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Carbohydrate Research 330 (2001) 391 – 399
Solid state NMR and X-ray diffraction studies of
-D-galacturonic acid monohydrate
Hui-Ru Tang,
a
Peter S. Belton,
a,
* Sian C. Davies,
b
David L. Hughes
b
a
Institute of Food Research, Norwich Laboratory, Norwich Research Park, Colney, Norwich NR47UA, UK
b
Nitrogen Fixation Laboratory, John Innes Centre, Norwich Research Park, Colney, Norwich NR47UH, UK
Received 3 August 2000; received in revised form 17 October 2000; accepted 21 November 2000
Abstract
Crystalline -D-galacturonic acid monohydrate has been studied by
13
C CPMAS NMR and X-ray crystallography.
The molecular dynamics were investigated by evaluating
13
C spin-lattice relaxation in the rotating frame (T
1
) and
chemical-shift-anisotropy properties of each carbon. Only limited molecular motions can be detected in the low
frequency ( 10
4
Hz) range by
13
C relaxation time measurements (T
1
) and changes of chemical shift anisotropy
properties as a function of temperature. X-ray analysis (at both ambient temperature and 150 K) shows that the acid
has the usual chair-shaped, pyranose ring conformation, and that the acid and water molecules are linked, through
all their OH groups, in an extensively hydrogen-bonded lattice. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: -D-Galacturonic acid; X-ray structure; Molecular dynamics; Chemical shift anisotropy; Spin-lattice relaxation;
CPMAS
1. Introduction
-D-Galacturonic acid (GA), is the
monomer of polygalacturonic acid which
forms the backbone of pectin in the plant cell
walls.
1
The molecular details of the structure
and dynamics of pectin are of vital importance
to the understanding of plant cell wall proper-
ties. However, little is known even about these
aspects of its monomer, galacturonic acid.
Crystal structures of its sodium – calcium and
– strontium salts
2,3
and of methyl galacturonic
acid methyl ester
4
have been determined, but
not of the parent acid.
The
1
H relaxation behaviour of -D-galac-
turonic acid showed
5,6
that the motions of the
exchangeable protons, i.e., those of the sugar
hydroxyl groups and water of crystallisation,
were the major relaxation mechanisms for the
spin-lattice relaxation. The sugar ring does not
have substantial motion in the solid state.
However, the
1
H relaxation behaviour of both
plant cell walls
6–8
and pectin itself
9
showed
that there were some other motions present in
addition to those of exchangeable protons in
both the high frequency (10
8
Hz at 360 K)
and low frequency (10
5
Hz at 240 K) re-
gions.
13
C MAS studies showed that there
were substantial differences in the dynamics of
pectin and cellulose in plant cell wall materi-
als,
6,10–13
and in mixtures of cellulose and
pectin.
14
In addition, hydration effects on cel-
lulose and pectin were also different.
6,8–14
With such a complex system, solid state NMR
(high resolution and proton relaxation studies)
has, so far, resulted only in the identification
of fractions usually labelled ‘mobile’ or
* Corresponding author. Tel.: +44-1603-458939; fax: +
44-1603-458939.
E -mail address: peter.belton@bbsrc.ac.uk (P.S. Belton).
0008-6215/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII:S0008-6215(00)00307-4