Microfibril diameter in celery collenchyma cellulose: X-ray scattering and NMR evidence

Cellulose isolated from celery collenchyma
is typical of the low-crystallinity celluloses
that can be isolated from primary cell-walls of
higher plants, except that it is oriented with high
uniformity. The diameter of the microfibrils of
celery collenchyma cellulose was estimated by
three separate approaches: 13C NMR measurement
of the ratio of surface to interior chains;
estimation of the dimensions of the crystalline
lattice from wide angle X-ray scattering (WAXS)
measurements using the Scherrer equation; and
the observation that microfibrils of this form of
cellulose have the unusual property of packing
into an irregular array from which small angle
X-ray scattering (SAXS) shows features of both
form and interference functions. The interference
function contributing to the SAXS pattern implied
a mean microfibril centre-to-centre distance
of 3.6 nm, providing an upper limit for the
diameter. However modelling of the scattering
pattern from an irregular array of microfibrils
showed that the observed scattering curve could
be matched at a range of diameters down to
2.4 nm, with the intervening space more or less
sparsely occupied by hemicellulose chains. The
lateral extent of the crystalline lattice normal to
the 200 plane was estimated as a minimum of
2.4 nm by WAXS through the Scherrer equation,
and a diameter of 2.6 nm was implied by the
surface: volume ratio determined by 13C NMR.
The WAXS and NMR measurements both
depended on the assumption that the surface
chains were positioned within an extension of the
crystalline lattice. The reliability of this assumption
is uncertain. If the surface chains deviated
from the lattice, both the WAXS and the NMR
data would imply larger microfibril diameters
within the range consistent with the SAXS
pattern. The evidence presented is therefore all
consistent with microfibril diameters from about
2.4 to 3.6 nm, larger than has previously been
suggested for primary-wall cellulose. Some degree
of aggregation may have occurred during the
isolation of the cellulose, but the larger microfibril
diameters within the range proposed are a consequence
of the novel interpretation of the experimental data from WAXS and NMR and
are consistent with previously published data if
these are similarly interpreted.