Deceleration of Proton Exchange between Monosaccharides and Water by Metallocryptand and Related Compounds

We report an unexpected yet powerful ability of a tris(salen) nickel(II) metallocryptand LNi3 to dramatically decelerate proton exchange between monosaccharides and water. In the presence of only 0.025 equiv of LNi3, the exchange rates of glucose hydroxy protons with water decreased by up to 74-fold. Similar deceleration effects were also observed for mannose, fructose, galactose, and methanol. Comparative studies using a mononuclear analogue (LmNi) and a metal-free analogue (H6L) suggest that the salen framework itself reorganizes the local hydrogen-bonding network. These findings identify salen-based molecules as a new class of proton exchange modulators under nonphysiological conditions and provide a conceptual basis for new opportunities to study hydration and for molecular-level control of dynamic processes in glycobiology.