PCA), an unsupervised tech-
nique used to reduce the dimensionality of data space.[18]
Such a chemiometric tool allowed a new set of variables to
be created, named principal components (PC), to explain
the variance of the system. A set of five duplicate samples
were collected for each amino acid, and the CD spectra
were recorded in duplicate for each sample. In our system,
the different amino acids showed effective separation, allow-
ing for discrimination based on the absolute configuration
(Figure 4). The first principal component PC1, which ac-
counts for almost 62% of the whole variance, is mainly con-
nected to the amino acid configuration; all the d-amino
acids were located on the negative projection of PC1,
whereas all the l-amino acids were located on the positive
projection of PC1, with the only exception being aspartic
acid. Within a certain configuration, PC2 is mainly connect-
ed to the aliphatic or aromatic nature of the backbone, that
is, in the plot PC2 versus PC1, d-Phe, d-Phg, d-Trp and d-
Tyr are on the positive projection of PC2 and all the other
d-amino acids are on the negative projection, whereas l-
Phe, l-Phg, l-Trp and l-Tyr are on the negative projection
and all the other l-amino acids on the positive projection,
with the only exception again being aspartic acid. In this
way, a global visualization could be obtained of the differen-
ces among the spectra and, as a consequence, among the be-
havior of the different amino acids on the base of their
chemical features. The behavior of the system in the pres-
ence of aspartic acid is unclear; it is probably due to the
presence of two carboxylate groups, which can compete in
the binding to ZnII, lowering the overall dichroic signal.
lacking the formyl group, which, in presence of ZnACTHUNTRGNE(UNG ClO4)2
and d-Phe, did not show any meaningful dichroic signal. To
test whether the observed behavior was general and could
be used as a probe for ee determination, we built a calibra-
tion curve by using the CD molar ellipticity values at
256 nm for a series of samples of known enantiopurity. A
good linear fit was observed, and reproducibility was con-
firmed employing four replicates for each data point. This
demonstrated that 2a could be used for the analysis and de-
termination of unknown enantiopurity of Phe (Figure 3).
Figure 3. Values of V at 256 nm for samples at different ee of Phe. The
calibration curve was built using four independent measurements.
The particular features of this recognition system also
offer the possibility of using multicomponent assemblies of
the species through one-pot reaction between ligand 1a, Zn-
ACHTUNGTRENNUNG(ClO4)2, and amino acid d-Phe (Scheme 2). A mixture of the
three components in methanol at 508C underwent complete
conversion in five hours, as shown by the CD spectrum
which was superimposable on that resulting from stepwise
assembly. It is interesting to note that the situation was dif-
ferent in case of imine formation prior to metallation
(Scheme 2). In this case the formation of imine 4 was ex-
tremely slow and did not allow its use as a probe for ee de-
termination. All of these observations, in addition to the ca-
pability of using 1a in a multicomponent assembly, point to
the fact that the zinc is acting as a catalyst for the condensa-
tion reaction.
To test the generality of the protocol, the system was ex-
amined with the other 18 couples of proteogenic amino
acids and phenylglycine with both enantiopure forms. As for
Phe, all of the d-enantiomers showed a positive first Cotton
effect, with the only variance being the magnitude and
shape of the curve, whereas all of the l-enantiomers showed
a negative first Cotton effect that was the mirror image of
Conclusion
We have demonstrated that by combining the features of
Tpy metal complexes and the presence of a highly reactive
aldehyde group it is possible to obtain an extremely versa-
tile system that can be used to determine the enantiopurity
of simple proteogenic amino acids. The new Tpy system
shows a remarkable enhancement of the CD signal upon
binding with a-amino acids, allowing quantitative ee analysis
with great accuracy. The ligand is easy to synthesize and the
system undergoes rapid self-assembly under mild conditions,
so that no further purification steps are required. Moreover,
an accurate chemiometric analysis showed that the self-as-
sembled systems can be used to discriminate among differ-
ent amino acids, thus opening new perspectives for a fast
screening of the nature and absolute configuration of a dif-
ferent class of molecules.
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C. Zonta et al.
formed with an Applied Biosystems
ESI-TOF Mariner Biospectrometry
Workstation (methanol as mobile
phase
with
internal
standards).
MALDI-TOF analyses were carried
out with an AB-SCIEX TOF-TOF
4800 instrument. Microanalyses were
performed with a Flash 2000 Thermo
Scientific Analyser. For new com-
pounds, satisfactory determinations
were obtained: CÆ0.3, HÆ0.27.
1-(6-Bromopyridin-2-yl)-N,N-bis(pyri-
din-2-ylmethyl)methanamine: 6-Bro-
mopyridine-2-carboxyaldehyde
(1 g,
5.4 mmol) and 2-dipicolylamine
(968 mL, 5.4 mmol) were stirred in an-
hydrous THF (40 mL) under an N2 at-
mosphere. After 1 h, sodium triace-
toxy borohydride (1.14 g, 5.4 mmol)
was added and the mixture was stirred
under an N2 atmosphere overnight at
RT. The mixture was washed three
times with saturated NaHCO3 solution
and the organic phase was dried over
Na2SO4. Evaporation of the solvent
under reduced pressure gave the pure
product (1.82 g, 4.9 mmol, 92%) as
a brownish solid. 1H NMR (200 MHz,
CDCl3): d=8.53 (d, J=4.9 Hz, 2H;
PyrH), 7.59 (m, 6H; PyrH), 7.30 (d,
J=7.5 Hz, 1H; PyrH), 7.15 (dd, J=
7.1, 4.9 Hz, 2H; PyrH), 3.90 ppm (s,
6H; CH2); 13C NMR
(62 MHz,
CDCl3): d=161.10, 158.90, 149.02,
141.20, 138.71, 136.46, 126.16, 122.99,
122.06, 121.55, 60.04, 59.38 ppm; MS
(ESI): m/z calcd for C18H17BrN4:
368.06;
found:
369.2
m/z
368.0637;
[M+H]+;
MALDI-TOF:
C18H17BrN4:
369.0016.
calcd
for
found:
[6-(3-Formylphenyl)-2-pyridylmethyl]-
bis(2-pyridylmethyl)amine (1a): 1-(6-
Bromopyridin-2-yl)-N,N-bis(pyridin-2-
ylmethyl)methanamine
(1.8 g,
4.9 mmol), 3-formyl-phenyl boronic
acid (1.1 g, 7.3 mmol), Na2CO3 (1.3 g,
12.2 mmol), and PdACHTNUGTRNEUNG(PPh3)4 (564 mg,
0.49 mmol) were dissolved in degassed
H2O/toluene/CH3OH (1:1:0.5, 50 mL)
under an N2 atmosphere. The mixture
was stirred for 10 h at 908C, then the
solvent was evaporated under reduced
pressure and the solid obtained was
dissolved in CHCl3. The organic phase
was extracted three times with 5% aq.
HCl solution. The aqueous phase was
then basified with NaOH and extract-
ed three times with CHCl3. The organ-
Figure 4. PCA: PC1 versus PC2 plot. In the upper panel, d-amino acids are all within the solid ellipse, l-amino
acids are all within the dashed ellipse. In the bottom panel, aromatic amino acids are all within the solid ellip-
~
&
*
^
*
ses, aliphatic amino acids are all within the dashed ellipse. Legend: Ala, Arg, Asn, Asp, + Cys,
~
~
&
^
*
^
Gln, & Glu, His, ꢂ Ile, Leu, – Lys, Met, + Phe, Phg, – Pro, Ser, Thr, Trp, Tyr, ꢂ Val.
*
*
Experimental Section
ic phase was dried over Na2SO4 and then evaporated under reduced pres-
sure and gave the pure product (1.87 g, 4.7 mmol, 97%) as a brownish
oil. 1H NMR (200 MHz, CDCl3): d=10.03 (s, 1H; CHO), 8.47 (m, 3H;
PyrH+ArH), 8.23 (d, J=7.8 Hz, 1H; ArH), 7.83 (d, J=7.6 Hz, 1H; Ar
H), 7.61 (m, 8H; PyrH+ArH), 7.08 (dd, J=7.0, 4.9 Hz, 2H; PyrH), 3.93
(s, 2H; CH2), 3.90 ppm (s, 4H; CH2); 13C NMR (62 MHz, CDCl3): d=
191.96, 159.11, 159.04, 154.58, 148.71, 139.95, 137.05, 136.44, 136.18,
132.39, 129.39, 129.07, 127.95, 122.60, 121.73, 121.65, 118.37, 59.88,
59.73 ppm; MS (ESI): m/z calcd for C25H22N4O: 394.2; found: 395.4
[M+H]+.
General remarks: Chemicals were purchased from Aldrich, Fluka or
Acros and used without further purification. CD spectra were recorded
with a Jasco J-715 instrument. IR spectra were recorded with a Perkin–
Elmer FTIR 1650. 1H and 13C{1H} NMR spectra (referenced to the sol-
vent residual peak) were recorded at 301 K with Bruker AC-300 or
250 MHz instruments. ESI-MS experiments were carried out in positive
mode with an Agilent Technologies LC/MSD Trap SL AGILENT instru-
ment (mobile phase methanol). HRMS (ESI-TOF) analyses were per-
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Enantiopurity and Absolute Configuration of Amino Acids
FULL PAPER
(6-Phenyl-2-pyridylmethyl)bis(2-pyridylmethyl)amine (1b): The reaction
was conducted by following the procedure described for 1a, but using
phenylboronic acid instead of 3-formyl-phenyl boronic acid. The final
yield was 97%. 1H NMR, 13C NMR and ESI+MS data corresponded to
those reported in the literature.[19]
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imum amount of anhydrous acetonitrile and ZnACHTNUGTRNEG(UN ClO4)2·6H2O (1 equiv)
was added. The solution was allowed to stand at RT for 5 min and then
the solvent was evaporated under reduced pressure, giving the desired
products as a yellow solid in 80% yield. Caution! Perchlorate salts of
metal complexes with organic ligands are potentially explosive. They
should be handled in small quantities and with caution. 1H NMR
(200 MHz, CD3CN): d=4.33 (m, 6H; CH2), 7.62 (m, 5H; PyrH+ArH),
7.81 (m, 4H; PyrH+ArH), 8.17 (m, 4H; PyrH+ArH), 8.45 (d, J=
6.0 Hz, 2H; PyrH), 10.03 ppm (s, 1H; CHO); 13C NMR (62 MHz,
CD3CN): d=57.23, 57.45, 124.75, 126.01, 126.51, 127.06, 129.60, 131.40,
133.11, 135.49, 140.62, 142.87, 142.96, 149.31, 155.77, 157.55, 159.88,
192.79 ppm; MS (ESI): m/z calcd for C25H22N4OZn: 458.11; found: 229.1
[M2+/2]; MALDI-TOF: m/z calcd for C25H22N4OZn
ACHTNUTRGENNG(U ClO4): 557.0565;
found: 556.8926.
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General procedure for CD measurements: For amino acids that were
soluble in methanol, a solution of 2a in methanol (0.0075m) and a solu-
tion of amino acid in methanol (0.01m) were mixed in ratio 1:1 (v/v) and
kept overnight at 508C in presence of molecular sieves. For amino acids
that were moderately soluble in methanol, a solution of 2a in methanol
(0.0075m) and a solution of amino acid in methanol (0.001m) were mixed
in ratio 0.1:1 (v/v) and kept overnight at 508C in the presence of molecu-
lar sieves. For all other amino acids, a solution of 2a in methanol
(0.0075m) and amino acid (1.25 equiv) were kept overnight at 508C in
the presence of molecular sieves. The compounds were used without fur-
ther purification. For CD measurements, suitable dilutions were per-
formed, to obtain a final concentration in the cuvette of imine equal to
0.75·10À4 m.
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Acknowledgements
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L. Dovico is acknowledged for initial experiments in the project. Prof. L.
Di Bari (Pisa) is acknowledged for useful discussions. The research was
funded by the Universitꢀ di Padova (PRAT CPDA099121 and
CPDA123307, Assegni di Ricerca 2010 CPDR103930/10 FAS), Progetto
Attrezzature Scientifiche finalizzate alla Ricerca 2010 (MALDI TOF/
TOF), MIUR (PRIN-2010-11 2010CX2TLM_002), and Fondazione
CARIPARO (NANO-Mode).
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Received: July 12, 2013
Published online: October 31, 2013
Chem. Eur. J. 2013, 19, 16809 – 16813
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