Determination of enantiomeric excess and concentration of unprotected amino acids, amines, amino alcohols, and carboxylic acids by competitive binding assays with a chiral scandium complex

Article abstract of DOI:10.1021/ja0636486

A practical UV-vis sensing method for enantioselective microanalysis of unprotected amino acids, amines, amino alcohols, and carboxylic acids in aqueous solution has been developed. Both concentration and enantiomeric composition of a wide range of chiral

Full text of DOI:10.1021/ja0636486

Published on Web 09/14/2006
Determination of Enantiomeric Excess and Concentration of Unprotected
Amino Acids, Amines, Amino Alcohols, and Carboxylic Acids by Competitive
Binding Assays with a Chiral Scandium Complex
Xuefeng Mei and Christian Wolf*
Department of Chemistry, Georgetown UniVersity, Washington, D.C. 20057
Received May 24, 2006; E-mail: cw27@georgetown.edu
The ever-increasing demand for enantiopure chemicals has been
accompanied by significant progress in asymmetric synthesis and
combinatorial methods that can produce large numbers of chiral
compounds in a short time. Enantioselective analysis often remains
the bottleneck in these processes because it usually entails laborious
and time-consuming chromatographic techniques. The need for
charge transfer band at 410 nm. Titration of a water/acetonitrile
-
5
(1:1) solution containing 0.98 × 10 M of Sc[(+)-1]
2
with (R)-
and (S)-valine revealed that the ligand displacements proceed with
remarkable stereoselectivity. We observed that (S)-valine is more
effective in replacing N,N′-dioxide ligands from the metal center
2
than the (R)-enantiomer (Figure 1). However, when Sc[(-)-1] was
1
faster methods has shifted the focus to MS, UV and colorimetric
employed in the same UV titration experiment, ligand substitution
occurred more readily with (R)-valine, thus proving enantioselective
UV-vis sensing based on competitive ligand exchange as previ-
2
3
4
assays, fluorescence spectroscopy, IR thermography, circular
5
6
7
dichroism, NMR spectroscopy, capillary electrophoresis, and
biochemical assays.
8
2b-d
ously described by Anslyn.
We have recently introduced 1,8-diheteroarylnaphthalene-derived
fluorosensors for chiral recognition of carboxylic acids and N-
protected amino acids. A remaining drawback of this approach is
the limited sensitivity due to relatively low association constants
between these sensors and chiral hydrogen bond donating substrates.
Anslyn’s and Corradini’s groups have demonstrated the usefulness
of diaminocyclohexane- and â-cyclodextrin-derived copper(II)
The UV-vis sensing method is based on a two-step ligand
exchange process involving the formation of intermediate diaster-
eomeric complexes. Displacement of the first N,N′-dioxide ligand
from the metal center by (R)- or (S)-valine generates Sc[(+)-1(R)-
2] and Sc[(+)-1(S)-2]. The first step can be monitored by the initial
decrease of the charge transfer band at 410 nm and is described by
individual exchange constants K1S and K1R, respectively. Further
addition of the substrate then results in the replacement of the
second N,N′-dioxide ligand and formation of equienergetic valine-
derived scandium complexes and free ligand 1. The latter step
results in the disappearance of the charge transfer band and is
described by individual exchange constants K2S and K2R. The
9
complexes for enantioselective colorimetric and fluorescent sensing
of amino acids.2
b-d,10
Encouraged by Anslyn’s success with
indicator-displacement assays,2
b-d
we decided to employ N,N′-
dioxide 1 in competitive binding assays. Herein, we report that Sc-
N,N′-dioxide 1] can be used for accurate and highly reproducible
[
2
UV-vis quantification of both enantiomeric composition and total
amount of unprotected amino acids, amines, amino alcohols, and
carboxylic acids in aqueous solutions.
conversion of Sc[(+)-1] to the ternary Sc[(+)-1(R)-2] complex
2
upon addition of (R)-2 to a solution of the sensor in water/
acetonitrile (1:1) was detected by ESI-MS (Supporting Information).
The stepwise replacement of the N,N′-dioxide ligands from Sc-
Similar to thoroughly studied coordination complexes of pyridine
and acridine N-oxide,11 we rationalized that 1 should form chiral
metal complexes exhibiting characteristic UV charge transfer bands.
Stepwise replacement of N,N′-dioxide 1 from the metal center by
chiral substrates would then provide an entry to competitive binding
assays suitable for simple UV quantification of concentration and
enantiomeric excess of a range of compounds. We observed that
the UV absorption of 1 undergoes characteristic changes during
titration with Cu(II), Zn(II), In(III), Al(III), Sn(II), Sc(III), Gd(III),
and Yb(III) triflates. While absorbance decreases at 260-330 nm,
it increases at 240-270 nm and a new charge transfer absorption
band appears at 360-480 nm, the latter region being most strongly
enhanced in the presence of scandium(III). Further UV titration
experiments and Job plot analysis revealed formation of a 2:1
[(+)-1]
2
by (R)-valine can be described as follows:
K1R
Sc[(+)-1] + (R)-2 { }Sc[(+)-1(R)-2] + (+)-1 (1)
2
K2R
Sc[(+)-1(R)-2] + (R)-2 { }Sc[(R)-2] + (+)-1 (2)
2
We conducted competitive binding experiments with substrates
3-12 (Chart 1). Using Sc[(+)-1] as the sensor, we observed
2
stereoselective replacement of dextrorotatory 1 in all cases (see
Supporting Information). The titration results reveal two remarkable
2
features of enantioselective sensing with Sc[N,N′-dioxide 1] . First,
this sensor differentiates between the enantiomers of a range of
compounds and is suitable for UV analysis of unprotected amino
acids, amino alcohols, amines, and carboxylic acids in aqueous
solution. Second, minute amounts of the scandium complex are
sufficient for enantioselective sensing of micromolar concentrations
of chiral substrates.
The diastereomeric complexes Sc[(+)-1(S)-2] and Sc[(+)-1(R)-
2] have different thermodynamic stabilities and molar extinction
coefficients at 410 nm, whereas the final exchange products, Sc-
6
complex with Sc(III) having an association constant of 1.6 × 10
-2
M
(see Supporting Information).
We expected that titration of enantiopure Sc[N,N′-dioxide 1]
2
with chiral substrates would result in fast and reversible replacement
of the two diacridylnaphthalene N,N′-dioxide ligands.12 Substitution
of the first N,N′-dioxide ligand by either enantiomer of a chiral
substrate would generate diastereomeric complexes, while the
second exchange process would produce a scandium complex
devoid of ligand 1. Since Sc(III) complexes of amino alcohols,
amines, amino acids, and carboxylic acids did not show any UV
absorption above 350 nm, we anticipated that the ligand exchange
can easily be measured by the disappearance of the characteristic
2 2
[(S)-2] and Sc[(R)-2] , do not absorb at 410 nm and therefore do
not interfere with UV measurements. The exchange processes
described for (R)-2 in eqs 1 and 2 have been analyzed for all
substrates 2-12 as described by Connors based on the assumption
13326
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J. AM. CHEM. SOC. 2006, 128, 13326-13327
10.1021/ja0636486 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. UV absorbance of Sc[(+)-1]2 as a function of the enantiomeric
composition of valine 2 at three different concentrations (blue, calibration;
red, actual measurements).
2
solution. Using racemic and enantiopure Sc[N,N′-dioxide 1] in two
competitive binding assays, we observed superior sensitivity and a
wider application spectrum compared to that in previously reported
fluorescence assays. The intrinsic sensitivity and selectivity of this
new sensor simplifies quantitative analysis of minute sample
amounts. We believe that our method combines several attractive
features: it is suitable for microanalysis of a wide range of chiral
compounds; it allows determination of both concentration and
enantiomeric excess; it depends on two simple assays that provide
accurate results; it avoids substrate derivatization, and it utilizes
sensitive UV-vis spectroscopy, minimizing solvent waste.
Figure 1. Enantioselective UV sensing. Left: UV-vis spectra of Sc[(+)-
]2 obtained by titration with (R)-valine. Right: UV sensing of the
enantiomers of valine using Sc[(+)-1]2 and Sc[(-)-1]2. The sensor
concentration was 0.98 × 10 M in acetonitrile:H2O (1:1).
Chart 1. Substrates Tested
1
-
5
Acknowledgment. Funding from the National Science Founda-
tion (CAREER Award, CHE-0347368) and the Petroleum Research
Fund (PRF40897-G4) is gratefully acknowledged.
that Beer’s law is followed by all UV-active species (see Supporting
Supporting Information Available: Synthesis of 1 and details of
all UV and MS experiments. This material is available free of charge
via the Internet at http://pubs.acs.org.
Information).13 Accordingly, the first N,N′-dioxide ligand of Sc-
2
[(+)-1] is more readily replaced by the (S)-enantiomer of valine
because Sc[(+)-1(S)-2] is more stable than Sc[(+)-1(R)-2]. As a
result, ligand exchange occurs at lower concentrations of (S)-2 than
with (R)-2, which explains the enantioselective decrease of the
intensity of the charge transfer absorption at 410 nm shown in
Figure 1.
We then decided to develop a method that allows accurate
measurements of both the total amount and the enantiomeric excess
of chiral compounds. We found that this can be accomplished by
combination of two simple assays. First, the total concentration of
a chiral substrate is analyzed by UV-vis sensing with racemic Sc-
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