A R T I C L E S
Cocinero et al.
Figure 1. Structural representations of cellulose, the cellulose disaccharide unit (1), and its C-4′epimer, the lactoside (2).
structure calculations.9 Interestingly, density functional theoreti-
cal (DFT) calculations of cellobiose in Vacuo have predicted a
global minimum energy structure corresponding to an anti-φ/
syn-ψ conformation with dihedral angles (φΗ and ψΗ) [defined
as φH: H1′-C1′-O′-C4, ψH: C1′-O′-C4-H4] ≈ 180°, 0°,
respectively, and the two hydroxymethyl groups adopting a cis
arrangement, rather than the trans (syn/syn) conformation
adopted by the disaccharide in condensed phase environments.10
The range of possible conformations that might in principle,
be accessed by the disaccharide units, however, is very large,7
and their calculated relative energies often lie within the limited
accuracy of the computational methods. Without experimental
confirmation, identification of their true intrinsic conformational
structures remains open to doubt. The same critique also applies
to molecular modeling of explicitly hydrated (microsolvated)
carbohydrates in Vacuo although DFT calculations (at 0 K) of
microsolvated cellobiose, have predicted relative stabilization
of the trans configuration with increasing hydration.11
Fortunately, it is now possible to determine the intrinsic
structures of carbohydrates and their microhydrated complexes
in the gas phase experimentally, through analysis of their mass
and conformer selective vibrational spectra recorded under
molecular beam conditions coupled with a combination of
molecular mechanics, DFT and ab initio calculations.12-14 The
vibrational signatures associated with OH and NH stretching
modes are particularly informative since they are extremely
sensitive to the local hydrogen bonded conformational environ-
ment.15,16 This strategy has already been used to identify the
intrinsic, gas phase structure of benzyl ꢀ-lactoside,12 which turns
out to be cis rather than trans, and it has also exposed the
specificity and the consequences of explicit hydration in a series
of monosaccharides commonly found in biological systems,
including glucose, galactose, mannose, xylose, and fucose.14 The
precise new strategy is ideally suited therefore to address, for
the first time, the influence of hydration (and hence, microsol-
vation) on the glycosidic conformation of disaccharides (and
oligosaccharides17).
It is applied here to the cellulose disaccharide and, focusing
on the critical ꢀ1,4-linkage at the nonreducing end of the
growing cellulose polymer, its C-4′ epimer, to identify and
compare (1) the intrinsic structures of phenyl ꢀ-cellobioside
(phenyl ꢀ1,4-D-glucopyranosyl-ꢀ-D-glucopyranoside) 1 and its
singly hydrated complex and (2) the corresponding structures
of its isolated and singly hydrated epimer, benzyl ꢀ-lactoside
2, see Figure 1. [The phenyl and benzyl groups in 1 and 2
provide the ultraviolet chromophore required for mass selective
detection via the resonant two-photon ionization step in the
infrared ion-depletion (IRID) technique. Their presence has a
negligible influence on the conformational landscapes of the
two disaccharides.12,15] The changed hydroxyl configuration at
C-4′ in 2, from equatorial to axial, turns out to be highly
significant, influencing the selected hydration site, the structural
consequences of hydration and the rigidity of the ꢀ1,4-glycosidic
linkage.
Methods
Synthesis. The phenyl glycoside of the cellulose disaccharide
unit (1) was readily prepared from parent carbohydrate D-glucose
(Scheme 1) in >30% overall yield. Thus, the chromophoric phenyl
‘tag’ was installed into the peracetate of D-glucose using BF3 ·Et2O-
catalyzed glycosylation with phenol. Protecting group manipulation
allowed selective access to OH-4 in a suitable tribenzylated acceptor
monosaccharide. MeOTf-activated glycosylation of OH-4 using the
peracetylated benzyl thioglucoside donor allowed access to the
cellobioside framework with excellent (>98%) ꢀ-stereoselectivity
consistent with the presence of a participatory group at C-2 of the
donor sugar. Global deprotection (hydrogenolysis and methanolysis)
yielded 1. This route of seven steps (longest linear route), although
requiring the stereoselective formation of the interglycosidic ꢀ1,4
bond, compared favorably with previous syntheses from less readily
available starting block cellobiose18 (e.g., 16% overall yield)19 as
well as allowing greater potential flexibility in the design of
analogues.
(9) (a) French, A. D.; Johnson, G. P. Cellulose 2004, 11, 449–462. (b)
French, A. D.; Johnson, G. P. Mol. Simul. 2008, 34, 365–372.
(10) Strati, G. L.; Willett, J. L.; Momany, F. A. Carbohydr. Res. 2002,
337, 1833–1849.
(11) (a) Bosma, W. B.; Appell, M.; Willett, J. L.; Momany, F. A. J. Mol.
Struct. (THEOCHEM) 2006, 776, 1–19. (b) Bosma, W. B.; Appell,
M.; Willett, J. L.; Momany, F. A. J. Mol. Struct. (THEOCHEM) 2006,
776, 21–31.
The lactose disaccharide (2), ‘tagged’ with a benzyl group was
commercially available.
(12) Jockusch, R. A.; Kroemer, R. T.; Talbot, F. O.; Snoek, L. C.; C¸ arc¸abal,
P.; Simons, J. P.; Havenith, M.; Bakker, J. M.; Compagnon, I.; Meijer,
G.; von Helden, G. J. Am. Chem. Soc. 2004, 126, 5709–5714.
(13) C¸ arc¸abal, P.; Jockusch, R. A.; Hu¨nig, I.; Snoek, L. C.; Kroemer, R. T.;
Davis, B. G.; Gamblin, D. P.; Compagnon, I.; Oomens, J.; Simons,
J. P. J. Am. Chem. Soc. 2005, 127, 11414–11425.
Computation. Initial structures were generated through an
extensive conformational search using a combination of the Large-
scale Low Mode (which uses low frequency modes to construct
conformational changes) and Monte Carlo Multiple Minimization
(14) Cocinero, E. J.; Stanca-Kaposta, E. C.; Scanlan, E. M.; Gamblin, D. P.;
Davis, B. G.; Simons, J. P. Chem. Eur. J. 2008, 14, 8947–8955.
(15) Simons, J. P.; C¸ a¸r; cabal, P.; Davis, B. G.; Gamblin, D. P.; Hu¨nig, I.;
Jockusch, R. A.; Kroemer, R. T.; Marzluff, E. M.; Snoek, L. C. Int.
ReV.Phys. Chem. 2005, 24, 489–532.
(17) Stanca-Kaposta, E. C.; Gamblin, D. P.; Cocinero, E. J.; Frey, J.;
Kroemer, R. T.; Fairbanks, J. A.; Davis, B. G.; Simons, J. P. J. Am.
Chem. Soc. 2008, 130, 10691–10696.
(18) Montgomery, E. M.; Richtmyer, N. K.; Hudson, C. S. J. Am. Chem.
Soc. 1943, 65, 1848–1854.
(16) Cocinero, E. J.; Stanca-Kaposta, E. C.; Gamblin, D. P.; Davis, B. G.;
Simons, J. P. J. Am. Chem. Soc. 2009, 131, 1282–1287.
(19) Stanek, J.; Kocourek, J. Chem. Listy Vedu Prum. 1953, 47, 607–702.
9
11118 J. AM. CHEM. SOC. VOL. 131, NO. 31, 2009