Molecular-Level Origins of Biomass Recalcitrance: Decrystallization Free Energies for Four Common Cellulose Polymorphs

Cellulose is a crystalline polymer of β1,4-d-glucose that is difficult to deconstruct to sugars by enzymes. The recalcitrance of cellulose microfibrils is a function of both the shape of cellulose microfibrils and the intrinsic work required to decrystallize individual chains, the latter of which is calculated here from the surfaces of four crystalline cellulose polymorphs: cellulose Iβ, cellulose Iα, cellulose II, and cellulose III<sub>I</sub>. For edge chains, the order of decrystallization work is as follows (from highest to lowest): Iβ, Iα, ΙΙΙ<sub>Ι</sub>, and II. For cellulose Iβ, we compare chains from three different locations on the surface and find that an increasing number of intralayer hydrogen bonds (from 0 to 2) increases the intrinsic decrystallization work. From these results, we propose a microkinetic model for the deconstruction of cellulose (and chitin) by processive enzymes, which when taken with a previous study [Horn et al. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 18089] identifies the thermodynamic and kinetic attributes of enzyme and substrate engineering for enhanced cellulose (or chitin) conversion. Overall, this study provides new insights into the molecular interactions that form the structural basis of cellulose, which is the primary building block of plant cell walls, and highlights the need for experimentally determining microfibril shape at the nanometer length scale when comparing conversion rates of cellulose polymorphs by enzymes.