The effect of the microstructure of cellulose fibrils on recalcitrance

The origin of the recalcitrance of highly crystalline cellulose fibrils is yet unsolved.  Comparative studies in our group show contrasting digestibility of cellulose Iα fibrils from Gluconacetobacter xylinus (CI 60-70%) and Cladophora aegagropila (CI ~90%) by purified Trichoderma reesei Cel7A (TrCel7A) (reducing-end specific cellobiohydrolase). Where the bacterial cellulose (BC) fibrils from G. xylinus can be rapidly hydrolyzed to near completion by TrCel7A (> 99%), under identical reaction conditions, TrCel7A hydrolysis of the algal cellulose (AC) fibrils from C. aegagropila is slow and limited (~35%). The differences in the recalcitrance between BC and AC fibrils cannot be explained by crystalline morphology or crystallinity.  Additionally, we demonstrate that neither fibril lengths nor average degrees of polymerization (i.e. reducing end concentrations) account for the contrasting reactivities. We recently demonstrated that BC fibrils undergo extensive fibrillation where larger fibrils (20-80 nm) dissociate into microfibrils of ~ 3-5 nm during hydrolysis by TrCel7A. We speculated that this microstructural change in the fibrils play a role in improving productive binding by TrCel7A thereby contributing to the observed rapid hydrolysis.  In contrast, AC fibrils have not been observed to fibrillate. Rather, the hydrolysis of AC fibrils by TrCel7A roughens the topography of the fibril surfaces in a manner suggesting erosion of microfibrils at the fibril surface. Moreover, we find a surprising but significant widening of AC fibrils hydrolyzed by TrCel7A. Studies are underway to understand if fibril widening is due to fibril swelling as microfibril-microfibril interactions loosen, or to lateral association of multiple fibrils during hydrolysis.