hance chiral separation with b-cyclodextrin (CD).[27] Conclu-
sively, ILs provide a new approach for the optimization of
HPLC and CE separations.
Results
Current research interest in ILs includes the design of ILs
with controllable chemical and physical properties or even
specific functions (i.e., so-called task-specific ILs).[28,29] One
interesting example is the design and synthesis of chiral ILs
and their use as chiral solvents for optical resolution and
asymmetric synthesis.[30–34] Chiral ILs have also been synthe-
sized and used as chiral selectors in CE.[35–37] Unfortunately,
we have found no reports on the application of chiral ILs in
HPLC. On the other hand, despite their potential, chiral ILs
are not commercially available and their synthesis often re-
quires expensive reagents and complicated synthetic
schemes.[30–33] These limitations severely hinder their applica-
tions.
Enantioseparations by HPLC with AAILs as chiral ligands:
In our HPLC study, the AAILs coupled with CuII were used
as chiral mobile-phase additives. The influencing factors
were studied with dl-Phe as a model analyte. Herein,
AAILs are used instead of conventionally used amino acid
ligands. Our first concern was whether or not the amino
acids can still serve as chiral ligands when being made into
AAILs and what the function is of the AAIL cation in the
separation. Thus, we first performed the separation using
AAILs with different alkyl chain lengths in the cation. As
shown in Figure 1, good baseline separations of dl-Phe en-
Another interesting possibility with task-specific ILs is to
introduce natural amino acids into the cations or anions of
ILs.[38–43] Amino acids have many advantages including low
cost, biocompatibility, and ease of chemically modification.
In most cases, amino acid based ILs are also chiral ILs.
Amino acids provide a valuable approach for functional
design of ILs. Specifically, the term “amino acid ionic
liquid”, or “AAIL”, was coined for the ILs with amino acids
as anions; they were first synthesized by the Ohno group
using an anion-exchange method.[44,45] Because the amino
acids have various functional groups and keep their nature
in AAILs,[46] it is possible to design AAILs for a wide range
of tasks. For instance, Chen et al. have used AAILs as a sup-
port for metal scavenging and heterogeneous catalysis.[47]
However, the study of AAILs is still in its infancy, and the
current research mainly focuses on their synthesis and char-
acterization.[48–51] To our knowledge, there have been no re-
ports of using AAILs for chromatographic separation or
chiral separation.
In this paper, we demonstrate for the first time the appli-
cation of AAILs in chiral separation. Four underivatized
amino acids were used as model analytes. Because the
amino acids maintain their nature in AAILs,[46] we per-
formed the separations based on the ligand-exchange princi-
ple.[52–54] The l-proline ILs coupled with CuII were used as
chiral selectors because l-proline has been proven to be an
outstanding ligand in organocatalysis and organometallic
catalysis.[55] Chiral recognition is achieved based on the for-
mation of ternary mixed-metal complexes between the
AAIL ligand and the analyte ligand. The different complex
stability constants of the mixed complexes with d and l en-
antiomers result in the enantioseparation of racemic amino
acids. The method was validated by means of two major
chiral separation techniques, HPLC and CE. In both tech-
niques, AAILs show a significant superiority over the con-
ventionally used amino acid ligands. Specifically, we have
made an elaborate comparison between the HPLC and CE
results. On the basis of the comparison, the detailed separa-
tion mechanisms, including the function of AAIL cation, are
discussed in this paper.
Figure 1. Separation factor (a; shown in white) and resolution (Rs; shown
in gray) of dl-Phe for different chiral selectors in HPLC. Pure l-Pro, l-
Pro+ACTHNUGTRENUNG[C4mim][Br], and four Pro ILs with different alkyl chain lengths
coupled with CuII were used as chiral mobile-phase additives. Mobile
phase: 1 mm chiral mobile-phase additive, 0.5 mm Cu(Ac)2, and MeOH
(15% v/v) in water; column: Ultimate XB C18 (150ꢁ4.6 mm i.d.); flow
rate: 1 mLminÀ1; temperature: 258C; fluorescence detection: lex/lem
=
215 nm/295 nm.
antiomers are obtained with all the four l-Pro ILs (separa-
tion factor (a) and resolution (Rs)>2.0), thereby indicating
that the AAILs are indeed effective chiral ligands. For com-
parison, we also performed the separation with l-Pro as a
chiral ligand under the same conditions. Although the base-
line separation is also obtained with l-Pro (a=1.82 and
Rs =2.70), its enantioselectivity is evidently poorer than that
of l-Pro ILs. Another interesting control experiment is the
separation using l-Pro and a common IL, 1-butyl-3-methyli-
midazolium bromide, [C4mim][Br], as a binary mobile-phase
additive. Surprisingly, the obtained resolution (Rs =1.46) is
even lower than that of pure l-Pro. Therefore, just the pres-
ence of the [Cnmim] cation in the mobile phase does not
cause the better separation, and it is essential to combine
the [Cnmim] cation and l-Pro into AAILs. Notably, by in-
creasing the alkyl chain length of [Cnmim] from C2 to C8,
the enantioselectivity is significantly enhanced (i.e., Rs in-
9890
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 9889 – 9896