Evaluation of chiral amino acid discrimination by a permethylated cyclic tetrasaccharide, cyclo-{→6)-α-D-Glcp-(1→3)-α-D-Glcp- (1→6)-α-D-Glcp-(1→3)-α-D-Glcp-(1→}, using FAB mass spectrometry

Article abstract of DOI:10.1246/cl.2008.1054

The novel cyclic oligosaccharide, permethylated tetrasaecharide (CTS), determinates the enantiomers of chiral amino acid isopropyl ester hydrochlorides. Copyright

Full text of DOI:10.1246/cl.2008.1054

1054
Chemistry Letters Vol.37, No.10 (2008)
Evaluation of Chiral Amino Acid Discrimination by a Permethylated Cyclic Tetrasaccharide,
cyclo-{!6)-ꢀ-D-Glcp-(1!3)-ꢀ-D-Glcp-(1!6)-ꢀ-D-Glcp-(1!3)-ꢀ-D-Glcp-(1!},
Using FAB Mass Spectrometry
Motohiro Shizuma,ꢀ1 Taro Kiso,1 Hisashi Terauchi,2 Yoshio Takai,3 Hitoshi Yamada,3 Tomoyuki Nishimoto,4
Daisuke Ono,1 Osamu Shimomura,2 Ryoki Nomura,2 Yoshikatsu Miwa,4
Masaki Nakamura,1 and Hirofumi Nakano1
1Department of Biochemistry, Osaka Municipal Technical Research Institute, Joto-ku, Osaka 536-8553
2Department of Applied Chemistry, Osaka Institute of Technology, Asahi-ku, Osaka 535-8585
3The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047
4Hayashibara Biochemical Laboratories, INC., Shimoishii, Okayama 700-0907
(Received June 20, 2008; CL-080626; E-mail: shizuma@omtri.city.osaka.jp)
The novel cyclic oligosaccharide, permethylated tetrasac-
charide (CTS), determinates the enantiomers of chiral amino
acid isopropyl ester hydrochlorides.
6
O
5
O
O
RO
RO
4
A
1
OR
2
3
OR
RO
OR
6
O
4
O
RO
Chiral recognition is one of the fundamental and important
processes in living systems, and chirality and the related technol-
ogy are one of the most significant subjects in pharmacy, bio-
chemistry, organic chemistry, etc.1,2 It is well known that some
kinds of cyclic oligosaccharide derivatives such as cyclodextrin
(CD)3 and cyclofructan (CF)4 show chiral discrimination toward
chiral molecules and/or ions, and the CD derivatives have been
applied as the chiral stationary phase in gas and liquid chroma-
tography.5,6 However, chiral discrimination of cyclic tetrasac-
charide (CTS) (Chart 1),7 which is a cyclic oligosaccharides,
has not as yet been examined. As CTS has a polyether ring
moiety (–O–C–C–O–C–O–C–C–C)2 in the molecular center,
CTS is expected to associate with cation guests via charge–
dipole electrostatic interactions. As is well known, chiral crown
ether derivatives, which are cyclic polyethers, have been report-
ed as having high enantioselectivity toward primary chiral
ammonium ions, and are applied to chiral separations.8 With
the application of the FAB mass spectrometry (MS)-enantiomer
labeled (EL) guest method,9,10 which is one of the methods for
estimation of the chiral recognition ability of new hosts,11 we
found for the first time that permethylated CTS (MeCTS) exhib-
its various degrees of chiral discrimination toward ammonium
ions of amino acid isopropyl esters. A MeCTS host (H) is com-
plexed with a 1:1 amino acid guest mixture of an unlabeled R
enantiomer (GRþ) and a deuterium-labeled S enantiomer
(GS-Dnþ, n: number of deuterium atoms). The enantioselectivity
of MeCTS is quantitatively evaluated from the relative peak
3
OR
RO
RO
OR
B
5
CTS
R =H
2
OR
O
O
MeCTS
O
R =Me
1
RO
O
RO
RO
OR
OR
OR
OR
RO
O
O
O
O
RO
O
O
O
OR
O
RO
O
O
RO
OR
RO
RO
OR
O
CF
R =H
OR
Me-α -CF
R =Me
OR
Chart 1. Cyclic tetrasaccharide (CTS) and ꢀ-cyclofructan
(ꢀ-CF) derivatives.
mixture was used for obtaining the FAB mass spectra. A typical
mass spectrum is shown in Figure 1, and the IR=IS-Dn values
obtained are summarized in Table 1 with the data of Me–ꢀ-
CF. MeCTS showed larger chiral discrimination ability than
Me–ꢀ-CF except for the case of Trp–O–i-Prþ. MeCTS showed
S selectivity toward the primary ammonium ions of the given
guests. The enantionselectivity for Val–O–i-Prþ and Pgly–O–
i-Prþ is large among the primary ammonium ions. For secondary
ammonium ion Pro–O–i-Prþ MeCTS showed R selectivity
clearly. Val–O–i-Prþ and Pgly–O–i-Prþ have a secondary
carbon atom neighboring on the each stereo center. But, Pro–
O–i-Prþ is also very constrained around the stereo center. It is
suggested that when MeCTS binds to chiral ammonium ions
to make complexes, the steric interactions between the host
and guests occur near the stereo center of the guests.
þ
intensity value [I(H + GR)þ/I(H + GS-Dn
) = IR/IS-Dn] of
the two host–guest diastereomeric complex ion peaks in the
FAB mass spectrum.
CTS was permethylated12 so that its complex ion was sensi-
tively detected by FABMS. Amino acid isopropyl ester hydro-
chlorides were used as guests so that natural abundance correc-
tion was unnecessary. All S enantiomer guests were labeled with
deuterium (isopropyl ester: n ¼ 6 or 7). A 1:1 racemic mixture
solution of enantiomer guests was prepared by mixing together
an equal amount of a 0.67 M MeOH solution of each enantiomer.
A 10 mL aliquot of the guest solution and a 5 mL aliquot of a
0.20 M host MeOH solution were added to 15 mL of the a 3-
nitirobenzyl alcohol (NBA) matrix. A 1 mL sample of the final
In NMR studies, the spectral changes of MeCTS by
resulting from addition of ammonium salts of amino acid esters
were too small to estimate the association constants (K <
Copyright ꢀ 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.10 (2008)
1055
Table 2. 1H NMR induced downfield shifts (ppm) of the peaks
þ
of MeCTS by complexation with NH4 (SCNꢁ) in acetone-d6
at 298 K
Proton
Limiting shift/ppm
Proton
Limiting shift/ppm
H1A
H2A
H3A
H4A
H5A
H6A
H60A0
Me2A
Me3A
Me4A
0.05
0.06
—
0.04
ꢁ0:18b
0.22
0.07
0.02
H1B
H2B
H3B
H4B
H5B
H6B
H60B0
Me2B
Me4B
Me6B
0.17
0.19
a
ꢁ0:02b
a
—
0.08
a
—
—
a
0.17
0.00
0.02
0.01
0.03
Figure 1. Example of mass spectra of complex ions of MeCTS
with Val–O–i-Prþ in the FABMS/EL-guest method. NBA was
þ
used as the matrix. Molecular weights of H, GRþ, and GS-Dn
are 816, 160, and 167, respectively.
aThe shifts did were not estimated exactly because of the overlaps
in other peaks. The limiting shift of each peak was calculated from
the association constant and the induced shift under certain host
and guest concentration conditions. bNegative values show upfield
shifts.
Table 1. The relative peak intensity (IR=IS-Dn values) of the
complex ions (H + GR)þ and (H + GS-Dn
þ
)
of MeCTS or
Me–ꢀ-CFþ(H) with ammonium ions of amino acid isopropyl
esters (GR and GS-Dnþ) in FAB mass spectrometrya
O
O
(a)
(b)
O
O
Permethylated cyclic oligosaccharide
Amino acid
isopropyl ester
O5A
O
M
O5A
O
S
O
O2B
H O
O
O2B
+H O
O6A
O
O6A
O
MeCTS
Me–ꢀ-CF
(O1B) O
(O1B) O
O
O
H
Ala–O–i-Prþ
Val–O–i-Prþ
Pro–O–i-Prþ
Met–O–i-Prþ
Phe–O–i-Prþ
Trp–O–i-Prþ
Pgly–O–i-Prþ
0.90
0.33
3.72
0.54
0.68
0.81
0.48
0.94
1.28
1.08
1.04
1.00
1.38
0.99
H
O
N
N+
O
S
O
O
L
O2OB M
L
O
O
H
H
O O6A
O O6A
O2OB
O
(O1B)
(O1B)
O
O
O
O
O
O5A
O5A
Steric interaction
(Weak)
Steric interaction
(Strong)
O
O
O
O
O
O
Figure 2. Illustrations of complexes of MeCTS with chiral or-
ganic ammonium ions. (a) Complex of CTS with one enantiomer
of guest; (b) with another one.
aThe counter anions of the chiral ammonium ion guests were
chloride (Clꢁ). The matrix of FABMS was 3-nitrobenzyl alcohol
(NBA). The accuracy of the IR=IS-Dn values were 1:00 ꢂ 0:04.
Pgly = phenylglycine (2-amino-2-phenylacetic acid).
We are grateful to Associate Professor Takeyuki Suzuki
(Material Analysis Center, ISIR, Osaka University) for admit-
ting the use of instrumental analyzers and Mr. Hiroshi Adachi
(Osaka University, Faculty of Science) for measurements of
high-resolution ESI-TOF-MS of MeCTS.
1 Mꢁ1). The specific shifts of MeCTS proton signals were
induced by adding ammonium thiocyanate in acetone-d6 at
298 K (log K ¼ 1:0 ꢂ 0:1). The limiting downfield shifts are
summarized in Table 2. The protons of H5A, H6A, H1B,
H2B, and Me2B showed the relatively large shifts, which
suggest that ammonium ions locate close to the protons and bind
to the oxygen atoms neighboring the protons. Thus, the oxygen
atoms of O5A, O6A (or O1B), and O2B are binding points of
ammonium ions. When potassium thiocyanate was used as a
guest, the proton signals responded in the same manner. The
structure of the complex between MeCTS and the potassium
ions calculated and optimized by MM (force field: CVFF)
based on the crystalline structure of CTS13 supports the proposed
structure on the basis of the limiting shifts.
The ammonium ion moiety fixes at the oxygen atoms as
shown in Figure 2. As MeCTS adopts a C2-symmetry, the space
around the stereo center divides into two parts. The three sub-
stituents of the ꢀ-carbon of the guest locate in different steric
environments. Therefore, each enantiomer of the guest was
distinguished effectively. In the case of Me–ꢀ-CF complex, as
each substituent locates in similar environments because of C3
(C6)-symmetry, the chiral discrimination ability toward chiral
amino acid esters becomes smaller than that of MeCTS.14
We are studying in detail the enantioselective complexation
of CTS derivatives. CTS and the its derivatives can be applied
in to chiral separation technology in the future.
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Published on the web (Advance View) September 6, 2008; doi:10.1246/cl.2008.1054