Cooperative Activation of Cellulose with Natural Calcium.

Naturally occurring metals, such as calcium, catalytically activate the intermonomer β-glycosidic bonds in long chains of cellulose, initiating reactions with volatile oxygenates for renewable applications. In this work, the millisecond kinetics of calcium-catalyzed reactions were measured via the method of the pulse-heated analysis of solid and surface reactions (PHASR) at high temperatures (370-430 °C) to reveal accelerated glycosidic ether scission with a second-order rate dependence on the Ca2+ ions. First-principles density functional theory (DFT) calculations were used to identify stable binding configurations for two Ca2+ ions that demonstrated accelerated transglycosylation kinetics, with an apparent activation barrier of 50 kcal mol-1 for a cooperative calcium-catalyzed cycle. The agreement of the mechanism with calcium cooperativity to the experimental barrier (48.7 ± 2.8 kcal mol-1) suggests that calcium enhances the reactivity through a primary role of stabilizing charged transition states and a secondary role of disrupting native H-bonding.