Synthesis, characterization, and application of chiral ionic liquids and their polymers in micellar electrokinetic chromatography

Article abstract of DOI:10.1021/ac060878u

Two amino acid-derived (leucinol and N-methylpyrrolidinol) chiral ionic liquids are synthesized and characterized in both monomeric and polymeric forms. Leucinol-based chiral cationic surfactant is a room-temperature ionic liquid, and pyrrolidinol-based chiral cationic surfactant melts at 30-35 °C to form an ionic liquid (IL). The monomeric and polymeric ILs are thoroughly characterized to determine critical micelle concentration, aggregation number, polarity, optical rotation, and partial specific volume. Herein, we present the first enantioseparation using chiral IL as a pseudostationary phase in capillary electrophoresis. Chiral separation of two acidic analytes, (±)-α- bromophenylacetic acid and (±)-2-(2-chlorophenoxy)propanoic acid (±)-(2-PPA) can be achieved with both monomers and polymers of undecenoxycarbonyl-L-pryrrolidinol bromide (L-UCPB) and undecenoxycarbonyl-L- leucinol bromide (L-UCLB) at 25 mM surfactant concentration using phosphate buffer at pH 7.50. The chiral recognition seems to be facilitated by the extent of interaction of the acidic analytes with the cationic head-group of chiral selectors. Polysodium N-undecenoxycarbonyl-L-leucine sulfate (poly-L-SUCLS) and polysodium N-undecenoxycarbonyl-L-leucinate (poly-L-SUCL) were compared at high and low pH for the enantioseparation of (±)-(2-PPA). AtpH 7.5, poly-L-SUCLS, poly-L-SUCL, and (±)-(2-PPA) are negatively charged resulting in no enantioseparation. However, chiral separation was observed for (±)-(2-PPA) using poly-L-SUCLS at low pH (pH 2.00) at which the analyte is neutral. The comparison of chiral separation of anionic and cationic surfactants demonstrates that the electrostatic interaction between the acidic analyte and cationic micelle plays a profound role in enantioseparation.

Full text of DOI:10.1021/ac060878u

Anal. Chem. 2006, 78, 7061-7069
Synthesis, Characterization, and Application of
Chiral Ionic Liquids and Their Polymers in Micellar
Electrokinetic Chromatography
Syed Asad Ali Rizvi and Shahab A. Shamsi*
Department of Chemistry, Center of Biotechnology and Drug Design, Georgia State University, Atlanta, Georgia 30303
to produce different pharmacological effects. Presently, a majority
of commercially available drugs are synthetic and chiral. Most of
these chiral drugs are obtained as a mixture of two enantiomers
during synthesis.4 To avoid the possible undesirable effects of
enantiomeric impurity in chiral drug, it is inevitable that only a
therapeutically active form be marketed. Hence, there is a
continuous need to develop technologies that have the ability to
separate enantiomers.
Very recently, ionic liquids (ILs) have found great applications
in efficient and environmentally benign chemical processing and
chemical analysis.5,6 By definition, the ILs are organic salts with
melting points (mp) below 100 °C or more often even lower than
room temperature.7-11 These compounds possess the dual capabil-
ity of dissolving both polar and nonpolar species, and the most
useful feature is that they do not evaporate even at high
temperatures.12-15 Most commonly, ILs are based on nitrogen-
rich, alkyl-substituted heterocyclic cations, with a variety of anions
(e.g., 1-ethyl-3-methylimidazolium tetrafluoroborate). Although, the
reasons for low melting points of ILs are not clear, it is stated
that ILs consist of bulky inorganic anions with delocalized charged
organic cations, which prevents the formation of a stable crystal
lattice or random molecular packing resulting in lower melting
points.16 Due to these remarkable characteristics, ILs have been
used as medium for liquid-liquid extractions,17-19 mobile-phase
Two amino acid-derived (leucinol and N-methylpyrroli-
dinol) chiral ionic liquids are synthesized and character-
ized in both monomeric and polymeric forms. Leucinol-
based chiral cationic surfactant is a room-temperature
ionic liquid, and pyrrolidinol-based chiral cationic sur-
factant melts at 30-35 °C to form an ionic liquid (IL).
The monomeric and polymeric ILs are thoroughly char-
acterized to determine critical micelle concentration,
aggregation number, polarity, optical rotation, and partial
specific volume. Herein, we present the first enantiosepa-
ration using chiral IL as a pseudostationary phase in
capillary electrophoresis. Chiral separation of two acidic
analytes, (()-r-bromophenylacetic acid and (()-2-(2-
chlorophenoxy)propanoic acid (()-(2-PPA) can be achieved
with both monomers and polymers of undecenoxycarbo-
nyl-
carbonyl-
L
-pryrrolidinol bromide (
L
-UCPB) and undecenoxy-
L
-leucinol bromide (
L-UCLB) at 25 mM surfac-
tant concentration using phosphate buffer at pH 7.50. The
chiral recognition seems to be facilitated by the extent of
interaction of the acidic analytes with the cationic head-
group of chiral selectors. Polysodium N-undecenoxycar-
bonyl-
N-undecenoxycarbonyl-
pared at high and low pH for the enantioseparation of (()-
(2-PPA). At pH 7.5, poly- -SUCLS, poly- -SUCL, and (()-
L
-leucine sulfate (poly-
L-SUCLS) and polysodium
L
-leucinate (poly-
L-SUCL) were com-
L
L
(2-PPA) are negatively charged resulting in no enantio-
separation. However, chiral separation was observed for
(()-(2-PPA) using poly-L-SUCLS at low pH (pH 2.00) at
(3) Williams, D. A.; Lamke, T. L.; Foye, W. O. Foye’s Principles of Medicinal
Chemistry; Lippincott Williams & Wilkins: Philadelphia, 2002.
(4) Silverman, R. B. The Organic Chemistry of Drug Design and Drug Action;
Academic Press: San Diego, 2004.
which the analyte is neutral. The comparison of chiral
separation of anionic and cationic surfactants demon-
strates that the electrostatic interaction between the acidic
analyte and cationic micelle plays a profound role in
enantioseparation.
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The separation of chiral compounds is currently the center of
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* Corresponding author. Phone: 404-651-1297. Fax: 404-651-2751. E-mail:
chesas@langate.gsu.edu.
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10.1021/ac060878u CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/07/2006
Analytical Chemistry, Vol. 78, No. 19, October 1, 2006 7061

additives in high-performance liquid chromatography (HPLC),20,21
electrolytes in capillary electrophoresis (CE),22-26 matrixes for
matrix assisted laser desorption ionization time-of-flight mass
spectrometry (MALDI-TOF MS),27,28 stationary phases for gas
chromatography29-32 and as modifiers in micellar electrokinetic
chromatography (MEKC).33,34 However, there is no report in the
literature about the use of ILs as chiral selector in CE.
Cationic surfactants are referred to as compounds containing
at least one long hydrophobic chain attached to a positively
charged nitrogen. These quaternary ammonium group-containing
surfactants are well known for displaying emulsifying properties,
antimicrobial activity, components in cosmetic formulations,
anticorrosive effects, and phase-transfer catalyst and as a chiral
induction medium (if chiral cationic surfactant) in organic
reactions.35-41 As with the case of chiral anionic surfactants, amino
acid-based (both monomeric and polymeric) and ephedrine-based
(monomeric) chiral cationic surfactants have been used as chiral
selectors in MEKC.42,43 However, unlike chiral anionic polymeric
surfactants, chiral cationic polymeric surfactants have not found
great application so far, and only one report of chiral cationic
polymeric surfactants as pseudostationary phase in MEKC is
reported.42
employed chiral anionic surfactants at basic pH. As a result, still
a large number of acidic analytes could not be resolved by MEKC.
The cationic surfactant undecenoxycarbonyl-
UCLB) is an ionic liquid at room temperature, while undecenoxy-
carbonyl- -pyrrolidinol bromide ( -UCPB) is a greasy solid that
L-leucinol bromide (L-
L
L
melts to form an ionic liquid at 30-35 °C. In our case, quaternized
nitrogen (chiral headgroup) is surrounded by a hydrophobic tail
and leucinol or pyrrolidinol side chain, which presumably prevent
the proper packing of the cations and anions in regular three-
dimensional patterns to form ionic liquids.
The current report is the first demonstration of MEKC chiral
separation of several anionic compounds such as phenoxypropi-
onic acid herbicide, (()-(2-PPA), and a very useful synthetic
intermediate (-R-bromophenylacetic acid, (()-(R-BP-AA),44,45 us-
ing two synthetic chiral ionic liquids, L-UCLB and L-UCPB, as well
their polymers. Chiral separation of acidic analyte is compared
using polymeric anionic surfactants containing similar headgroups
under both acidic and basic pH conditions.
MATERIALS AND METHODS
Standards and Chemicals. The analytes (()-(R-BP-AA) and
(()-(2-PPA) were obtained as a racemic mixture from Sigma
Chemical Co. (St. Louis, MO) and Aldrich (Milwaukee, WI),
respectively. Chemicals used for the synthesis of surfactants
included ω-undecylenyl alcohol, triphosgene, pyridine, dichlo-
In this study, we report the synthesis, characterization, and
application of novel IL-type surfactants and their polymers for
chiral separation of acidic analytes in MEKC. Acidic analytes due
to inherent negative charge poorly interact with most commonly
romethane, 2-bromoethylamine hydrobromide, L-leucinol, N-me-
thylpryrrolidinol, 96% formic acid, 37% formaldehyde, and 2-pro-
panol (HPLC grade), were also obtained from Aldrich, and were
used as received.
(19) Carmichael, A. J.; Earle, M. J.; Holbrey, J. D.; McCormac, P. B.; Seddon,
K. R. Org. Lett. 1999, 1, 997-1000.
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Chromatogr. 1986, 352, 407-425.
Synthesis and Characterization of Monomeric Surfactants
and Micelle Polymers. Choloroformate has been synthesized
as reported earlier46 by reacting triphosgene with unsaturated
alcohol (step 1, Figure 1). The carbamate-functionalized alkenyl
bromide (step 2, Figure 1) was synthesized by dropwise addition
of (10 mmol) choloroformate over an equimolar aqueous solution
of 2-bromoethylamine hydrobromide and Na2CO3 and the resultant
mixture was stirred for 2 h. The resulting solution was extracted
twice with dichloromethane, which then was washed three times
with H2O, dried over Na2SO4, and concentrated by evaporating
solvent to yield product 1 (89-93%). The N,N-dimethylleucinol
(product 2, step B, Figure 1) was synthesized by reductive
alkylation of primary amine of leucinol using the well-known
Eschweiler-Clark reaction (yield 55-70%).47-49 The chiral ionic
liquids were synthesized by refluxing the carbamate-functionalized
alkenyl bromide (product 1) with N,N-dimethylleucinol or N-
methylpryrrolidinol for 48 h in 2-propanol (IPA). After 48 h, the
reaction mixture was concentrated by evaporating IPA, and the
resulting fluid was dissolved in water and extracted with ethyl
acetate. The aqueous solution of ionic liquids (products 3 and 4,
Figure 2) was lyophilized (yield 40-55%) at -50 °C collector
temperature and 0.05 mbar pressure for 14 days (to ensure
(21) Poole, S. K.; Shetty, P. H.; Poole, C. F. Anal. Chim. Acta 1989, 218, 241-
264.
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766-770.
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640, 463-471.
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(25) Yanes, E. G.: Gratz, S. R.: Stalcup, A. M. Analyst 2000, 125, 1919-1923.
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Anal. Chem. 2001, 73, 3838-3844.
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3679-3686.
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Spectrom. 2003, 17, 553-560.
(29) Armstrong, D. W.; He, L.; Liu, Y. S. Anal. Chem. 1999, 71, 3873-3876.
(30) Berthod, A.; He, L.; Armstrong, D. W. Chromatographia 2001, 53, 63-68.
(31) Anderson, J. L.; Armstrong, D. W. Anal. Chem. 2003, 75, 4851-4858.
(32) Ding, J.; Welton, T.; Armstrong, D. W. Anal. Chem. 2004, 76, 6819-6822.
(33) Mwonngela, S. M.; Numan, A.; Gill, N.; Agbaria, R. A.; Warner, I. M. Anal.
Chem. 2003, 75, 6089-6096.
(34) Laamanen, P. L.; Lahtinen, S. B. M.; Matilainen, R. J. Chromatogr., A 2005,
1095, 164-171.
(35) Cross, J., Singer, E. J., Eds. Cationic Surfactants; Marcel Dekker Inc.: New
York, 1994.
(36) Dwars, T.; Paetzold, E.; Oehme, G. Angew. Chem., Int. Ed. 2005, 44, 7174-
7199.
(37) Goldberg, S. I.; Baba, N.; Green, R. L. J. Am. Chem. Soc. 1978, 100, 6768-
6769.
(38) Borocci, S.; Ceccacci, F.; Galantini, L.; Mancini, G.; Monti, D.; Scipioni, A.;
Venanzi, M. Chirality. 2003, 15, 441-447.
(39) Davidson, T. A.; Mondal, K.; Yang, X. J. Colloid Interface Sci. 2004, 276,
468-502.
(40) Diego-Castro, M. J.; Hailes, H. C.; Lawrence, M. J. J. Colloid Interface Sci.
2001, 234, 122-126.
(41) Diego-Castro, M. J.; Hailes, H. C. J. Chem. Soc., Chem. Commun. 1998,
15, 1549-1550.
(42) Dobashi, A.; Hamada, M.; Yamaguchi, J. Electrophoresis 2001, 22, 88-96.
(43) Dey, J.; Mohanty, A.; Roy, S.; Khatua, D. J. Chromatogr., A 2004, 1048,
127-132.
(44) Hamdouchi, C.; Martinez, C. S.; Gruber, J.; Prado, M. D.; Lopez, J.; Rubio,
A.; Heinz, B. A. J. Med. Chem. 2003, 46 (20), 4333-4341.
(45) Procopio, A.; Alcaro, S.; Cundari, S.; De Nino, A.; Ortuso, F.; Sacchetta, P.;
Pennelli, A.; Sindona, G. J. Med. Chem. 2005, 48 (19), 6084-6089.
(46) Rizvi, S. A. A.; Shamsi, S. A. Electrophoresis 2003, 24, 2514-2526.
(47) Eschweiler, W. Ber. Dtsch. Chem. Ges. 1905, 38, 880.
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7062 Analytical Chemistry, Vol. 78, No. 19, October 1, 2006

Figure 1. Synthesis of the carbamate-functionalized (A) alkyl
bromide and (B) N,N-dimethyl-L-leucinol.
complete removal of water from both products). 1H NMR spectra
of L-UCPB, L-UCLB and their polymers were recorded on a Varian
Unity+ 300-MHz spectrometer using D2O as the solvent. The
surfactants were characterized and checked for purity by MALDI-
1
TOF MS (Figure 3, H NMR, and elemental analysis.
L
-UCPB.
1H NMR: δ 0.759-0.893 (b, 6H), 1.170 (m, 12H), 1.471 (m, 2H),
1.767 (m, 2H), 1.883 (b, 1H), 2.085 (m, 2H), 3.06 (b, 2H), 3.239-
3.613 (b, 8H), 3.777-3.844 (m, 1H), 4.052 (d, J ) 14.7, 2H), 4.379
(b, 2H), 4.789 (m, 2H), 5.626 (m, 1H). Anal. Calcd for C20H39N2O3-
Br + 2H2O: C, 50.95; H, 9.19; N, 5.94; Found: C, 51.56; H, 10.07;
Figure 2. Synthesis and polymerization of leucinol- and pryrrolidinol-
derived ionic liquid and their polymers.
specific volume of both monomer and polymer was determined
using previously reported procedure.46 The optical rotation of
monomeric and the polymeric surfactants was obtained by an
Autopol III automatic polarimeter (Rudolph Research Analytical,
Flanders, NJ) by measuring the optical rotation at 589 nm of a 10
mg/mL solution of each monomer and polymer in triply deionized
water at 25 °C.
MEKC Instrumentation. All experiments were performed
using an Agilent CE system (Agilent Technologies, Palo Alto, CA)
equipped with 0-30-kV high-voltage power supply, a diode array
detector for UV detection, and Chemstation software (V 9.0) for
system control and data acquisition. The fused-silica capillary was
obtained from Polymicro Technologies (Phoenix, AZ). The total
length of the capillary used with an Agilent CE system was 64.5
cm (56.0 cm from inlet to detector, 50-µm i.d., 350-µm o.d.),
prepared by burning ∼3-mm polyimide coating to create a
detection window.
Capillary Electrophoresis Procedures and Calculations.
The capillaries for all MEKC experiments were prepared by
flushing with 1 M NH4OH for 60 min at 50 °C followed by a 30-
min rinse with triply deionized water at 20 °C. Between each
injection, the capillary was flushed with 0.1 M NH4OH and H2O
for 3 min each. Separations began after a 2-min rinse with the
running buffer, followed by a 5-min flush with the running buffer
Analytical Chemistry, Vol. 78, No. 19, October 1, 2006 7063
N, 5.88. L
-UCLB. 1H NMR: δ 1.170 (b, 12H), 1.468 (b, 2H), 1.766-
1.992 (b, 4H), 1.992-2.164 (b, 2H), 3.032 (b, 2H), 3.147 (b, 2H),
3.472 (b, 3H), 3.506-3.619 (b, 3H), 3.830 (b, 2H), 4.376 (b, 2H),
4.804 (m, 2H), 5.658 (m, 1H). Anal. Calcd for C22H45N2O3Br +
2H2O: C, 52.68; H, 9.85; N, 5.59; Found: C, 52.14; H, 9.00; N,
6.87.
The critical micelle concentration (cmc) was determined using
a Sigma 703 digital tensiometer (KVS Instruments USA, Monroe,
CT), by the Du NoU¨ y ring method at room temperature. Complete
olymerization of the synthesized ionic liquids was achieved by
continuous 60Co γ-irradiation (8 Mrad/h) of a 100 mM aqueous
1
solution for 30 h. The H NMR indicated the disappearance of
double bond protons signal in the region of 4.8-5.0 and 5.7-5.9
ppm. After irradiation, the polymeric surfactant solutions were
filtered and dialyzed against triply deionized water using a
regenerated cellulose dialysis membrane (Spectrum Laboratories,
Inc., Rancho Dominguez, CA) with a 1000-Da molecular mass
cutoff for 24 h. Finally, the dialyzed solutions were lyophilized to
obtain the dried polymeric surfactants. Further characterization,
such as aggregation number and polarity of the amphiphilic ionic
liquids (monomers and polymers), was determined by using the
pyrene emission vibronic fine structure method.46 The partial

Figure 3. MALDI-TOF mass spectra (positive mode) with proposed cleavages and the corresponding fragment masses for IL-type surfactants
(A) undecenoxycarbonyl-L-leucinol bromide (L-UCLB) and (B) undecenoxycarbonyl-L- pryrrolidinol bromide (L-UCPB) after freeze-drying on a
lyophilizer at -50 °C collector temperature and 0.05 mbar pressure for 14 days. The mass spectra were obtained using R-cyano-4-hydroxycinnamic
acid as MALDI matrix.
containing ionic liquids. All separations were performed at -20
kV and at 20 °C. All surfactants (both monomers and polymers)
were run with a new capillary (cut to the same length from the
same capillary bundle) and were preconditioned using the identical
flushing procedure as mentioned above. Chiral resolution (Rs) of
acidic analytes (()-(2-PPA) and (()-(R-BP-AA) were calculated
by Chemstation software using the peak width at half-height
method:
(2.35/2)(tr - tr )
2
1
Rs )
W50(1) + W50(2)
7064 Analytical Chemistry, Vol. 78, No. 19, October 1, 2006

W50(1) and W50(2) are the widths at 50% height for peaks 1 and 2,
respectively. The selectivity (R) is defined as t2/t1, where t1 and
t2 are the migration times of the first- and second-eluting
enantiomers. Methanol was used as the t0 marker and was
measured from the time of injection to the first deviation from
the baseline. Dodecanophenone was used as tracer for tmc at 100
mM surfactant concentration of each monomer and polymer. The
effective electrophoretic mobility of the monomers and polymers
of ionic liquids was calculated by the following equation:
Table 1. Physicochemical Properties of the Monomers
and Polymers of Chiral Amino Acid-Derived Cationic
Surfactants L-UCLB and L-UCPB
characteristic of the
IL-type monomeric
surfactants
L-UCPB
L-UCLB
cmca (mM)
1.15 ( (0.01)g
95 ( (0.09)
0.84 ( (0.05)
97 ( 0.04
aggregation numberb
polarity (I1/I3) ratioc
optical rotationd
1.095 ( (0.001)
-2.35 ( (0.02)
0.8281 ( (0.0036)
-2.83 × 10-4
1.180 ( 0.040
+21.67 ( 0.03
0.7185 ( 0.00
-2.42 × 10-4
(( 5.31 × 10-6
1.94 × 10-4
partial specific volumee
electroosmotic mobility
µep ) -µapp - (-µeof
)
µeof (cm2 V-1 s-1
)
(( 1.56 × 10-5
)
)
)
f
effective electrophoretic
2.08 × 10-4
mobility
µep (cm2 V-1 s-1
)
(( 1.54 × 10-5
3.79
(( 0.20)
)
(( 7.49 × 10-7
f
where µep, µeof, and µapp are effective electrophoretic mobility,
electroosmotic mobility, and apparent electrophoretic mobility,
respectively. The negative sign of µapp and µeof is due to the fact
that monomeric and polymeric ionic liquids coat the capillary wall
and result in anodic electroosmotic flow; therefore, negative
voltage (-20 kV) has to be applied for separation.
Preparation of MEKC Buffers and Analyte Solutions. For
all MEKC experiments, the final background electrolyte (BGE)
consisted of 25 mM each of Na2HPO4/NaH2PO4 buffered at pH
7.5. The desired pH value was obtained by using 1 M NaOH. The
pH of BGE was adjusted before the addition of ILs (monomers
and polymers). This BGE solution is finally filtered through a 0.45-
µm Nalgene syringe filter (Rochester, NY). The running MEKC
buffer solution was prepared by addition of 25 mM IL-type
surfactants to the BGE, followed by ultrasonication for about 25-
30 min. The analytes prepared in 50/50 (v/v) of MeOH/H2O at
various concentrations were injected at a pressure of 50 mbar for
1-5 s. The dodecanophenone was prepared in 100% MeOH at 3
mg/mL (stock solution), diluted to 1.8 mg/mL in 60:40 MeOH/
H2O, and injected at a pressure of 50 mbar for 10 s.
migration time window
5.09
(tmc/to)f
(( 0.38)
characteristic of the
polymeric surfactants
poly-L-UCPB
poly-L-UCLB
aggregation numberb
polarity (I1/I3) ratioc
optical rotationd
34 ( (0.780)
25 ( (0.034)
1.219 ( (0.001)
-7.84 ( (0.04)
1.22 ( (0.007)
+17.45 ( (0.64)
partial specific volumee
0.8408 ( (0.0075)
0.7634 ( (0.0008)
electroosmotic mobility
-2.54 × 10-4
-2.34 × 10-4
f
µeof (cm2 V-1 s-1
)
(( 3.67 × 10-6
)
)
(( 3.12 × 10-6
)
effective electrophoretic
2.02 × 10-4
1.91 × 10-4
mobility
µep (cm2 V-1 s-1
)
(( 2.96 × 10-6
4.87
(( 0.16)
(( 3.52 × 10-6)*
5.38
(( 0.53)
f
migration time window
(tmc/to)f
a Critical micelle concentration is determined by the surface tension
measurements. b Aggregation number is determined by the florescence
quenching experiment using pyrene as a probe and cetylpyridinium
chloride as a quencher. c Polarities of the surfactants are determined
using ratio of the fluorescence intensity (I1/I3) of pyrene. d Optical
rotations of 10 mg/mL monomer and micelle polymers were deter-
mined in triply deionized water snd were obtained at 589 nm (sodium
D line). e Partial specific volumes were determined by the density
measurements at different surfactant concentrations. f The µep values
for all monomer and polymeric ionic liquids were determined using
methanol as to marker and dodecanophenone as tmc tracer. Experi-
mental conditions: 64.5 cm (56-cm effective length) × 50 µm i.d.
capillary with an applied voltage of -20 kV at 25 °C using a running
buffer of 25 mM each of NaH2PO4 and Na2HPO4, 100 mM monomer
and polymeric ionic liquids; dodecanophenone introduction, 50 mbar
for 10 s (1.5 mg/mL in 50:50 MeOH/H2O). g Standard deviations are
given in parentheses.
RESULTS AND DISCUSSION
Physicochemical Properties. Table 1 represents the phys-
icochemical properties of the synthesized enantiomerically pure
chiral surfactants
-UCPB (IL) (mp 30-35 °C) as well as their polymers, poly-
UCLB and poly- -UCPB. The -UCLB exhibited higher polarity,
lower cmc, partial specific volume (Vh), significantly higher optical
rotation, but similar aggregation number (A) compared to -UCPB.
A similar trend was also observed for the poly- -UCLB and poly-
-UCPB, except that the A value was higher for the former
L-UCLB (room-temperature ionic liquid) and
L
L-
L
L
L
cations (contain quaternary nitrogen of the cationic surfac-
tants).50,51
L
L
The electrophoretic parameters of monomeric and polymeric
ionic liquids were also examined (Table 1) at 100 mM surfactant
concentrations (at lower surfactant concentration, tmc marker was
not observed even after 3 h). The reversed electroosmotic flow
(-µeof) and effective electrophoretic (µep) mobilities of both poly-
polymer. Comparing physicochemical properties of monomeric
and polymeric cationic surfactants, it can be noticed that A is
always lower for the polymers than monomers, while polarity and
Vh is always higher for polymeric surfactants.
Figure 3 shows the MALDI-TOF MS of both -UCLB (A) and
L
L
-UCPB and poly-
time window (tmc/t0) was greater compared to their respective
monomers. In addition, the monomer and polymer of -UCLB
-UCPB have lower
L-UCLB were slightly lower, while the migration
L
-UCPB (B) in positive mode. Both -UCLB and L-UCPB surfac-
L
tants showed the molecular ion peak (base peak) at mass-to-charge
ratios (m/z) of 385.3 and 355.3, respectively along with a fragment
L
compared to the monomer and polymer of
L
generated by the loss C5H11O. For
L-UCLB, the masses at 386.2
µep and provided larger tmc/t0.
Enantioseparation of Acidic Analytes. The optimization of
chiral resolution of (()-(R-BP-AA) and (()-(2-PPA) was performed
and 299.2 m/z, and for -UCPB, the masses at 356.3 and 268.2
L
m/z, are generated due to the 13C isotope related to the molecular
ion and the generated fragment ion, respectively. The generation
of the cationic fragments (Z2) for both ionic liquids as shown in
Figure 3 is in accord with the previous observations that most of
the fragments generated from cationic surfactants bear preformed
(50) Morrow, A. P.; Kassim, O. O.; Ayorinde, F. O. Rapid Commun. Mass
Spectrom. 2001, 15, 767-770.
(51) Tuiman, A. A.; Cook, K. D.; Magid, L. J. J. Am. Soc. Mass Spectrom. 1990,
1, 85-91.
Analytical Chemistry, Vol. 78, No. 19, October 1, 2006 7065

Figure 4. Comparison of 25 mM L-UCPB (A), poly-L-UCPB (B), 25 mM L-UCLB (C), and poly-L-UCLB (D) for enantioseparation of (()-(R-
BP-AA) (2.5 mg/mL in MeOH/H2O). MEKC conditions: 50 mM NaH2PO4/Na2HPO4, pH 7.5, pressure injection 50 mbar 5 s, -20 kV, 20 °C, and
UV detection at 214 nm.
by studying pH of the BGE, type and concentration of BGE,
organic modifiers, and surfactant concentration. After optimizing
the chiral MEKC conditions, chiral separation of (()-(R-BP-AA)
Although chiral resolution (Rs) at a lower pH range (4.00-6.00)
do not differ drastically, maximum Rs was obtained at pH 7.5, but
no Rs at pH 2.00, and at pH > 7.5, Rs deteriorates (data not shown).
One plausible explanation of Rs deterioration at a pH of >7.5 could
be the excess hydroxide ions (originated from the use of NaOH
to adjust the BGE pH), which competes with the anionic chiral
analyte for the positively charged ionic liquid at basic pH. The
absence of any Rs at pH 2.00 and lower Rs at intermediate pH
suggest that electrostatic interaction indeed contributes signifi-
cantly to chiral recognition. It has been reported in the literature
that, in the presence of certain organized media (e.g., micelles),
the pKa of the organic acid is altered up to more than 4 pH
units.43,52 Therefore, it is reasonable to believe that the amphiphilic
and (()-(2-PPA) were compared using
L-UCPB,
L-UCLB, and their
respective polymers (poly- -UCPB and poly-
L
L-UCLB) to get insight
on the factors affecting analyte-micelle interactions and ultimately
chiral separation.
Enantioseparation of (()-r-Bromophenylacetic Acid. Pan-
els A and C in Figure 4 show the chiral separation of (()-(R-BP-
AA) at optimum separation conditions with L-UCPB and L-UCLB,
respectively. Since (()-(R-BP-AA) has a dissociable carboxylic acid
group with pKa ) 2.40 ((0.10), the effect of pH on enantiosepa-
ration was evaluated from pH 2.00 to 8.50 (data not shown).
7066 Analytical Chemistry, Vol. 78, No. 19, October 1, 2006

Figure 5. Comparison of 25 mM L-UCPB (A), poly-L-UCPB (B), 25 mM L-UCLB (C), and poly-L-UCLB (D) for enantioseparation of (()-(2-PPA)
(0.5 mg/mL in MeOH/H2O). MEKC conditions are the same as in Figure 4.
ionic liquids might have increased the pKa of (()-(R-BP-AA) such
that maximum ionization occurs around pH 7.50. Hence, greater
electrostatic interaction with the positively charged ionic liquids
provided maximum chiral Rs at pH 7.50. On the other hand,
L-UCPB, which apparently allows maximum interaction via three-
point interaction with (()-R-Br-Ph-AA.53 The Rs trend is consistent
with the findings of Thiobodeaux et al.,54 who observed that
surfactants derived from L-proline (a rigid amino acid) provided
L
-UCPB and L-UCLB concentrations, as well as the use of organic
better chiral separation for rigid chiral molecules (e.g., BNP). The
analyte (()-(R-BP-AA) has a chiral center, which is adjacent to a
bromo group and a carboxylate group. Thus, it appears that the
chiral recognition was greatly facilitated not only by electrostatic
interactions between carboxylate group of the analyte and the
positively charge nitrogen but the presence of a bromo group
modifiers (e.g., methanol, acetonitrile) did not show any significant
variations in Rs.
As depicted,
for (()-(R-BP-AA) compared to
possible explanation for enhanced chiral resolution provided by
-UCPB over -UCLB could be due to the rigid ring system of
L
-UCPB provided almost twice as high chiral Rs
L
-UCLB (Figure 4A vs C). One
L
L
(53) Davankov, V. A. Pure Appl. Chem. 1997, 69, 1469-1474.
(54) Thibodeaux, S. J.; Billiot, E.; Warner, I. M. J. Chromatogr., A 2002, 966,
179-186.
(52) Bertschinger, A. T.; Perry, C. S.; Galland, A.; Prannkerd, R. J.; Charman,
W. N. J. Pharm. Sci. 2003, 92, 2217-2228.
Analytical Chemistry, Vol. 78, No. 19, October 1, 2006 7067

Figure 6. Comparison of 25 mM poly-L-SUCLS (A), 25 mM poly-L-SUCL (B), and 50 mM poly-L-SUCLS (C) for enantioseparation of chiral
(()-(2-PPA) (0.5 mg/mL in MeOH/H2O). MEKC conditions (A, B): pH 8.00, 25 mM NH4OAc/25 mM TEA, 15 °C, pressure injection 50 mbar s,
+20 kV applied for separations, and UV detection at 200 nm. (C) MEKC conditions same as Figure 5A except pH 2.00, 25 mM NaH2PO4/25 mM
CH3COONa, and -20 kV applied for separations.
adjacent to the chiral center might also hydrogen bond with the
OH group of the ionic liquids (Figure 2).
thought to be the key factors ensuring maximum enantioselec-
tivity. However, in case of (()-(2-PPA), the chloro group on the
benzene ring is farther away from the chiral center. Furthermore,
Comparing the electropherograms in Figure 4A versus (C) and
(B) versus (D), it is obvious that monomers of both
L
-UCPB and
the nonrigidity of
hydrogen-bonding interactions between the chloro group on the
benzene ring and the primary alcohol of the -leucinol. Comparing
Figure 5 panels (A) versus (B) and (C) versus (D), it is clear that
monomers and polymers of -UCPB and -UCLB show very similar
stereoselective interactions with (()-(2-PPA) as evident from the
Rs and R values.
It is interesting to compare the enantioseparation capability
between two polymeric chiral anionic surfactants [polysodium
L-UCLB might have resulted in favorable
L-UCLB provided better chiral resolution, selectivity, and efficiency
compared to the corresponding polymers. The probable reason
behind this observation could be the polydispersity of the
polymers,55 which usually is the case when surfactants are
polymerized at a concentration higher than the cmc.55,56
Enantioseparation of (()-2-(2-Chlorophenoxy)propanoic
Acid. As discussed above, in the case of (()-(R-BP-AA), maximum
chiral Rs was obtained at pH 7.50 and no Rs at pH 2.00. O’Keeffe
et al.57 and Haynes et al.58 have reported the separation of (()-
(2-PPA) at pH 5.00-6.00 with a cationic substituted â-cyclodextrin.
Similar to the case of (()-(R-BP-AA) separation, the variation in
surfactant concentration and addition of organic modifier showed
no significant effects on chiral Rs of (()-(2-PPA).
L
L
L
N-undecenoxycarbonyl-L-leucine sulfate (poly-L-SUCLS) and poly-
sodium N-undecenoxycarbonyl-L-leucinate (poly-L-SUCL)] with the
chiral cationic surfactants discussed earlier for racemic anionic
analyte. The chiral separation of (()-(2-PPA) with both sulfated
and carboxylated headgroup polymeric surfactants was investi-
gated at basic pH (Figure 6A and B). As we have mentioned
earlier, anionic compounds are usually difficult to separate with
anionic surfactant due to the electrostatic repulsion between
similar charges. Hence, as expected, no chiral resolution was
Panels A and C in Figure 5 show the chiral separation of (()-
(2-PPA) at optimum MEKC parameters using -UCPB and
L
L-
UCLB, respectively. The nonrigid leucine-based (L-L-UCLB) chiral
selector (Figure 5C) provided significantly higher chiral Rs of (()-
(2-PPA) than -UCPB. This resolution trend is opposite to the
L
obtained for (()-(2-PPA) at pH 8.00. Since poly-L-SUCLS has a
separation of (()-(R-BP-AA) (Figure 4A, C). As stated, the
proximity of the bromo and carboxylate group to the chiral center
of (()-(R-BP-AA) as well as the rigidity of the chiral selector was
sulfated headgroup, it can be used at any pH without any solubility
problem. Therefore, we performed MEKC at pH 2.00 (Figure 6C)
in order to minimize dissociation of the carboxylic acid group of
(()-(2-PPA) (pKa 3.11 ( 0.10). As can be seen in Figure 6C, partial
chiral separation of (()-(2-PPA) was achieved at pH 2.00. However,
we could not improve this chiral Rs any further even after fine-
tuning of the MEKC parameters (data not shown). Hence,
(55) Tarus, J.; Agbaria, R. A.; Morris, K.; Mwongela, S.; Numan, A.; Simuli, L.;
Fletcher, K. A.; Warner, I. M. Langmuir 2004, 20, 6887-6895.
(56) Mileva, E. J. Colloid Interface Sci. 2000, 232, 211-218.
(57) O’Keeffe, F.; Shamsi, S. A.; Darcy, R.; Schwinte, P.; Warner, I. M. Anal.
Chem. 1997, 69, 4773-4782.
comparing the chiral separation of (()-(2-PPA) with poly-
L-UCLB
(58) Hanes, J. L., III; Shamsi, S. A.; O’Keeffe, F.; Darcy, R.; Warner, I. M, III J.
Chromatogr., A 1998, 803, 261-271.
(Figure 5D), poly- -SUCLS (Figure 6A, C), and poly-
L
L-SUCL
7068 Analytical Chemistry, Vol. 78, No. 19, October 1, 2006

(Figure 6B), it is clear that indeed electrostatic attraction interac-
tion plays a dominant role in chiral recognition. In addition,
structural features (e.g., rigidity and charges) of both analyte and
chiral selector also seem to affect the chiral recognition.
strate the enantioseparation of a large number of acidic analytes,
we still believe that our findings will guide the future research in
MEKC separation of acidic analytes with intelligently designed
synthetic chiral ionic liquids.
CONCLUSIONS
ACKNOWLEDGMENT
This paper is the first demonstration of successful chiral
This work was supported by a grant from the National
Institutes of Health (Grant GM 62314).
separation of acidic analytes with synthetic chiral ILs in CE. Both
L-UCLB and L-UCPB ionic liquid-type surfactants were thoroughly
characterized before and after the polymerization. It was found
that chiral separation of the acidic analytes with the chiral ILs
and their polymers is strongly dependent on the presence of
opposite charge as well as the structural compatibility between
chiral selector and the analyte. Even though we did not demon-
Received for review May 12, 2006. Accepted August 16,
2006.
AC060878U
Analytical Chemistry, Vol. 78, No. 19, October 1, 2006 7069