Ab initio and experimental studies on the protonation of glucose in the gas phase

Protonations of alpha- and beta-D-glucopyranose in the gas phase were investigated using the ab initio molecular orbital approach at the HF/6-31G* level with full geometry optimization. Minimum-energy structures of three neutral and six protonated species for each anomer were calculated, Geometries, energies, and intramolecular hydrogen bonding in these structures are discussed. For the neutral species at 298 K the order of stability for the hydroxymethyl conformers is calculated to be GT > CG > TG for the a anomer and GG > CT > TG fur the beta anomer. Protonated species that least disrupt the internal hydrogen bonding network in the neutral species are considered: these include protonations on the oxygen sites labeled as the hydroxymethyl O6, the ring O5, and the exocylic hydroxyl O4. The O6 protonation in the TG conformation is electronically most favored. Energy corrections fur basis-set deficiency and electron-correlation omission in the adopted theoretical procedure were estimated from high-level calculations on ethanol, 2-propanol, and dimethyl ether. In addition, the gas-phase basicity (GB) of glucose was measured by proton transfer reactions in a Fourier transform ion cyclotron resonance mass spectrometer. The experimental GB values for both anomers were determined to be 188 +/- 3 kcal/mol. The experimental values are compared with the ab initio estimates of 178-190 and 177-189 kcal/mol for the respective alpha and beta anomers. Theoretical structures for the lowest-electronic-energy protonated species in the three hydroxymethyl conformations of each anomer are also presented to serve as reference data for postulating various kinetic pathways.