Isomerization of glucose to fructose and mannose

Glucose is the most abundant basic building block for plant biomass. Acid or base catalyzed glucose reactions typically have multiple pathways forming condensation, dehydration and isomerization products. This is due to the multiple reactive hydroxyl groups on the glucose molecule as well as its conformational complexity. Here glucose isomerization reactions to fructose and mannose are investigated. Car-Parrinallo ab initio molecular dynamics simulations (CPMD) coupled with metadynamics (MTD) simulations were conducted to investigate glucose isomerization reactions in acidic aqueous solution. The mechanisms and the rate-limiting steps were elucidated. Both proton-catalyzed glucose to fructose and mannose isomerizations are initiated by protonation of C2-OH on the glucose ring. The subsequent steps involve the reorganization of the carbocations produced.  In the case of glucose to fructose isomerization reaction, a hydride shift from C2 to C1 occurs to form a tertiary carbocation. In the case of glucose to mannose isomerization, a hydroxyl group shift from C1 to C2 occurs to form a stable C1 carbocation. The free energy surfaces and associated barriers were determined.