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Ibbett, Roger and Gaddipati, Sanyasi and Hill, Sandra and Tucker, Gregory A. (2013) Structural reorganisation of cellulose fibrils in hydrothermally deconstructed lignocellulosic biomass and relationships with enzyme digestibility. Biotechnology for Biofuels, 6 (1). 33/1-33/15. ISSN 1754-6834

Abstract

Background: The investigation of structural organisation in lignocellulose materials is important to understand

changes in cellulase accessibility and reactivity resulting from hydrothermal deconstruction, to allow development

of strategies to maximise bioethanol process efficiencies. To achieve progress, wheat straw lignocellulose and

comparative model wood cellulose were characterised following increasing severity of hydrothermal treatment.

Powder and fibre wide-angle X-ray diffraction techniques were employed (WAXD), complemented by enzyme

kinetic measurements up to high conversion.

Results: Evidence from WAXD indicated that cellulose fibrils are not perfectly crystalline. A reduction in fibril

crystallinity occurred due to hydrothermal treatment, although dimensional and orientational data showed that

fibril coherency and alignment were largely retained. The hypothetical inter-fibril spacing created by hydrothermal

deconstruction of straw was calculated to be insufficient for complete access by cellulases, although total digestion

of cellulose in both treated straw and model pulp was observed. Both treated straw and model pulps were

subjected to wet mechanical attrition, which caused separation of smaller fibril aggregates and fragments,

significantly increasing enzyme hydrolysis rate. No evidence from WAXD measurements was found for preferential

hydrolysis of non-crystalline cellulose at intermediate extent of digestion, for both wood pulp and hydrothermally

treated straw.

Conclusions: The increased efficiency of enzyme digestion of cellulose in the lignocellulosic cell wall following

hydrothermal treatment is a consequence of the improved fibril accessibility due to the loss of hemicellulose and

disruption of lignin. However, incomplete accessibility of cellulase at the internal surfaces of fibrillar aggregates

implies that etching type mechanisms will be important in achieving complete hydrolysis. The reduction in

crystalline perfection following hydrothermal treatment may lead to an increase in fibril reactivity, which could

amplify the overall improvement in rate of digestion due to accessibility gains. The lack of preferential digestion of

non-crystalline cellulose is consistent with the existence of localised conformational disorder, at surfaces and

defects, according to proposed semicrystalline fibril models. Cellulases may not interact in a fully selective manner

with such disordered environments, so fibril reactivity may be considered as a function of average conformational

states.

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