(-)/(+)-Sparteine induced chirally-active carbon nanoparticles for enantioselective separation of racemic mixtures

Article abstract of DOI:10.1039/c6cc02525k

Chiral carbon nanoparticles (CCNPs) were developed by surface passivation using the chiral ligand (-)-sparteine or (+)-sparteine (denoted (-)-SP/CNP and (+)-SP/CNP, respectively). The chirality of the prepared CCNPs was demonstrated by circular dichroism

Full text of DOI:10.1039/c6cc02525k

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DOI: 10.1039/C6CC02525K
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(-)/(+)-Sparteine Induced Chirally-active Carbon Nanoparticles for
Enantioselective Separation of Racemic Mixtures†
Guru Raja Vulugundam,‡ Santosh K. Misra,‡ Fatemeh Ostadhossein, Aaron S. Schwartz-Duval,
Enrique A. Daza and Dipanjan Pan*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Chiral carbon nanoparticles (CCNPs) were developed by surface the environment when used in large scale and biological
passivation using the chiral ligand (-)-sparteine or (+)-sparteine toxicity.5 To avoid such disadvantages, another category of
(named (-)-SP/CNP and (+)-SP/CNP, respectively). The chirality of chiral nanomaterials have been reported as chiral polymeric
the prepared CCNPs was demonstrated by circular dichroism and particles that were explored for potential applications in chiral
polarimetry and employed as an enantioselective separation resolution by crystallization and stereo selective synthesis.6 But
platform for representative racemic mixtures.
restriction of using polymer based particles with many reactive
racemic mixtures, directed the search for a commercially
exploitable base material with better universality in regard to
having lesser chances of reacting with racemic mixtures.
Chirality is an important phenomenon in materials of both
synthetic and natural origin. Carbohydrates, peptides and
amino acids are some of the common chiral molecules present
in living systems. Chiral recognition and separation has a vital
relevance in various fields like agrochemicals, consumer
products and especially in pharmaceutical industry where one
of the enantiomers of the same molecule may exhibit
remarkable disparity in terms of pharmacological potency,
pharmacokinetics, toxicity, metabolic pathways and recognition
by immune system.1 Thus it is highly desirable to separate a
specific enantiomer from their racemic mixture before using for
any decided application.
Considerable effort has been expended in the last few years
for the development of nanoscale structures with chirality for
diverse applications like asymmetric synthesis, liquid crystals
and chiral recognition.2 Chiral particles in the nanometric scale
would increase the area of interaction significantly and open up
various advantages associated in process of chiral separation.
Metallic chiral nanoparticles and quantum dots (QDs) have
been previously prepared using chiral molecules like DNA,
peptides, or small organic ligands like cysteine, glutathione and
penicillamine as capping agents.3 The chirality induced in
nanoparticles play a crucial role in their application as
biosensors, recognition of chiral molecules and fabrication of
chiroptical nanomaterials.4 In spite of various emerging
beneficial applications of the metal based nanoparticles, they
are plagued with potential disadvantages related to toxicity to
Carbon nanoparticles (CNPs) provide an environmentally
benign alternative by virtue of their tunable biocompatibility,
reactivity, ease of preparation and cost effectiveness.7 To the
best of our knowledge, an optically active, chiral carbon
nanoparticles (CCNPs) for separation of enantiomer from
racemic mixtures has yet to be explored. With this unmet need,
our approach introduces chirality in CNPs by controlled surface
passivation with chiral molecules, i.e. sparteine. In its both
enantiomeric form the compound has long been used as a chiral
base for various asymmetric reactions often leading to highly
enantioselective transformations.8 The choice of sparteine to
induce chirality into the CNPs is due to its unique structural
feature. Sparteine contains a rigid bisquinolizidine structure
with the asymmetric center present adjacent to the chiral
amines. Both the amines are conformationally locked and
would not allow easy chiral inversion. Consequently, this
property would restrict inversion of chirality of prepared CCNPs
under conditions of enantiomeric separation.
Both (-)-sparteine and (+)-sparteine were used to bestow
chirality on the CNPs through surface passivation and generate
(-)-SP/CNP and (+)-SP/CNP, respectively. Synthesized CCNPs
were demonstrated to be a novel chiral recognition platform
and as an efficient adsorbent for separation of enantiomers.
Sucrose was used as the carbon source for the synthesis of
CNPs and the surfaces of the CNPs were passivated by the chiral
a.Department of Bioengineering, Beckman Institute, Materials Science and
Engineering, University of Illinois at Urbana-Champaign, 502 N. Busey, Urbana, IL,
61801, USA.E-mail: dipanjan@illinois.edu
†Electronic Supplementary Information (ESI) available: Detailed procedure for
preparation and characterization of CCNPs, See DOI: 10.1039/x0xx00000x
‡ These authors contributed equally to this work.
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ligands, (-)- and (+)-sparteine by pre-passivation using a facile number-averaged hydrodynamic diameters of aqueous
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and economical microwave irradiation route. During the course suspensions of (-)-SP/CNP and (+)-SP/CDNOPI:(1s0p.1a0r3t9e/iCn6eC:sCu0g2a52r5=K
of microwave irradiation, the colorless solution changed to dark 1:10) were 21±5nm and 24±5 nm respectively, while the pristine
brown solution followed by evaporation of water, finally turning CNPs without any passivation showed 50±7nm (Fig 2A). The
into a black residue (Fig. 1).
aqueous suspensions of CCNPs prepared from higher ratios of
sparteine:sugar resulted in particles in the size range of 50-200
nm. Electrophoretic mobility studies for various suspensions of
CCNPs revealed that with increasing feed ratio of sparteine, the
resultant CCNPs possessed more negative zeta potentials (Table
S1). TEM was performed to study the morphology of the CCNPs
and the anhydrous state particle diameter of (-)-SP/CNP (1:10)
was measured to be 14±2nm (Fig 1E-H).
AFM images were acquired from drop casted samples on
clean and polished glass slides to study the surface pattern of
these CCNPs. The average height value (Hav) of representative
samples was found to be 71 nm (Fig 1I). The UV-Vis absorption
spectra of (-)-sparteine revealed a peak at 220 nm which is
characteristic of n-σ* due to the tertiary amine. Pristine CNP
displays a characteristic strong absorption peak at 282 nm
related to the n-π* transition of carbonyl group. This peak is
slightly red-shifted to 283 nm for (-)-SP/CNP. In the fluorescence
spectra, when excited at λex = 360 nm, (-)-SP/CNP showed a
broad peak in the range of 450-600 nm.
The FT-IR spectrum for the (-)-SP/CNP exhibited stretching
vibrations of C-OH at 3350 cm-1, C-H at 2928 cm-1 and 2860 cm-
1, and the C-N stretch at 1280 cm-1. The peak at 1064 cm−1 can
be identified as stretching peak of the C–O–C bond. (Fig 2C) We
also observed the low frequency C-H stretching bands in the
region 2800-2500 cm-1 called “trans bands” which are typical for
sparteine that depends on its conformational and
configurational arrangement.9 Raman spectra of CNP, (-)-
SP/CNP and (+)-SP/CNP are shown in Figure 2D. We observed
the characteristic D (~1370 cm-1) and G (~1600 cm-1) bands of
CNPs. The G band originates due to the in-plane vibration of sp2
carbon atoms while D (defect/disorder) band is related to a
hybridized vibrational mode of graphene edges and is an
indicator of some disorder in the graphene structure. We found
that the intensity ratio (IG/ID) was slightly increased after
passivation of CNPs with (-)/(+)-sparteine. The marginal
increase in IG/ID of (-)-/(+)-SP/CNP than pristine CNP is plausibly
arising from induced graphitic nature due to presence of
sparteine during preparation.
To demonstrate the chiroptical properties of CCNPs, we
performed circular dichroism measurements of the aqueous
suspensions of (-)- and (+)-SP/CNPs. CD spectra of CCNPs
suspended in water showed clear CD responses which are
mirror images of each other in the region of 195-220 nm, while
the aqueous CNP suspensions without passivation showed no
CD signal (Figure 2E). Optical activity of the CCNPs was also
probed by polarimetry. The optical rotation of (-)-SP/CNP and
(+)-SP/CNP prepared using sparteine:sugar = 1:10 was found to
be -0.08° and +0.108°, respectively (Table 1). With the
increasing passivation by the chiral ligand, a corresponding
increase in the optical rotation of the respective CCNPs has
been observed. CD and polarimetry studies revealed that the
achiral CNPs were transformed into CCNPs by surface
passivation of chiral ligand. We also examined the effect of
Fig.
1
Schematic
representation for the (A) synthesis of (-)- and
(+)-sparteine passivated CCNPs, (-)-SP/CNP and (+)-SP/CNP; (B) Optical
photograph of aqueous suspension of CCNPs; (C, D) schematic representation and
optical photograph of CCNPs as chiral adsorbent in a column. Representative
transmission electron microscopy images of (-)-SP/CNP 1:10 (E), particle size
distribution curve from TEM (F), (-)-SP/CNP 2:1 (G) and (+)-SP/CNP 2:1 (H) (scale
bar 200 nm); atomic force microscopy image of (-)-SP/CNP 1:10.
The interaction between the carbon nanoparticles and (-)-/(+)-
sparteine is purely physical and electrostatic in nature,
presumably arising from the tertiary amines of sparteine and
surface abundant carboxylic acids groups in CNPs.7b,e,f The (-)-
SP/CNP and (+)-SP/CNP were well dispersed in aqueous
medium and were stable for several months without any
noticeable precipitation. The morphology and chemical
integrity of the prepared (-)-/(+)-SP/CNP nanoparticles were
extensively characterized by transmission electron microscopy
(TEM), dynamic light scattering (DLS), atomic force microscopy
(AFM), electrophoretic potential (Zeta), Raman spectroscopy,
Fourier Transform infrared (FT-IR), nuclear magnetic resonance
(NMR), UV-Vis, Fluorescence and circular dichroism (CD) and
optical rotation measurements, which confirmed the successful
preparation of CCNPs presenting chiral ligands at the nanoscale.
Three different weight ratios of (-) or (+)-sparteine and sucrose
(1:1, 1:2, and 1:5 respectively) were attempted for surface
passivation.
DLS measurements revealed that CCNPs were
monodispersed in nanopure water (0.2 µM) and there was an
increment in the size of CCNPs upon increase in the
concentration of sparteine on the surface (Table S1). The
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microwave irradiation of (-)-sparteine by H-NMR and found from the reaction mixture by ultra-centrifugation (95,000 rpm).
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DOI: 10.1039/C6CC02525K
that the structural integrity was retained even after microwave The CD spectrum of supernatant was measured and the amount
treatment (Fig 2F). Figure 2G shows the 1H-NMR of (+)-SP/CNP. of adsorption was calculated from the change in the peak
We observed the presence of a multiplet at δ 1.1-1.2 in the
intensity at 202 nm. Alternatively, a prototype column with the
solid CCNPs was prepared through which a solution of L-/D-
cysteine was passed and comparable result was obtained in
both the cases.
(A)
(B)
(C)
(D)
Fig. 3 Circular dichroism spectra and adsorption ratio of L-/D-cysteine by (A, B) (-
)-SP/CNP and (C, D) (+)-SP/CNP. Absorbance of L- and D- cysteine solution (circles
and squares, respectively) by (-)-SP/CNP (A) and (+)-SP/CNP (C). Blank symbols
correspond to signals from the solution of 2 mM concentration before treatment
while filled symbols correspond to signals of the spectra after treatment with (-)-
SP/CNP or (+)-SP/CNP. Chiral CNPs were prepared from (-)- or (+)-sparteine: sugar
= 2:1. Bar chart of the adsorption ratio for both the chiral CNPs (B, D).
(F)
(E)
(F)
1
2
Fig. 3A shows that (-)-SP/CNP has adsorbed 33% of L-
cysteine enantiomer from the solution, whereas only 14% of D-
cysteine was adsorbed. This results in an enantiomeric excess of
19% ee for L-cysteine. In the case of (+)-SP/CNP, the selectivity
of adsorption was found to be in opposite direction. (+)-SP/CNP
showed enantiomeric excess of 15% ee for D-cysteine
enantiomer (Fig 3B). As a control, we also checked if the
unmodified CNPs bind to L-/D-cysteine. We found there was no
binding by pristine CNPs (data not shown). We chose two other
ratios of (-)- or (+)-sparteine:sugar (1:5 and 1:10) during the
preparation of (-)-SP/CNP and (+)-SP/CNP to study the effect of
surface passivation of CNPs on the specific binding of the
enantiomers. We found that enantiospecific adsorption by the
chiral carbons decreased with the decrease in the amount of the
chiral ligand deposited on the CNPs (Fig. S3). As the initial ratio
of (-)-sparteine:sugar varied from 1:5 to 1:10, % ee for L-
cysteine reduced from 19.3% to 11%. Similarly, (+)-SP/CNP
showed enantiomeric excess of 9% and 4.7% towards D-
cysteine for the ratios of (+)-sparteine:sugar = 1:5 and 1:10
respectively. However, when an excess of the chiral passivating
agent was used as in the case of (+)-SP/CNP prepared from an
initial ratio of (+)-sparteine:sugar of 10:1, there was only a
marginal improvement in the % ee for D-cysteine up to 17.4%
(Fig. S4). To further verify the enantiospecific adsorption
capability of CCNPs, we measured the optical rotation before
and after treatment of the L-/D- cysteine with the chiral CNPs
(Table 1). L-Cys (25 mM) showed an optical rotation of +0.021°
which reduced to +0.015° after its treatment with (-)-SP/CNP
showing the adsorption of 28.1%. However, D-Cys induced an
optical rotation of -0.022° and -0.019° before and after
treatment with (-)-SP/CNP, showing a lesser adsorption of
11.4%. In order to check the versatility of the enantioselective
(G)
Fig.
2 Physico-chemical characterization of the chiral carbon nanoparticles
(CCNPs) prepared from sparteine:sugar = 1:10. (A) Hydrodynamic diameter; (B)
UV-Vis spectroscopy and Normalized fluorescence emission spectra (λex = 360
nm) (C) FT-IR; (D) Raman spectra and (E) Circular dichroism spectra (F) 1H-NMR
spectra of (-)-sparteine before (1) and after (2) microwave treatment showing no
significant effect and (G) 1H-NMR of different prepared formulations.
spectrum of (+)-sparteine which retained in (+)-SP/CNP,
revealing the presence of (+)-sparteine in CCNP formulation.
However, other important peaks of sparteine got overlapped by
those of CNPs.
Then we proceeded to investigate the enantioselective
interactions of chiral molecules with CCNPs. In this study, L- and
D-cysteine were chosen as a representative model system to
demonstrate the chiral recognition ability of the CCNPs. In order
to quantify the adsorption of each enantiomer of L-/D-cysteine
by CCNPs individually, chiral adsorption studies were
performed. Adsorption studies were performed by preparing
aqueous solutions of L- or D-cysteine (2 mM) and measured
their CD responses. Then (-)-SP/CNP or (+)-SP/CNP was added
such that the final concentration of CCNPs was 0.5 mg/mL and
the mixture was stirred for 12 h. The CCNPs were separated
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binding of CCNPs, chiral adsorption measurements of proline points of chiral inversion, (-)-/(+)-sparteine. The physico-
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DOI: 10.1039/C6CC02525K
enantiomers were also performed onto the (-)- and (+)-SP/CNP chemical integrity of the obtained CCNPs was investigated by
(Fig. 4). The adsorption pattern and ratio were similar to those UV-Vis, fluorescence, FT-IR, NMR, Raman, DLS, AFM, TEM and
of cysteine. (-)-SP/CNP resulted in 17% ee of L-proline zeta potential studies. As a result of the passivation using (-)-
enantiomer whereas (+)-SP/CNP resulted in 14% ee for D- /(+)-sparteine, the CNPs were rendered chiral. Their chiroptical
proline enantiomer. These results were also supported by response was confirmed using circular dichroism and
polarimeter studies (Table 1).
polarimetry techniques. We demonstrated that these CCNPs
could be used as a chiral adsorbent for enantiospecific binding.
Table 1. Polarimetry study of adsorption of cysteine The % ee obtained in this work is better or comparable than that
enantiomers into (-)-SP/CNP.a
of similar adsorbants.7g The method excels due to its simplicity
and low cost of the materials used. The general method to
prepare chiral CNPs described here opens a new route to
prepare a broad variety of chiral CNPs with other chiral
passivating agents and we can envision enantiomeric
enrichment and separation of racemic mixtures.
Notes and references
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Fig. 4 Circular dichroism spectra and adsorption ratio of L-/D-Proline by (A, B) (-)-
SP/CNP and (C, D) (+)-SP/CNP. Absorbance of L- and D- cysteine solution (circles
and squares, respectively) by (-)-SP/CNP (A) and (+)-SP/CNP (C). Blank symbols
correspond to signals from the original solution of 2mM concentration before
treatment while filled symbols correspond to signals of the spectra after
treatment with (-)-SP/CNP or (+)-SP/CNP. Chiral CNPs were prepared from (-)- or
(+)-sparteine:sugar = 2:1. Bar chart of the adsorption ratio for both the chiral CNPs
(B, D).
7
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report the development of chiral CNPs by surface passivation
with a chiral ligand of high asymmetric response with restricted
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