Bending trisaccharides by a chelation-induced ring flip of a hinge-like monosaccharide unit

Full text of DOI:10.1021/ja984062p

J. Am. Chem. Soc. 1999, 121, 5089-5090
5089
Bending Trisaccharides by a Chelation-Induced Ring
Flip of a Hinge-Like Monosaccharide Unit
Hideya Yuasa* and Hironobu Hashimoto*
Department of Life Science
Faculty of Bioscience and Biotechnology
Tokyo Institute of Technology
Nagatsuta, Midori-ku
Yokohama 226-8501, Japan
ReceiVed NoVember 25, 1998
Chelation-induced conformational change of polypeptides is a
useful strategy in de novo protein design,1 which ultimately aims
at the construction of artificial proteins with tailor-made structures
and functionalities. Employing this strategy with oligosaccharides
may lead to the development of novel sugar-based architectures.
Although the addition of metal ions to solutions of some natural
polysaccharides causes coil-helix transitions2,3 and some monosac-
charides undergo shifts in the ring conformation equilibrium by
the addition of metal ions,2,4 the extent to which metal ions
influence these conformational properties is usually small, and
no extraordinary conformations are created.
Figure 1. Structures of hinge sugar 1 and the hinged trisaccharides 2
and 3. Upon chelation of a metal ion (Hg2+ or Zn2+), a ring flip of the
hinge unit is induced to give the turn structure shown in the upper right.
In the frame is the global minimum of the turn structure of 2-Zn2+
,
In this study, we created a novel turn structure of the
preliminarily calculated by molecular dynamics simulation (MM2 force
field, 300 K); all hydrogen atoms are omitted for clarity.
4
trisaccharides 2 and 3 by a hinge-like C1-to-1C4 ring flip of the
2,4-diamino-2,4-dideoxy-â-D-xylopyranoside unit (Figure 1). This
ring flip was enabled by the flexible ring structure5 and the strong
chelating ability of diamino groups in the 1,3-diaxial orientation.6
Conformational behavior of the methyl glycoside 1 and that of
the hinge unit on addition of metal ions were investigated by 1H
NMR.
Scheme 1a
The methyl glycoside 1 was synthesized in five steps from the
known compound 47 (Scheme 1). The methods for synthesizing
2,4-diazido-2,4-dideoxy derivatives of glucopyranoside8 and
R-xylopyranoside9 were employed for the synthesis of the key
intermediate 7. Birch reduction of 7 gave the compound 1. The
trisaccharides 2 and 3 were designed to mimic the trisaccharide
sequence (Galâ1-3GlcNAcâ1-2ManR)10 of complex N-linked
glycans, in which the central sugar, GlcNAc, is replaced with
the hinge unit. Introduction of the mannose units at the reducing
end of 1 was accomplished by conversion of 7 into the
1-O-trichloroacetimidate 10 and then conversion of 10 by the
Schmidt method11 to give disaccharides 11 and 12. Galactose
residues were incorporated into 11 and 12 by the trichloroace-
(1) (a) Kohn, W. D.; Kay, C. M.; Sykes, B. D.; Hodges, R. S. J. Am. Chem.
Soc. 1998, 120, 1124-1132. (b) Schneider, J. P.; Kelly, J. W. J. Am. Chem.
Soc. 1995, 117, 2533-2546. (c) Kohn, W. D.; Hodges, R. S. Trends
Biotechnol. 1998, 16, 379-389. (d) Schneider, J. P.; Kelly, J. W. Chem. ReV.
1995, 95, 2169-2187.
a (a) MsCl, pyr; then 88% AcOH, 80%. (b) NaOMe. (c) MsCl, pyr,
90% in 2 steps. (d) NaN3, Bu4NBr, toluene-H2O (1:1), 140 °C, 7, 51%;
8, 21%. (e) Ac2O, H2SO4, 89%. (f) H2NNH2‚AcOH, DMF, 50 °C. (g)
Cl3CCN, Cs2CO3, ClCH2CH2Cl, 58% in 2 steps. (h) Na, liq. NH3, 88%.
(i) M-OH, TMSOTf, CH3CN, -40 °C; then NaOMe. 11, 63%; 12, 53%
(R-isomer: 23%). (j) G-OC(dNH)CCl3, BF3‚OEt2, MS4A, CH2Cl2, -78
°C f rt. 13, 71%; 14, 86%. (k) NaOMe; then H2S, pyr-H2O (1:1); then
Na, liq. NH3, -78 °C. 2, 50%; 3, 63%.
(2) Whitfield, D. M.; Stojkovski, S.; Sarkar, B. Coord. Chem. ReV. 1993,
122, 171-225.
(3) Wittgren, B.; Borgstro¨m, J.; Piculell, L.; Wahlund, K.-G. Biopolymers
1998, 45, 85-96.
(4) (a) Angyal, S. J. AdV. Carbohydr. Chem. Biochem. 1989, 47, 1-43.
(b) Whitfield, D. M.; Sarkar, B. J. Inorg. Biochem. 1991, 41, 157-170. (c)
Symons, M. C. R.; Benbow, J. A.; Pelmore H. J. Chem. Soc., Faraday Trans.
1 1984, 80, 1999-2016.
timidate method to give trisaccharides 13 and 14, which were
subjected to deprotection and reduction of the azido groups to
give the desired trisaccharides 2 and 3.
(5) See, for example: Kim, J. M.; Roy, R. J. Carbohydr. Chem. 1997, 16,
1281-1292.
(6) Hausherr-Primo, L.; Hegetschweiler, K.; Ru¨egger, H.; Odier, L.;
Hancock, R. D.; Schmalle, H. W.; Gramlich, V. J. Chem. Soc., Dalton Trans.
1994, 1689-1701.
The 1H NMR spectrum of 1 showed the J-values characteristic
of the 4C1 chair conformation (Figure 2a), and these did not change
within the temperature range 298-353 K. Addition of diamagnetic
metal ions, Hg(OAc)2 and Zn(OAc)2, caused a line-broadening
at 298 K (Figure 2b). This line-broadening can be explained by
the relatively slow exchange process between different structures
when there are more than two structures, because acceleration of
the process by increasing the temperature up to 348 K resulted
(7) Helm, R. F.; Ralph, J.; Anderson, L. J. Org. Chem. 1991, 56, 7015-
7021.
(8) Paulsen, H.; Koebernick, H.; Stenzel, W.; Ko¨ll, P. Tetrahedron Lett.
1975, 1493-1494.
(9) Janairo, G.; Malik, A.; Voelter, W. Liebigs Ann. Chem. 1985, 653-
655.
(10) Gal, GlcNAc, and Man denote galactose, N-acetyl-glucosamine, and
mannose, respectively.
(11) Schmidt, R. R.; Behrendt, M.; Toepfer, A. Synlett 1990, 694-696.
10.1021/ja984062p CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/15/1999

5090 J. Am. Chem. Soc., Vol. 121, No. 21, 1999
Communications to the Editor
amino groups. Indeed, when the amino group at C-4 of 1 was
replaced with an acetamido group, metal ion additions caused no
signal broadening or decreases in the J-values in 1H NMR spectra.
Moreover, the titration of 1 with the metal ions afforded
hyperbolic curves for the chemical shifts,16 from which a 2:1
stoichiometry of the 1-metal ion complex was also deduced.
1
The H NMR spectra of the trisaccharides 2 and 3 showed
that the hinge sugar unit maintains a 4C1 chair conformation within
the temperature range 298-353 K. Addition of 0.5 equiv Hg2+
or 1-3 equiv Zn2+ ion caused a line-broadening for the hinge
sugar unit at 298 K in a manner similar to that of 1. On the other
hand, signals for Gal and Man underwent comparatively slight
line-broadening. The difference in the extent of line-broadening
suggests that the deformation of the ring structure occurs only at
the hinge sugar unit and this ring flip permits a harmonic
movement of the peripheral sugar units. Indeed, decreases of the
J-values at 353 K compared with those for 2 or 3 was observed
only for the hinge sugar unit. The populations of the 1C4
conformation of the hinge sugar unit of 2 or 3 that complexes
with a metal ion, estimated from the J-values, are shown in Table
1. These results indicate that the trisaccharides 2 and 3 were
obviously bent to afford turn structures by the addition of the
metal ions. There is likely to be an attractive force such as the
van der Waals force between Gal and Man in the turn structure
of 3, because the disaccharide formed by removing Gal from 3
hardly underwent a ring flip at the hinge sugar (1C4 < 5%) on
addition of metal ions.
Figure 2. 1H NMR spectra (400 MHz) of compound 1 (26 mM) in the
absence and presence of Hg(OAc)2 (0.5 equiv) in 50 mM AcONa-d3 buffer
(pH 7.0). Methyl signals were adjusted to 3.584 ppm. (a) 1 at 298 K. (b)
1 with Hg(OAc)2 at 298 K. (c) 1 with Hg(OAc)2 at 348 K.
Table 1. J-Values (Hz)a for the Vicinal Ring Protons of the
1
Xylose Unit and the Calculated C4 Populations (%) of Compounds
1, 2, 3, and Their Complexes with Metal Ionsb
In conclusion, by adding metal ions, we were able to bend
trisaccharides having a hinge diaminosugar unit in the center. In
compd
J1,2
J2,3
9.6
6.4
7-8c
9.6
6.9
8.1
9.8
8.7
7.9
J3,4
J4,5a
J4,5b
1C4
1
8.1
5.0
5.6
8.2
5.3
6.1
8.1
7.2
6.6
9.6
6.6
7.5
9.5
7.2
8.1
9.6
9.0
8.1
5.1
4.0
4.0
5.2
4.3
4.0
5.3
4.7
4.5
10.7
6.9
7.5
10.8
7.0
9.0
10.8
9.0
8.5
0
41
33
0
39
21
0
1
analogy with this hinge sugar, the equilibrium between the C4
1-Hg2+
1-Zn2+
2
and 2S0 conformations of the iduronic acid unit of heparin shifts
1
toward the C4 conformation on addition of calcium chloride.16
2-Hg2+
2-Zn2+
3
However, the interconversion of the conformations of the iduronic
acid unit hardly affect the end-to-end distance of the heparin
chain.13 In this respect, the hinged trisaccharides dramatically
change their shape from straight to bent. This flexibility and the
novel turn structure may be applied to the fabrication of novel
artificial architectures containing oligosaccharide building blocks.
3-Hg2+
3-Zn2+
17
23
a 400 MHz NMR at 70-80 °C. b Hg2+, 0.5 equiv; Zn2+, 1-3 equiv.
c Unclear assignment due to slight line-broadening at H-2 and H-4
signals.
Acknowledgment. We thank the Sumitomo Foundation and the
Chemical Research Foundation of Mitsubishi Yuka Co. (now Mitsubishi
Chemical Co.) for financial support.
in sharp signals that are indicative of time-averaged resonances
of each proton (Figure 2c). The line-broadening may be due, in
part, to the restricted motions of the whole molecule, if the
complex is extremely large.12
Supporting Information Available: Experimental procedures and
full characterization of the reported compounds. Plots of the 1C4
populations and chemical shifts of 1 as a function of amounts of added
metal ions. 400 MHz 1H NMR spectra of 2 and 2-Zn2+ (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
The J-values at 348 K for the mixture of 1 and 0.5 equiv of
Hg(OAc)2 are significantly smaller than those for 1 only (Table
1), suggesting structural deformation by chelation. A similar
tendency was observed when 1.4 equiv of Zn(OAc)2 were added
to 1. To estimate the conformations of 1 in the presence of these
metal ions, least-squares fitting of the J-values, computed for all
possible conformers, to the observed J-values was performed.13,14
The calculation indicated that, in the presence of metal ions, 1
exists as an equilibrium mixture of the 4C1 and 1C4 conformations
only. The 1C4 populations are listed in Table 1. The partial
formation of the 1C4 conformation is most likely due to an
intramolecular chelation bridge between the two axially oriented
JA984062P
(14) Molecular dynamics simulation was carried out for the compounds 1
and 1-Zn2+ (MM2 force field, 1000 K) by Cache 3.9 software (Oxford
Molecular Group, Inc.). The generated ring structures (4C1, 1C4, 2S0, 3S1, 03B)
that exceed 1 ppm in population were further optimized by MM2, and the
J-values were calculated by the generalized Karplus equation.15 The least-
squares fitting of the calculated J-values to the observed ones was performed
by Sigma Plot 5.0 software (Jandel Corporation).
(15) (a) Haasnoot, C. A. G.; De Leeuw, F. A. A. M.; Altona, C. Bull. Soc.
Chim. Belg. 1980, 89, 125-131. (b) Haasnoot, C. A. G.; De Leeuw, F. A. A.
M.; Altona, C. Tetrahedron 1980, 36, 2783-2792.
(12) See, for example: Hausser, K. H.; Kalbitzer, H. R. NMR in Medicine
and Biology; Springer-Verlag: Berlin, Heidelberg, 1991.
(13) Ferro, D. R.; Provasoli, A.; Ragazzi, M.; Torri, G.; Casu, B.; Gatti,
G.; Jacquinet, J.-C.; Sinay¨, P.; Petitou, M.; Choay, J. J. Am. Chem. Soc. 1986,
108, 6773-6778.
(16) (a) Ferro, D. R.; Provasoli, A.; Ragazzi, M.; Casu, B.; Torri, G.;
Bossennec, V.; Perly, B.; Sinay¨, P.; Petitou M.; Choay, J. Carbohydr. Res.
1990, 195, 157-167. (b) van Boeckel, C. A. A.; van Aelst, S. F.; Wagenaars,
G. N.; Mellema, J.-R.; Paulsen, H.; Peters, T.; Pollex, A.; Sinnwell, V. Recl.
TraV. Chim. Pays-Bas 1987, 106, 19-29.