Hydration effects on spacing of primary-wall cellulose microfibrils: a small angle X-ray scattering study

Celery collenchyma cell walls are typical
of primary plant cell walls in their composition but
contain unusually well-oriented cellulose microfibrils
that are packed with more regularity than normal,
permitting small-angle X-ray scattering (SAXS)
experiments that would not otherwise be possible.
Small-angle scattering data were obtained for the cell
walls in essentially their native state and for isolated
cellulose, in a fibrous form that retained the physical
shape and microfibril orientation of the native cell
walls. The scattering patterns showed a distinct peak
attributed to the interference contribution to the
convolution of form and interference functions. The
position of the peak attributed to the interference
function implied a mean centre-to-centre microfibril
spacing of approximately 3.2 nm in dry isolated
cellulose and 3.8 nm in dry cell walls. Hydration
increased the mean microfibril spacing in the cell
walls to 5.4 nm but had only a small effect on the
mean microfibril spacing of isolated cellulose. In the
scattering profile from intact, hydrated cell walls it
was just possible to discern the position of the first
Bessel minimum, from which a microfibril diameter
in the range 3.1–3.6 nm may be estimated. This
estimate is likely to include attached hemicellulose
chains. Porod plots of scattering intensity indicated a
relatively sharp interface between microfibrils and
their immediate surroundings. The SAXS data imply
that cellulose microfibrils 2.6–3.0 nm in diameter are
not quite in lateral contact with one another in the
isolated cellulose and are augmented by hemicelluloses
and separated by readily hydrated matrix
polysaccharides in the native plant cell wall.