Separation of enantiomers with magnetic silica nanoparticles modified by a chiral selector: Enantioselective fishing

Article abstract of DOI:10.1039/b908349a

Magnetic silica nanoparticles modified by a chiral selector were demonstrated to be useful in magnetic field induced separation of enantiomers.

Full text of DOI:10.1039/b908349a

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Separation of enantiomers with magnetic silica nanoparticles
modified by a chiral selector: enantioselective fishingw
Hee Jung Choi and Myung Ho Hyun*
Received (in Cambridge, UK) 27th April 2009, Accepted 14th September 2009
First published as an Advance Article on the web 1st October 2009
DOI: 10.1039/b908349a
Magnetic silica nanoparticles modified by a chiral selector were
demonstrated to be useful in magnetic field induced separation of
enantiomers.
The two enantiomers of racemic chiral compounds, including
chiral drugs, often show different pharmacological, toxicological
and/or metabolic activities in living systems. Consequently,
individual enantiomers of chiral compounds should be
examined for their own biological activities particularly during
the process of developing or marketing new chiral drugs.1 For
this purpose, methods of separating enantiomers are essential.
There are various methods that can be applied for the separation
of enantiomers and/or for the determination of the enantio-
meric composition of chiral compounds.2 For example,
various chromatographic methods including high performance
liquid chromatography (HPLC),3 gas chromatography (GC),4
supercritical fluid chromatography (SFC),5 simulated moving
bed (SMB) chromatography,6 various electromigration
capillary methods,7 enzymatic or non-enzymatic dynamic
kinetic resolutions8 and classical chemical resolutions9 have
been successfully utilized. As another method of separating
enantiomers, in this study, we directed our attention to the
magnetic field induced separation of enantiomers with the use
of magnetic silica nanoparticles (MSNPs).
Fig. 1 Schematic representation of the separation of enantiomers
using (a) surface modified MSNPs (black solid circle) only, and (b)
surface modified MSNPs and surface modified NMSPs (white empty
circle) at the same time.
Our new strategy for the separation of enantiomers
with MSNPs is the utilization of inorganic–organic hybrid
materials based on MSNPs and an appropriate chiral selector
(CS). MSNPs tagged with an appropriate CS (MSNPs/CS) are
expected to select a single enantiomer from a racemic mixture
and the resulting MSNPs/CS–enantiomer complexes can be
easily removed by using a magnet as shown in Fig. 1a.
Another strategy is to use both MSNPs/CS and non-magnetic
silica particles tagged to the antipode CS (NMSPs/CS) at the
same time (Fig. 1b). Then the MSNPs/CS and NMSPs/CS
particles are expected to enantioselectively interact with one of
the two enantiomers; the resulting MSNPs/CS–enantiomer
complexes are separated by using a magnet and the other
NMSPs/CS–enantiomer complexes are separated by simple
decantation after centrifugation. The enantiomer separation
process shown in Fig. 1 looks like fishing and our new strategy
for the separation of enantiomers is termed as ‘enantioselective
fishing’.
MSNPs, spherical silica nanoparticles containing magnetite
(Fe3O4), have attracted much attention because of their
superparamagnetic property, low cytotoxicity, and chemically
modifiable surfaces.10 Because of the superparamagnetic
property, MSNPs are attracted to a magnetic field, but retain
no residual magnetism after the field is removed. Consequently,
MSNPs attached to specific molecules of interest can be
removed from a medium using a magnetic field. Previously,
MSNPs have been shown to be useful in magnetic field
induced bioseparations,10a,b,11 and magnetite nanoparticles
(MNPs) immobilized with an appropriate chiral catalyst or
enzyme have also been successfully utilized for asymmetric
reactions or enzymatic kinetic resolutions.12 However, MSNPs
have not been utilized in the direct separation of enantiomers
to the best of our knowledge. In this study, we demonstrate
that MSNPs can be used in the direct separation of enantiomers.
The MSNPs used in this study were prepared via a
procedure which is similar to, but slightly modified from that
reported previously.10b The average particle size of MSNPs
prepared in this study was 300 nm (see Fig. S1 of the ESIw) and
the measured magnetization saturation (MS) was 15.5 emu gÀ1
(see Fig. S2 of the ESIw). NMSPs (Kromasil silica gel, 5 mm)
used in this study were available from Eka chemicals
(Bohus, Sweden). (R)- and (S)-N-(2,2-dimethyl-4-pentanoyl)-
proline-3,5-dimethylanilide, which have been shown to be
useful for the resolution of N-(3,5-dinitrobenzoyl)-a-amino
acid N-propylamides as a HPLC chiral stationary phase
(CSP) when bonded to silica gel,13 were selected as the CS.
Department of Chemistry and Chemistry Institute for Functional
Materials, Pusan National University, Busan 609-735,
Republic of Korea. E-mail: mhhyun@pusan.ac.kr;
Fax: (+82) 51 516 7421; Tel: (+82) 51 510 2245
w Electronic supplementary information (ESI) available: Experimental
details, SEM and TEM images of the magnetic silica nanoparticles,
magnetic properties of the magnetic silica nanoparticles and chromato-
grams for the determination of the enantiomeric excess of the
separated enantiomers. See DOI: 10.1039/b908349a
MSNPs/(S)-CS and NMSPs/(R)-CS shown in Fig.
2
were prepared by treating MSNPs and NMSPs with (S)- or
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MSNPs/(S)-CS–enantiomer complexes were washed with
ethanol in order to retrieve the separated enantiomer. The
enantiomeric composition of the separated enantiomers
present in the ethanol solution was analyzed by HPLC with
a (R,R)-Whelk-O 1 chiral column (Regis Tech., USA).
The enantiomer separation results are summarized in
Table 1. Racemic N-(3,5-dinitrobenzoyl)alanine, valine and
leucine N-propylamides were subjected to separation. In each
case, the (S)-enantiomer was selectively taken by MSNPs/
(S)-CS even though the enantioselectivity was not so great.
The selectivity for the (S)-enantiomers by MSNPs/(S)-CS
might be rationalized by the enantioselective p–p and two
hydrogen bonding interactions between the (S)-CS and two
enantiomers as proposed previously for the resolution of
N-(3,5-dinitrobenzoyl)-a-amino acid N-propylamides on an
HPLC CSP.13 When the percentage of 2-propanol in hexane
was decreased, the enantiomer separations were improved
(see entry 1, 2, 3, and 4 in Table 1), but solubility problems
were encountered for valine and leucine derivatives. Under
less-polar conditions, the stereoselective interaction between
the (S)-CS and the (S)-enantiomers seems to be more
significant. When the concentration of racemic N-(3,5-dinitro-
benzoyl)alanine N-propylamide in 30% 2-propanol in hexane
was increased, the enantiomer separation for alanine
derivative was also improved (see entry 5 in Table 1), but
solubility problems were again encountered for valine and
leucine derivatives.
Fig. 2 Schematic presentation for the structures of (a) MSNPs/(S)-CS
and (b) NMSPs/(R)-CS.
(R)-N-(2,2-dimethyl-5-ethoxydimethylsilylpentanoyl)-proline-
3,5-dimethylanilide, which in turn were prepared from (S)- or
(R)-proline in refluxing toluene for 72 h, via a previously
reported procedure.13 Elemental analysis of MSNPs/(S)-CS
(C, 13.76%; H, 2.22%; N, 2.32%) and NMSPs/(R)-CS
(C, 6.47%; H, 1.50%; N, 1.40%) showed a loading of
0.52 mmole of (S)-CS per gram of MSNPs/(S)-CS and a
loading of 0.25 mmole of (R)-CS per gram of NMSPs/(R)-CS,
respectively.
MSNPs/(S)-CS was utilized in the separation of the two
enantiomers of N-(3,5-dinitrobenzoyl)-a-amino acid N-propyl-
amides. As per the method shown in Fig. 1a, MSNPs/(S)-CS
(400 mg) and a 10 mL solution of racemic N-(3,5-dinitrobenzoyl)-
As an alternative method, shown in Fig. 1b, MSNPs/(S)-CS
(300 mg), NMSPs/(R)-CS (300 mg) and 10 mL solution
of racemic N-(3,5-dinitrobenzoyl)alanine N-propylamide
(0.5 mg mLÀ1 or 2.0 mg mLÀ1) in 2-propanol–hexane
(10 : 90 or 30 : 70, v/v) were added to a 20 mL vial. The mixed
heterogeneous solution was shaken with a vortex mixer for 5 min
at room temperature. MSNPs/(S)-CS–enantiomer complexes
were collected by a magnet and the NMSPs/(R)-CS–enantiomer
a-amino acid N-propylamide (0.5 mg mLÀ1 or 2.0 mg mLÀ1
)
dissolved in 2-propanol–hexane (10 : 90, 30 : 70, 50 : 50 or
80 : 20, v/v) were added to a 20 mL vial. The mixed hetero-
geneous solution was shaken with a vortex mixer for 5 min at
room temperature and then MSNPs/(S)-CS–enantiomer
complexes were collected by using a magnet. The collected
Table 1 Enantiomeric excess values of N-(3,5-dinitrobenzoyl)alanine (Ala), valine (Val) and leucine (Leu) N-propylamide enantiomers separated
with MSNPs/(S)-CS in 2-propanol–hexanea
Entry
Percentage of 2-propanol in hexane
Ala
Val
Leu
1
2
3
4
5
10% 2-propanol (0.5 mg mLÀ1 b
)
50.1% ee in (S)
38.2% ee in (S)
29.6% ee in (S)
18.9% ee in (S)
63.8% ee in (S)
—
—
—
30% 2-propanol (0.5 mg mLÀ1 b
50% 2-propanol (0.5 mg mLÀ1 b
80% 2-propanol (0.5 mg mLÀ1 b
30% 2-propanol (2.0 mg mLÀ1 b
)
)
)
)
29.3% ee in (S)
28.1% ee in (S)
16.4% ee in (S)
—
—
13.8% ee in (S)
—
a
Enantiomeric excess was checked with HPLC chiral column [(R,R)-Whelk-O 1 (250 Â 4.6 mm ID]. Mobile phase: 30% 2-propanol in hexane.
Flow rate: 1.0 mL minÀ1. Temperature: 20 1C. Detector: 254 nm UV. Concentration of racemic N-(3,5-dinitrobenzoyl)-a-amino acid
N-propylamides in 2-propanol–hexane.
b
Table 2 Enantiomeric excess values of N-(3,5-dinitrobenzoyl)alanine N-propylamide enantiomers separated by using MSNPs/(S)-CS and
NMSPs/(R)-CS at the same time in 2-propanol–hexanea
Entry
Percentage of 2-propanol in hexane
MSNPs/(S)-CS
NMSPs/(R)-CS
1
2
3
10% 2-propanol (0.5 mg mLÀ1 b
)
62.0% ee in (S)
51.1% ee in (S)
68.1% ee in (S)
77.2% ee in (R)
66.2% ee in (R)
80.1% ee in (R)
30% 2-propanol (0.5 mg mLÀ1 b
30% 2-propanol (2.0 mg mLÀ1 b
)
)
a
Enantiomeric excess was checked with HPLC chiral column [(R,R)-Whelk-O 1 (250 Â 4.6 mm ID]. Mobile phase: 30% 2-propanol in hexane.
Flow rate: 1.0 mL minÀ1. Temperature: 20 1C. Detector: 254 nm UV. Concentration of racemic N-(3,5-dinitrobenzoyl)alanine N-propylamide in
2-propanol–hexane.
b
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complexes were collected by decantation after centrifugation.
The MSNPs/(S)-CS–enantiomer complexes and NMSPs/
(R)-CS–enantiomer complexes were washed with ethanol in
order to retrieve the absorbed enantiomers. Each ethanol
solution was analyzed by HPLC with an (R,R)-Whelk-O 1
chiral column and the enantiomer separation results are
summarized in Table 2. The trends of the enantioselectivity
with the variation of the percentage of 2-propanol in hexane
and the concentration of racemic N-(3,5-dinitrobenzoyl)-
alanine N-propylamide are consistent with those of the first
method as shown in Table 2. The enantioselectivity by
MSNPs/(S)-CS was improved when NMSPs/(R)-CS was used
at the same time, but the enantioselectivity by NMSPs/(R)-CS
is greater than that by MSNPs/(S)-CS. During the process of
separating MSNPs/(S)-CS–enantiomer complexes from
NMSPs/(R)-CS–enantiomer complexes by a magnet, a small
amount of NMSPs/(R)-CS–enantiomer complexes seems to be
embedded in MSNPs/(S)-CS–enantiomer complexes and
consequently, the enantiomeric purity of the enantiomer
separated by MSNPs/(S)-CS might be lower than that of the
enantiomer separated by NMSPs/(R)-CS.
This work was supported by a grant-in-aid for the
National Core Research Center Program from MEST/KOSEF
(R15-2006-022-03001-0).
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In conclusion, we have demonstrated that MSNPs tagged to
an appropriate chiral selector can be utilized in magnetic
field induced separation of enantiomers. The separation of
enantiomers were improved when both MSNPs/(S)-CS and
NMSPs/(R)-CS were used at the same time. Even though the
complete separation of the two enantiomers was not achieved
in this study, the method of magnetic field induced separation
of enantiomers with the use of MSNPs tagged to an
appropriate chiral selector is expected to be developed further
and utilized as a successful enantiomer separation technique in
the future.
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ꢀc
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6456 | Chem. Commun., 2009, 6454–6456