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Masami Sawada,* Yoshio Takai and Hitoshi Yamada
Materials Analysis Center, The Institute of Scientific
and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan
Muneyoshi Yoshikawa and Ryuichi Arakawa
Department of Applied Chemistry,
Kansai University, Suita, Osaka 564-0073, Japan
Hiroyuki Tabuchi, Mikio Takada and Jyuichi Tanaka
Faculty of Engineering, Osaka Institute of Technology, Asahi-
ku, Osaka 535-0002, Japan
Motohiro Shizuma*
Technochemistry Department, Osaka Municipal Technical Research Institute, Joto-ku, Osaka 536-
8553, Japan
Hiroshi Yamaoka
Faculty of Science, Osaka Women’s University, Sakai, Osaka 567-0035, Japan
Keiji Hirose, Kazuo Fukuda and Yoshito
Tobe
Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan |
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Chiral recognition in the host-guest complexation systems of
chiral crown ether hosts and amino ester guests was thoroughly examined using the electrospray ionization (ESI) mass spectrometry/enantiomer labeled (EL)-guest method. In this
method, the mass spectra of a mixture of three components in a solution, a chiral host (H), an equal amount of an (S)-enantiomer guest labeled with deuterium atoms
(GS-dn+) and an unlabeled (R)-enantiomer guest (GR+), were measured and the relative peak intensity value
[I(H + GR)+ / I(H + GS-dn)+ = IRIS] of the host-guest complex ions, observed with an excess
guest concentration, was taken to provide the chiral recognition ability of the host. In our earlier report (1996), we demonstrated that the apparent chiral recognition abilities using
a mass spectrometer with a homemade ESI interface were depressed by about one tenth compared with the corresponding abilities obtained by fast-atom bombardment (FAB) MS.
In the present study, the enantioselective complexation behaviors of various combinations of chiral crown hosts with chiral guests were further investigated in detail mainly using a
modern commercial ESI/ion trap (IT) mass spectrometer. Consequently, it was found that the apparent IRIS values from the ESI-MS/EL-guest method changed significantly,
depending upon the instrument used, and in particular, upon the ESI interfaces. Moreover, under the specific measuring conditions in ESI-IT-MS, the degrees of depression of the
apparent chiral recognition abilities are roughly grouped into three classes, depending upon the number (or probably the type) of the hydrophobic substituents of the hosts.
Representing the degrees by the slopes when plotting the apparent IRIS values in ESI-MS versus those in FAB-MS, the slopes for the three classes are (1) 1.0, (2) 0.7 and
(3) 0.3; the higher the hydrophobicity of the hosts (and then, the host-guest complex ions), the lower the slope (the apparent enantioselectivity). Strengthening the degree of
depression may be caused by an increase in the local concentration of the host close to the surface of the droplets produced during the electrospary ionization process. The chiral
recognition ability (KR / KS) in an equilibrated solution agrees quite well with the IRIS value in FAB-MS rather than that in ESI-MS. |
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Keywords:
chiral recognition, host–guest complexation, chiral crown ether, electrospray ionization, deuterium labeling |
