Carbohydrate Research 337 (2002) 851 – 861
www.elsevier.com/locate/carres
The crystal structure of the -cellobiose·2 NaI·2 H
2
O complex in
the context of related structures and conformational analysis
Zenaida Peralta-Inga,
a
Glenn P. Johnson,
b
Michael K. Dowd,
b
Jacob A. Rendleman,
c
Edwin D. Stevens,
a,
* Alfred D. French
b,
*
a
Department of Chemistry, Uniersity of New Orleans, New Orleans, LA 70148, USA
b
US Department of Agriculture, Southern Regional Research Center, 1100 Robert E. Lee Bouleard, New Orleans, LA 70124, USA
c
National Center for Agricultural Utilization Research, 1815 N Uniersity Street, Peoria, IL 61604, USA
Received 17 September 2001; received in revised form 6 February 2002; accepted 8 February 2002
Abstract
The crystal structure of -D-glucopyranosyl-(1 4)--D-glucopyranose (-cellobiose) in a complex with water and NaI was
determined with Mo K
radiation at 150 K to R =0.027. The space group is P 2
1
and unit cell dimensions are a =9.0188,
b =12.2536, c =10.9016 A , =97.162°. There are no direct hydrogen bonds among cellobiose molecules, and the usual
intramolecular hydrogen bond between O-3H and O-5 is replaced by a bridge involving Na
+
, O-3, O-5, and O-6. Both Na
+
have sixfold coordination. One I
-
accepts six donor hydroxyl groups and three CH···I
-
hydrogen bonds. The other accepts three
hydroxyls, one Na
+
, and five CH···I
-
hydrogen bonds. Linkage torsion angles
O-5
and
C-5
are -73.6 and -105.3°,
respectively (
H
=47.1° and
H
=14.6°), probably induced by the Na
+
bridge. This conformation is in a separate cluster in ,
space from most similar linkages. Both C-6OH and C-6OH are gg, while the C-6OH groups from molecules not in the
cluster have gt conformations. Hybrid molecular mechanics/quantum mechanics calculations show 1.2 kcal/mol strain for any
of the small-molecule structures. Extrapolation of the NaI cellobiose geometry to a cellulose molecule gives a left-handed helix
with 2.9 residues per turn. The energy map and small-molecule crystal structures imply that cellulose helices having 2.5 and 3.0
residues per turn are left-handed. © 2002 Elsevier Science Ltd. All rights reserved.
Keywords: -Cellobiose; Cellulose; Mercerization; Modeling; QM/MM; Salt complex; Crystal structure
1. Introduction
Substantial progress has been made in determining
the structure of exceptionally crystalline samples of
cellulose.
1
However, many questions about cellulose
structure remain when it is in industrial and biological
environments where it has lesser or altered crystallinity.
Therefore, it is important to understand its intrinsic
shape, its degree of flexibility, and the environmental
effects on these properties. One way to increase this
understanding is to continue to study di- and oligosac-
charides as model compounds.
Cellobiose, the smallest molecule that contains the
linkage and repeated glucose rings of cellulose, has been
studied by X-ray crystallography
2–4
in the usual -
anomeric form and by NMR spectroscopy.
5
About 30
years ago, the preparation of microcrystalline -cel-
lobiose was reported, along with a crystalline NaI
complex.
6
However, structures were not determined.
This paper reports the structure of a salt complex from
that work. In addition to its implications for the shape
properties of cellulose, interactions with Na
+
ions in
this complex provide insight on mercerization, a poorly
understood commercial treatment of cotton cellulose
with NaOH.
Because each experiment determines one, or at most
a few, molecular shapes, data from other molecules that
have small chemical differences from cellobiose are also
useful in understanding cellulose. The variations in
constitution inevitably cause differences in the crystal
* Corresponding authors. Tel.: +1-504-2864410 (A.D.F.);
tel.: +1-504-2806856 (E.D.S.).
E -mail addresses: estevens@uno.edu (E.D. Stevens),
afrench@srrc.ars.usda.gov (A.D. French).
0008-6215/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII:S0008-6215(02)00041-1