Keisuke Hirata^{a, c}，Shun-ichi Ishiuchi^{a, c}，Anne Zehnacker-Rentien^{b, c} and Masaaki Fujii^c
a) Dep. Chem., Schl. Science, Tokyo Tech, ^b)U. Paris-Saclay, ^c) WRHI, IIR

　In this article, I would like to introduce our joint research with Prof. Anne Zehnacker-Rentien in CNRS / U. Paris-Saclay, Profs Keisuke Hirata and Shun-ichi Ishiuchi in School of Science.

　Cyclodextrins (CDs) are cyclic oligosaccharides composed of D-glucose linked by a-1,4 glycosidic linkages, which form vacancies in the molecule (Fig. 1). Since CD has chirality, when a guest molecule has chirality, each enantiomer has a different affinity for CD. CD has been used as a stationary phase in liquid chromatography and has been put to practical use as a chiral column for splitting optical isomers [1]. Although β-CD, which is composed of seven D-glucose moieties, is widely adapted to small guest molecules such as amino acids, the pore size of β-CD is 6 Å, which is smaller than that of amino acids (2~3 Å). However, the pore size of β-CD is 6 Å, which is much larger than the size of amino acids (2~3 Å), and it is not clear why such a large host can recognize the chirality of small guests.

	Fig. 1　Structure of b-CD

　In this study, we investigated the interaction between two enantiomers of protonated tyrosine and methyl β-CD (β-MCD), in which all OH groups of β-CD are methoxylated, by using cryogenic ion-trap infrared spectroscopy. We attempted to elucidate the mechanism of chiral discrimination of CD from the difference of the host-guest interaction. β-MCD eliminates the complex OH stretching vibrational bands derived from CD, and the vibrational bands derived from protonated tyrosine can be clearly observed.

　The experimental setup is shown in Fig. 2 [2-3]. The complexes of tyrosine and β-MCD, protonated using electrospray (ESI) method, were extracted into the gas phase, selected for specific masses using a quadrupole mass spectrometer, and trapped in a cooled ion trap (~10 K).　 Hydrogen gas was introduced to the complex ion, and molecular hydrogen was attached to the complex ion. When the complex ion was irradiated with a tunable infrared (IR) laser light, the molecular hydrogen dissociated by vibrational excitation due to IR absorption. The fragment ions (complex ions) generated by this dissociation were detected by a time-of-flight mass spectrometer (TOF-MS). By scanning the wavelength of the IR light while monitoring the amount of fragment ions, the IR photodissociation (IRPD) spectrum corresponding to the IR absorption spectrum was measured.

	Fig. 2　Experimental Setup

　First, to investigate whether β-MCD is more selective for L- or D- tyrosine, we electrosprayed a solution containing equal amounts of L-tyrosine and D-tyrosine with β-MCD and measured the mass spectrum. Fig. 3 shows the mass spectrum of the complexes of protonated tyrosines and β-MCD, where the molecular weight of L-tyrosine is 10 times larger than that of D-tyrosine due to the isotope substitution (¹³C, ¹⁵N). The D-TyrH⁺ complex is three times stronger than the L-TyrH⁺ complex, indicating that β-MCD has a chiral selectivity for D-TyrH⁺.

　The IRPD spectra of each of the TyrH⁺ complexes were then measured in order to determine the structural basis for this difference in chiral selectivity. The IRPD spectra of the complexes of protonated tyrosine and β-MCD are shown in Fig. 4(c) and (d). From comparison with the spectra of tyrosine and β-MCD alone (Fig. 4(a) and (b)), bands at 3640 cm^-1, 3430 cm^-1, 3300~3400 cm^-1, 3020~3300 cm^-1 and 2700~2800 cm^-1 are assigned to free phenol OH stretching without hydrogen-bonding, hydrogen-bonded phenol OH stretching, free NH stretching, hydrogen-bonded NH stretching and hydrogen-bonded carboxylic acid OH stretching vibration, respectively. As can be seen in Fig. 4, the "amino-bonded" form, in which the carboxylic acid OH and amino group NH of tyrosine form hydrogen-bonds with b-MCD, was observed in both complexes. On the other hand, in the D-TyrH⁺ complex, a band of hydrogen-bonded phenol OH was observed at 3430 cm^‑1, suggesting that the "phenol-bonded" type, in which β-MCD and phenol OH are hydrogen-bonded, also coexists in addition to "amino-bonded" complex. These results are in agreement with the theoretical infrared spectra obtained by quantum chemical calculations.

	Fig. 3 Mass spectrum of complexes of b-MCD with L- and D-TyrH+

　From these results, we conclude that the high chiral selectivity of β-MCD for D-TyrH+ is due to the fact that only the D-TyrH⁺ complex can take the phenolic bonded form. If this is the case, the chiral selectivity should be decreased without phenol OH. To confirm this, we measured the mass spectrum of β-MCD with phenylalanine (PheH⁺), in which the phenol OH of tyrosine is replaced by hydrogen (Fig. 5). As a result, unlike TyrH⁺, the D- and L-PheH⁺ complexes were observed with almost the same intensity, showing that the chiral selectivity of β-MCD for D-, L-PheH⁺ was lost. Thus, it was experimentally demonstrated that the key to the chiral selection of -MCD for tyrosine is whether or not the phenol-bound form can be prepared.

Fig. 4	IR-UV dip spectra of (a) isomer A of L-TyrH+, (b) b-MCDH+，(c) L- and (d) D-TyrH+ with b-MCD complexes

	Fig. 5 Mass spectrum of b-MCD with L- and D-PheH+

【References】

[1]	D. W. Armstrong and W. Demond, J. Chromatogr. Sci., 1984, 22, 411.,
[2]	S. Ishiuchi, et al., J. Mol. Spectrosc., 2017, 332, 45.,
[3]	S. Ishiuchi, et al., Phys. Chem. Chem. Phys., 2019, 21, 561.