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`Measurement of chiral amino acid discrimination by cyclic oligosaccharides: a
`
`direct FAB mass spectrometric approach
`
`Masami Sawada,*‘1' Motohiro Shizurna,” Yoshio Takai,‘ Hiroshi Adachi,‘ Tokuji Takeda’’ and Takao
`Uchiyamad
`
`4' Institute ofthe Scientific and Industrial Research, Osaka Univemity, Ibaraki, Osaka 567, Japan
`l’ Technochemistry Department, Osaka Municipal Technical Research Institute, Joto-lm, Osaka 536, Japan
`5 Faculty ofScience, Osaka University, Toyonaka, Osaka 560, Japan
`4 Department ofBiology, Osaka Kyoiku University, Kashihara, Osaka 582, Japan
`
`The novel cyclic oligosaccharides, perrnethylated cycIofruc-
`tans MECF, lb and 2b, discriminate enantiomers of chiral
`amino acid ester hydrochlorides.
`
`Chiral discrimination of amino acids by cyclodextrins (CD) has
`been widely investigated in the field of liquid chromatographic
`and electrophoretic enantioseparations,‘“4 but that by cyclo-
`fructans (CF)5=5 has not as yet been examined. With the
`application of the FAB mass spectrometry (MS)—enantiomer
`labelled (EL) guest method,7—9 one of the rare methods for
`estimation of the chiral recognition ability of new hosts.” we
`found for the first
`time that perrnethylated cyclofiuctans
`(MECF) exhibit various degrees of chiral discrimination
`towards amino acid esters. This is the first successful detection
`
`of the chiral amino acid-discriminating ability of cyclic
`oligosacchaiides on a unified scale covering the two series,
`where various types of complexation mechanism, such as
`charge—dipole electrostatic, hydrophobic and hydrogen bonding
`interactions are operative. An oligosaccharide host (H) is
`complexed with a 1:1 amino acid guest mixture of an
`unlabelled (R)-enantiomer (GR+) and a deuterium labelled
`(S)-enantiomer (Gs_p,.+). The enantioselectivity of a given
`oligosaccharide (host) toward a given racemic amino acid
`(guest)
`is quantitatively estimated from the relative peak
`intensity value {I[(I-I + GR)+]/I[(‘H + G_;D,,)+] = IR/Is_D,,} of the
`two host—guest diastereomeric complex ion peaks in the FAB
`mass spectrum.
`The cyclic oligosaccharide hosts lb—5b were permethylated
`so that their complex ions were sensitively detected by FAB
`MS. Amino acid 2-propyl ester hydrochlorides“ were used as
`guests so that natural abundance correction7 was unnecessary.
`All (S)-enantiomers were labelled with deuterium (2-propyl
`ester: n = 6 or 7). A 1:1 racemic mixture solution of
`enantiomer guests was prepared by mixing an equal amount of
`a 0.67 M MeOH solution of each enantiomer. A 10 pl aliquot of
`the guest solution and a 5 pl aliquot of a 0.20 M host CHCI3
`solution were added to 15 pl ofthe 3-nitrobenzyl alcohol (NBA)
`matrix. A 1 pl sample of the final mixture was used for
`obtaining FAB mass spectra. A typical FAB mass spectrum is
`shown in Fig. 1, and the IR/I5-_D,, values obtained are summa-
`rized in Table 1. MECF6 lb and MECF7 2b showed higher
`(R)-enantioselectivity for [Trp-0-Pr*']* (IR/Is_]);. = 1.38-1.29;
`—AAGe..... = 150-190 cal mol-1) [—AAG,..,., = RI1n(IR/
`I5_D,,) was employed for interconversion]?-9 MECF6 lb for
`[Tle-O-Pri]+ and MECF7 2b for [Ser-O-Pr1‘]+ or [Pro-0-Pr1']+
`provided the next best (R)-enantioselectivities. On the other
`hand.
`IVIECF7 2b for
`[Pgly-O-Pr*']* showed an inverse
`(S)-enantioselectivity (IR/I5,-_D,, = 0.76). B-MECD 4b and
`y-MECD Sb indicated (R)-enantioselectivity for [Ser-O-Pri]+
`and [Pro-O-Pr*']* (IR/I_g_D,, = ca. 1.15; —AAG,.,,, = ca. 80 cal
`mol—1).
`Among many thermodynamic chiral discrimination studies,
`the chiral discrimination of simple amino acids by CDs on the
`
`PAGE 1 OF 2
`
`basis of the host—guest (1:1) intermolecular interactions has
`rarely been reported until now. In one of the reports. Lincoln
`determined the relative binding constant (KRIKS = 0.92 in water
`at 295.5 K) of t!-MECD 3b with 4-fluorophenylglycine
`hydrochloride using the 19F NMR titration method12J3 The
`enantioselectivity of or-MECD 3b for the corresponding [Pgly-
`O-Pr‘]+ under our FAB MS conditions (IR/Is_D,, = 0.94) was in
`a good agreement with the above KR/K5 value. It is worthwhile
`to note that this is further experimental evidence for the parallels
`between the IR/I5_D,. and KR/Ks values which have been
`described previously.7'9 The NOE behavior of at-CD 3a with
`Trp using 1H NMR analysis suggested (R)-enantioselectivity, 14
`
`R0
`
`OR
`
`R0
`
`0
`
`ORG
`0
`
`R0 R0
`
`0
`
`R0
`OR
`0
`
`0
`
`OR
`on
`
`R0
`
`0
`R0
`
`on
`
`0
`
`0
`
`R°
`
`OR
`
`on
`
`OR
`
`n
`
`OR
`1aCF6,n=1,R=H
`bMECF6, n= 1, R= Me
`2aCF7,n=2_R=H
`DMECF7 n=2_ R=Me
`
`OR
`
`Me
`
`3au—CD,n=1,R=H
`bu-MECD_n=1,R
`4a|5-CD_n=2_R=H
`b|S—MECD,n=2_R
`5ay-CD_n=3,R=H
`by-MECD,n3_
`=
`
`= Me
`
`Me
`
`Chem. Commun., 1998
`
`1453
`
`SENJU EXHIBIT 2046
`
`LUPIN V SENJU
`
`IPR2015—01100
`
`

`
`Table 1 IR/IS-Dn values of permethylated cyclic oligosaccharide hosts with amino acid ester hydrochloride guestsa
`
`Host
`
`Guest (counter
`anion: Cl2)
`
`MECF6
`1b
`
`MECF7
`2b
`
`a-MECD
`3b
`
`b-MECD
`4b
`
`g-MECD
`5b
`
`18-C-6
`
`[Trp-O-Pri]+
`[Pgly-O-Pri]+
`[Phe-O-Pri]+
`[Tle-O-Pri]+
`[Met-O-Pri]+
`[Ser-O-Pri]+
`[Pro-O-Pri]+
`[Gly-O-Pri]+
`
`1.38
`0.99
`1.00
`1.18
`1.04
`1.01
`1.08
`1.01
`
`1.29
`0.76
`1.01
`1.00
`0.95
`1.18
`1.16
`0.99
`
`1.29
`0.94
`1.02
`0.95
`0.91
`0.95
`1.07
`0.97
`
`1.23
`0.91
`1.01
`0.94
`0.91
`1.15
`1.07
`0.98
`
`1.17
`0.89
`1.00
`0.93
`0.92
`0.99
`1.14
`
`0.98
`0.99
`1.02
`0.97
`0.96
`0.96
`0.96
`0.99
`
`a 18-crown-6 (18-C-6) is the typical achiral host employed. Glycine 2-propyl ester (Gly-O-Pri)+ is the typical achiral guest employed. Averaged value (n = 4)
`of 10th, 20th, 30th and 40th scan data. Errors of the IR/IS-Dn values are estimated within ±0.04. Tle = tert-leucine, Pgly = phenylglycine.
`
`101
`
`205
`
`107
`
`247
`
`303
`
`345 409
`
`100
`
`200
`
`300
`
`400
`500
`)+
`(H + GR
`
`600
`
`700
`
`800
`
`900
`
`1000
`
`1472
`
`)+
`(H + GS– Dn
`
`1243
`
`40
`
`20
`
`0
`
`40
`
`20
`
`Relative abundance (%)
`
`butyl groups of the corresponding chiral guest. The difference in
`the stereochemical complementarity would be origin of the
`chiral discrimination of MECF6 1b.
`We are studying in detail the mechanism of chiral discri-
`mination of the cyclofructans. We hope that cyclofructans and
`their derivative hosts will be amenable to the further develop-
`ment of new chiral stationary phases and chiral selectors.
`We are very grateful to Mitsubishi Chemical Co. for the kind
`gift of cyclofructans (1a, 2a and the other oligomers).
`
`Notes and References
`† E-mail: m-sawada@sanken.osaka-u.ac.jp
`
`1 D. W. Armstrong, A. M. Stalcup, M. H. Hilton, J. D. Duncan,
`J. R. Faulkner and S. C. Chang, Anal. Chem., 1990, 62, 1610.
`2 A. Berthod, S. C. Chang and D. W. Armstrong, Anal. Chem., 1992, 64,
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`3 S. Fanali, J. Chromatogr., 1989, 474, 441.
`4 V. Schurig and H.-P. Nowotny, Angew. Chem., Int. Ed. Engl., 1990, 29,
`939.
`5 M. Kawamura, T. Uchiyama, T. Kuramoto, Y. Tamura and K. Mizutani,
`Carbohydr. Res., 1989, 192, 83.
`6 Y. Takai, Y. Okumura, T. Takana, M. Sawada, S. Takahashi, M. Shiro,
`M. Kawamura and T. Uchiyama, J. Org. Chem., 1994, 59, 2967.
`7 X. X. Zhang, J. S. Bradshaw and R. M. Izatt, Chem. Rev., 1997, 97,
`3133.
`8 M. Sawada, Y. Takai, H. Yamada, S. Hirayama, T. Kaneda, T. Tanaka,
`K. Kamada, T. Mizooku, S. Takeuchi, K. Ueno, K. Hirose, Y. Tobe and
`K. Naemura, J. Am. Chem. Soc., 1995, 117, 7726.
`9 M. Sawada, Mass Spectrom. Rev., 1997, 16, 73.
`10 M. Sawada, Y. Takai, H. Yamada, J. Nishida, T. Kaneda, R. Arakawa,
`M. Okamoto, K. Hirose, T. Tanaka and K. Naemura, J. Chem. Soc.,
`Perkin Trans. 2, 1998, 701.
`11 E. P. Kyba, J. M. Timko, L. J. Kaplan, F. de Jong, G. W. Gokel and
`D. J. Cram, J. Am. Chem. Soc., 1978, 100, 4555.
`12 C. J. Easton and S. F. Lincoln, Chem. Soc. Rev., 1996, 163.
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`2.
`14 K. B. Lipkowitz, S. Raghothama and J. Yang, J. Am. Chem. Soc., 1992,
`114, 1554.
`
`Received in Cambridge, UK, 23rd April 1998; 8/03023E
`
`0
`
`1100
`
`1200
`
`1300
`
`1400
`
`1500
`m / z
`Fig. 1 An example of a FAB mass spectrum from the FABMS/EL guest
`method. Host: MECF6 1b; guest: [Trp-O-Pri]+. The left peak (m/z 1472) is
`a complex ion (H + GR)+ and the right (m/z 1478) is (H + GS-Dn)+.
`
`1600
`
`1700
`
`1800
`
`1900
`
`2000
`
`which was also in line with our results obtained by FAB MS
`(a-MECD 3b with [Trp-O-Pri]+; IR/IS-Dn = 1.29).
`The crystalline structure of a complex of MECF6 1b with
`Ba2+ (counter anion: SCN2) that was reported previously
`showed that the cation was coordinated at ten points to the host,
`four 3-OMe oxygens of the furanose rings and six oxygens in
`the 18-crown-6 skeleton (Fig. 2).6 In the case of a complex of
`MECF6 with amino acid ester hydrochlorides, it is expected that
`the 3-OMe groups become chiral barriers for the indole or tert-
`
`Fig. 2 (a) Crystalline complex of 1b with barium cation (counter anion:
`SCN2). Hydrogen atoms are not shown. (b) Imaged illustration of a
`complex of 1b with an alkylammonium ion. The nitrogen cation of the
`alkylammonium replaced the barium from (a). The a-carbon of the
`alkylammonium ion was localized at a position of 1.49 Å from the nitrogen,
`an average C–N distance in amino acids.
`
`1454
`
`Chem. Commun., 1998
`
`Published on 01 January 1998. Downloaded on 12/10/2015 09:58:38.
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