Hemicellulose binding and the spacing of cellulose microfibrils in spruce wood

Lynne H Thomas,Anne Martel,Michael C Jarvis,Isabelle Grillo

doi:10.1007/s10570-020-03091-z

Lynne H Thomas, Anne Martel + Show 2 more

Open Access

https://doi.org/10.1007/s10570-020-03091-z

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Abstract

Cellulose microfibrils in conifers, as in other woody materials, are aggregated into loose bundles called macrofibrils. The centre-to-centre spacing of the microfibrils within these macrofibrils can be estimated from the position of a broad diffraction peak in small-angle neutron scattering (SANS) after deuteration. A known spacing of 3.0 nm, increasing with moisture content, is consistent with direct microfibril to microfibril contact. However recent evidence indicates that conifer microfibrils are partially coated with bound xylan chains, and possibly with lignin and galactoglucomannan, implying a wider centre-to-centre spacing as found in angiosperm wood. Delignification of spruce wood allowed a weak SANS peak to be observed without measurable change in spacing. By deuterating spruce wood in mildly alkaline D2O and then re-equilibrating with ambient H2O, deuterium atoms were trapped in a position that gave a 3.8 nm microfibril spacing under dry conditions as in angiosperm wood, instead of the 3.0 nm spacing normally observed in conifers. After conventional vapour deuteration of spruce wood a minor peak at 3.8 nm could be fitted in addition to the 3.0 nm peak. These observations are consistent with some microfibril segments being separated by bound xylan chains as in angiosperms, in addition to the microfibril segments that are in direct contact.

Highlights

Cellulose from higher plants forms partially crystalline microfibrils that, at the time of their formation, appear to contain about 18 chains (Newman et al 2013), which would give a diameter just under 3 nm (Jarvis 2018)
Where microfibrils cluster into aggregates, diameters across the hydrophilic faces can be estimated from the centre-to-centre spacing of the aggregated microfibrils measured by small-angle neutron scattering (SANS), with surface deuteration to provide neutron scattering contrast (Jarvis 2018)
Xylans bound to the hydrophilic faces of softwood microfibrils would be expected to increase the microfibril diameter beyond the 3 nm expected for bare microfibrils, as they do in angiosperm wood (Thomas et al 2014; Langan et al 2014)

Summary

Introduction

Cellulose from higher plants forms partially crystalline microfibrils that, at the time of their formation, appear to contain about 18 chains (Newman et al 2013), which would give a diameter just under 3 nm (Jarvis 2018). Where microfibrils cluster into aggregates (macrofibrils), diameters across the hydrophilic faces can be estimated from the centre-to-centre spacing of the aggregated microfibrils measured by small-angle neutron scattering (SANS), with surface deuteration to provide neutron scattering contrast (Jarvis 2018).

Results

Conclusion