The mutarotation kinetics and equilibrium of glucose in aqueous solution are perturbed by the presence of metal cations, with Ca2+ showing one of the strongest effects, significantly shifting the equilibrium to favor the α-anomer. In this work we quantify this effect using a combination of polarimetry and 1H NMR. In the NMR measurements we use DOSY filtering to suppress the interfering HDO peak which originates in the rapidly-exchanging glucose hydroxyl protons. NMR and polarimetry give closely comparable results on anomeric abundance, although in agreement with literature studies on the unperturbed mutarotation equilibrium in aqueous glucose solutions we find that KNMR is consistently slightly smaller than Kpol. Polarimetric measurements confirm that the effect is largely cationic in character. and show that a catalytic effect of the cation on the mutarotation rate is diminished by increased solution viscosity at higher concentrations. Computational results on the glucose-Ca2+ system indicate that the dominant ion-molecule structures in solution are relatively undistorted ones with anomeric couplings 3JHCCH that are not strongly perturbed from their values for aqueous glucose solutions.

Effect of metal cations on glucose mutarotation: Complementary studies using polarimetry and DOSY-filtered NMR

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