Cellulose crystallinity revisited with two-dimensional modeling of X-ray diffraction, moisture sorption, and isotopic exchange

Cellulose crystallinity has been repeatedly considered as a key factor for enzymatic digestibility, but consistent definition, measurement, and interpretation of “crystallinity” is often missing. We have been investigating cellulose crystallinity with a combination of three techniques: X-ray diffraction with modeling of two-dimensional patterns, water vapor sorption, and isotopic exchange in deuterated water. From several studies performed across wide spectrum of pretreated biomasses, two key conclusions have been reached. First, the primary effect that most leading pretreatments promote on the structure of plant cellulose is to increase the mean width of crystallites, most likely due to co-crystallization (fusion) of originally distinct crystallites. Increasing crystallite width may dominate the changes observed through “crystallinity indexes”. Second, for samples not submitted to intentional decrystallization (e.g., in ball milling) nor allomorph change (e.g., in ammonia pretreatment), cellulose is essentially crystalline, with the two following exceptions. i) Minor contents (<3%) of disordered cellulose may exist in domains along the fibrils. ii) Appreciable contents (up to 40%) of disordered cellulose exist due to intrinsic disorder in crystallite-crystallite interface regions, which are inherent to the lateral aggregation (bundling) of the crystallites. This intrinsic interfacial disorder is not immediately accessible to enzymes, but dominates the spectroscopic and diffraction signals usually assigned to disordered cellulose. These findings bring a new rationale for the role that cellulose “crystallinity” may have on enzymatic hydrolysis.