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submitted on 02.10.2020 and posted on 05.10.2020by Gregory G. Facas, Vineet Maliekkal, Cheng Zhu, Matthew Neurock, Paul Dauenhauer
occurring metals such as calcium catalytically activate the inter-monomer
β-glycosidic bonds in long chains of cellulose initiating reactions to volatile
oxygenates for renewable applications. In this work, the millisecond kinetics
of calcium catalyzed reactions were measured via the method of pulse-heated
analysis of solid/surface reactions (PHASR) at high temperature (370-430 °C) to
reveal accelerated glycosidic ether scission with a second order rate
dependence on 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 mechanism with calcium
cooperativity to the experimental barrier (48.7 ± 2.8 kcal mol-1)
suggests that calcium enhances reactivity through a dual role of disrupting
native H-bonding and stabilizing charged transition states.