Decrystallization Thermodynamics of Four Common Cellulose Polymorphs and α-Chitin

Cellulose and chitin are the two most abundant biological materials on Earth, and hold significant potential as feedstocks for the production of renewable fuels and chemicals.  In the biosphere, processive enzymes degrade both cellulose and chitin by decrystallizing individual polymer chains from the surfaces of polymer crystals. The ability of enzymes to decrystallize polymer chains from crystals is likely a function of the crystal polymorph and chain location on the crystal surface.  To that end, we calculate the thermodynamic cost of cellulose decrystallization as a function of cellulose morphology, polymorph type, and oligomer length at the atomic scale using large-scale, free energy simulations.  α-Chitin decrystallization is also evaluated as a function of morphology.  Our results provide insight into the molecular-level origins of cellulose and chitin insolubility; as well as how enzymes may have evolved to degrade biomass in the biosphere.  These results also suggest methods for increasing cellulose digestibility for biofuels applications; as well as new experimental measurements regarding the impacts microfibril shape and polymorph type have on the conversion kinetics.