Isotope Effect-Mapping of the Ionization of Glucose Demonstrates Unusual Charge Sharing

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Abstract
Isotopic substitution is known to affect kinetic rate constants and equilibrium constants in chemistry. In this study, we have used tritium substitution and high pH to probe the glucose ⇋ glucose- + H+ equilibrium. Passing partially ionized mixtures of [3H]- and [14C]glucose over anionic exchange resin has permitted the detection of subtle differences in pKa. We have found that, at pH 11.7 in an anionic exchange system, [3H]glucose always elutes ahead of the [14C]glucose, and we report isotope effects of 1.051 ± 0.0007, 1.012 ± 1.0005, 1.014 ± 0.0004, 1.024 ± 0.0003, 1.014 ± 0.0004, and 1.015 ± 0.0014 for [1-3H]-, [2-3H]-, [3-3H]-, [4-3H]-, [5-3H]-, and [6,6-3H2]glucose, respectively, as compared to either [2-14C]-or [6-14C]glucose. The elevated isotope effects at H1 and H4 imply unusual charge sharing in anionic aqueous glucose. Base titration of 13C-chemical shift changes demonstrates the dominance of pyranose forms at elevated pH, and ab initio isotope effect computations on gas-phase glucose anions are presented.