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Detecting Elusive Intermediates in Carbohydrate Conversion: A Dynamic Ensemble of Acyclic Glucose–Catalyst Complexes
journal contribution
posted on 2017-04-14, 00:00 authored by Sebastian Meier, Magnus Karlsson, Pernille Rose JensenThe role of acyclic carbohydrates
in pathways toward value-added
chemicals has remained poorly characterized due to the low population
of acyclic forms, and due to their instability under reaction conditions.
We conduct steady-state and pre-steady-state measurements by direct
reaction progress monitoring with sensitivity-optimized NMR spectroscopy
in the molybdate-catalyzed epimerization of glucose to mannose. We
detect an exchanging pool of at least 5 acyclic glucose–catalyst
complexes under near-optimum reaction conditions. In the presence
of catalyst, the acyclic glucose population increases within few seconds
prior to reaching a steady state. Exchange between the acyclic intermediates
increases under conditions that favor epimerization. Species accounting
for less than 0.05% of total glucose can be monitored with subsecond
time resolution to allow kinetic analysis of intermediate formation
and catalytic conversion. Epimerization occurs 2–3 orders of
magnitude faster than the binding of acyclic glucose to the catalyst
under near-optimum reaction conditions. The current study brings insight
into the nature of acyclic intermediate-catalyst complexes of very
low population and into experimental strategies for characterizing
very minor intermediates in carbohydrate conversion to value-added
compounds.
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Keywords
sensitivity-optimized NMR spectroscopyElusive Intermediatescarbohydrate conversionmolybdate-catalyzed epimerizationacyclic intermediate-catalyst complexesDynamic Ensembleacyclic glucoseCarbohydrate Conversionreaction progress monitoringacyclic intermediates increasesspecies accountingsubsecond time resolutionacyclic carbohydratesreaction conditionspre-steady-state measurementsfavor epimerizationacyclic glucose population increasesnear-optimum reaction conditionsacyclic forms
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